The Red Seal exam

• 135 questions, all multiple choice with four options (A to D), exactly one correct answer. They are not trick questions, but the wrong answers are not always obvious.
• 70% is the pass mark. You get 4 hours.
• The acronym list at the bottom of this section is handed out with the exam, so you don't have to memorize what DPF or SAHR stands for. You do have to know what they do.
• Questions are written from the Red Seal Occupational Standard (it replaced the old National Occupational Analysis, NOA) by people in the trade from across Canada.

Question types

Exam breakdown

• D Electrical and G Hydraulics are 50 of the 135 questions. Then B Engines (20). Study in that order.
• Each task covers "services, diagnoses and repairs". Expect questions on all three: maintenance intervals and procedures, reading symptoms and test results, and the right repair.

Strategy

• For cause-and-effect questions, ask: is this a flow problem (speed, cycle time) or a pressure problem (force, torque)? That one split answers a lot of hydraulic and transmission questions. For electrical: is it an open, a short, a short to ground or high resistance?
• Diagnostic order the notes repeat in nearly every system: gather information → talk to the operator → visual inspection (fluid level and condition first) → operational tests at operating temperature → pressure / flow / voltage-drop tests → find the root cause → repair → verify.
• "What is checked first?" is almost always the easiest, cheapest, most likely thing: fluid level, belt, battery state of charge, fuses, connections, filters.
• Read every option before answering. If two look right, pick the one that fixes the cause, not the symptom.

Acronyms handed out with the exam

Formula sheet

Key numbers

A-1 · Performs safety-related functions

Responsibility and hazard control

• Health and safety is shared by government, employers and employees.
• Worker duties: wear and inspect PPE, report hazards and injuries, refuse unsafe work, take part in safety programs.
• Hazard controls, best to worst: engineering (remove or guard the hazard) → administrative (procedures, training, signs) → PPE (last line of defence). Isolation sits with engineering controls.
• Hazard types: biological, chemical, ergonomic, physical. Slips and trips are the number one shop danger, so good housekeeping comes first.
• WCB is funded by employers (not workers or government).
• Both the technician and the employer are responsible for the safety of a vehicle that is serviced and returned to the road.

PPE

• PPE must meet the CSA standard. The employee wears and inspects it; the employer enforces it.
• Eye protection classes: 1 glasses, 2 goggles, 3 welding helmet, 4 welding hand shield, 5 hood, 6 face shield, 7 respirator.
• Boots: green triangle = grade 1 toe (125 J impact); yellow triangle = grade 2 (90 J).
• Hard hats: Class G (general) impact and penetration, non-conducting; Class E (electrical) same but tested to a higher voltage; Class C (conducting) impact and penetration only.

WHMIS

• Three key elements: worker education, labels, safety data sheets (SDS).
• Supplier: classify the product, label it, provide and update the SDS. Employer: train workers (job-specific and general) and make the SDS available. Worker: take part, use workplace labels, use PPE.
• Supplier label: product identifier, pictograms, signal word, hazard and precautionary statements, supplier ID. Workplace label: product name matching the SDS, safe handling, reference to the SDS.
• Pictograms are a red-bordered diamond; biohazard uses a black circle border. The SDS is the most complete source of information.

Fire

• Fire triangle: fuel, air (oxygen), ignition (heat). Remove one and the fire goes out.
• Classes: A ordinary combustibles (wood, paper), B flammable liquids, C electrical, D combustible metals, K cooking oils. An ABC (multipurpose dry chemical) extinguisher handles A, B and C.
• Call for help first. Never fight a fire if it could block your escape, you don't have the right extinguisher, it has spread, or the extinguisher is empty or not working.
• PASS: Pull the pin, Aim at the base, Squeeze the handle, Sweep side to side.
• Ground bulk fuel tanks with a rod into the earth; use a ground strap between the machine and the tank when refuelling. Gasoline cans are red, diesel yellow.

Shop hazards

• Carbon monoxide is colourless, odourless and tasteless; it displaces oxygen in the blood. Never run an engine indoors without exhaust extraction.
• Shop ventilation should give about 4 air changes per hour; local extraction captures fumes at the source.
• Confined space: small space with restricted entry and exit. Test the air with a gas detector before entry and have an easy way out.
• Lock-out / tag-out (LOTO): before work, shut the machine down, remove the key, isolate the energy (battery disconnect, pressure, stored hydraulic or spring energy), lock and tag it with your own lock, then try to start it to verify zero energy. Only the person who put the lock on removes it.
• Stored energy: lower attachments to the ground, block raised loads, relieve hydraulic pressure and accumulators, install articulation and steering lock bars.
• Check for hydraulic pinhole leaks with cardboard, never your hand (injection injury).
• Never work under a machine held only by jacks. Use stands, blocking or cribbing.
• When removing a battery, disconnect the ground cable first (reconnect it last).
• Hybrid and electric equipment have their own safety protocols (high voltage isolation). See section I.

A-2 · Uses and maintains tools and equipment

Hand tools

• Punches: prick punch 30° point (makes a mark); centre punch 60° (enlarges the mark for drilling); taper/starting punch (breaks a pin loose); parallel punch (same diameter full length; drives it out); aligning punch; roll pin punch.
• Chisels: flat (60–70°); cape chisel cuts keyways; diamond point; half round; rivet buster; bushing cutter.
• Hacksaw: teeth face away from the handle; at least 2 teeth on the material; 40–50 strokes per minute; more teeth per inch for thinner metal.
• Files: single cut (one row) and double cut (two rows). Clean with a file card; a scorer for stuck pieces.
• Twist drills: 118° included angle (59° each side), 12–15° lip clearance. Carbon steel bits run at about half the speed of high speed steel.
• Taps in order: taper (about 6 threads chamfered), plug, bottoming (no chamfer). Tap drill = diameter − pitch. Thread inserts (helicoil) are often stronger than the original thread.
• Reamers: always turn clockwise, in and out.
• Debur tube before flaring.
• Bench grinders: wheel rated above grinder RPM; tool rest and guard as close to the wheel as possible.

Measuring tools

• Imperial micrometer: 40 threads per inch, so one turn = 0.025 in; vernier micrometers read 0.0001 in.
• Inside micrometers start at about 2 in; small hole gauges about ⅛–½ in; telescoping gauges about 5/16–6 in (both transferred to an outside micrometer).
• Depth micrometer scale is reversed.
• Dial indicator: preload the stem to mid-travel before measuring.
• Feeler gauges: slight drag. Go/no-go (stepped) gauges.
• Out of round = biggest minus smallest at one spot (measure 90° apart). Taper = difference along the length. Max wear = worst reading vs spec.
• Torque wrench: pick one where the target torque is in the middle half of its scale. Extensions don't change torque; adaptors that add length do. Check accuracy yearly (4–6% is acceptable).
• Spring scale preload: scale reading × radius (lever length) = torque.
• Torque = force × lever length (N·m, lb-ft).

Shop and access equipment

• Extension ladders: at least 1 m above the landing; never stand on the top rung.
• Scaffold guardrails on any platform 3 m (10 ft) or higher; toe boards stop tools falling; scaffolds support about 4 × the load.
• Fall arrest: full body harness, inspected each shift; remove from service after a fall.
• Forklifts: pneumatic tires outside, solid inside; exhaust fumes are the indoor danger.
• Jacks: never use brake fluid in a hydraulic jack. Dunnage/blocking can widen a jack stand's base.

Hoisting and rigging

• Safe working load (SWL) is the most important factor when choosing rigging. Breaking strength = load that breaks it new.
• Safety factors: 5:1 general rigging, 10:1 lifting people, 20:1 elevators. Chain is usually 4:1 (minimum ultimate load vs working load limit).
• Shock loading: stress from sudden starts and stops.
• Web slings: discard if the red core yarn shows; must have the manufacturer's tag. Chain: discard for stretch, wear over 10% of link diameter, cuts, nicks or welds; the grade is embossed on the links.
• The employer inspects rigging at the start of each shift.
• Recommended sling angle 60° (lower angles raise the load in each leg). With a 4-leg bridle, assume only 2 legs carry the load.
• Shoulderless eyebolts: straight (90°) pulls only. Shackle pins are not interchangeable.
• All hooks need a safety latch (except grab and sorting hooks); otherwise mouse the hook with wire.
• Hitches: vertical, bridle, basket (U around the load), double wrap basket for pipe and smooth cylinders, choker (reduces SWL, not for loose bundles), double wrap choker draws loose bundles tight.
• Use tag lines; never touch the load. Centre of gravity = where the weight is concentrated.
• Signals: emergency stop = both arms swept out from the body at shoulder height.
• Sheaves: minimum diameter about 20 × rope diameter; check the groove with a gauge.

Wire rope

• Lay is viewed facing the end of the rope. Strands to the right = right lay, to the left = left lay. Wires same way as strands = lang lay; opposite = regular lay (wires look straight along the rope).
• Measure diameter across the crowns (widest point), not the flats. Classified as strands × wires per strand. Bigger wires = more abrasion resistance, less flexibility.
• Clips: "never saddle a dead horse" (U-bolt on the dead/cut end, saddle on the live end).
• Drum winding: right lay = right hand, left lay = left hand (overwind index finger up, underwind finger down). First wraps tight. Lubricate regularly, inspect weekly.
• IWRC (independent wire rope core) is stronger; fibre core is more flexible. Preformed rope resists unravelling.

Oxy-acetylene heating and cutting

• Oil or grease + oxygen can explode with no spark. Oxygen valve: right-hand thread, frangible (burst) disc, open fully or close fully.
• Acetylene: unstable above 15 psi, spontaneous above 28 psi. Lighter than air, garlic smell, flammable 2–89%. Cylinder filled with porous material and acetone; must be upright; draw no more than 1/7 of the cylinder per hour; fusible plugs melt about 100 °C. Left-hand thread.
• Propane is heavier than air (pools in low spots).
• Hoses: oxygen green, right-hand thread; fuel red, left-hand thread with a groove on the nut.
• Check valves stop gases mixing; flashback arrestors stop a flashback (heat-actuated shut-off).
• Leak test at 5 psi with approved leak detector. Shut down: close the cylinder, open the torch to bleed, release the regulator, close the torch (oxygen and fuel each).
• Flames: carburizing (white feather, excess fuel), neutral (feather just gone) for heating and cutting mild steel, oxidizing (short, loud, excess oxygen).
• Backfire: flame pops back into the tip (too little gas, dirty tip, too close). Sustained backfire = burning in the tip/mixer (screech, black smoke). Flashback: burning in the hoses (very wrong pressures, failure to purge).
• Non-ferrous metals can't be cut with oxy-fuel. Thinner material = more torch angle.

Electronic service tools

• Use a high-impedance digital multimeter (DMM) on electronics; test lights load circuits.
• Laptop service software connects through the diagnostic connector and a communication adapter to read codes, view live data, run tests and reprogram parameters within the OEM's limits.
• Before welding: disconnect the battery negative and unplug ECMs. Use a wrist strap (static) when handling modules.

A-3 · Performs routine work practices

Documentation and planning

• Service manuals, parts books, schematics, bulletins, work orders and the operator's report are your references. Schematics are drawn de-energized with components in their normal position; they show components, not wire lengths.
• Job plan: diagnose, find the parts and tools, schedule the downtime, do the repair, verify, document.
• Commercial Vehicle Inspection Program (CVIP): yearly inspection by a licensed inspector (renew every 5 years) at an approved facility (every 2 years). The inspector must refuse the certificate if the vehicle fails (owner has 10 days to repair). Decal on the front left of trailers or the windshield.

Maintenance systems

• Breakdown maintenance (fix it when it breaks) causes secondary damage, high cost and downtime.
• Preventive: scheduled by kilometres (on-road) or hours (off-road).
• Predictive / scheduled component replacement: parts changed on a set schedule, guided by oil sampling and condition monitoring.
• Oil sampling: same point, regular intervals, take it hot and mid-stream. Silicon = dirt (intake leak), iron = cylinder/ring wear, lead/copper = bearings.

Fluids and lubricants

• Viscosity = resistance to flow. High viscosity = slow flow, more power needed, sluggish; low = internal leakage and heat. Viscosity index: high VI = little change with temperature.
• Multigrade (10W-40): acts like SAE 10 cold, SAE 40 hot.
• Additives: detergents prevent deposits, dispersants keep dirt suspended, oxidation inhibitors, anti-rust, anti-foam, pour point depressants.
• API service: S = spark ignition, C = compression ignition. Modern diesels with aftertreatment need low-ash oil (CJ-4 and newer).
• Hydraulic oil jobs: transmit power, lubricate, seal, cool. Never mix oils. Higher temperature = faster oxidation.
• Grease is graded by NLGI number (stiffness): 000 very fluid to 6 very firm; 1–3 most common. Use water-resistant lithium grease where called for.

Hoses, tubing, piping and fittings

• Pipe: sized by nominal ID and schedule (wall thickness), can't be bent, seals on NPT threads. Tubing: sized by OD and wall, can be bent. Hose: sized by ID in dash sizes (1 dash = 1/16 in).
• NPT = tapered interference seal (needs sealant). JIC = 37° flare (hydraulics). SAE = 45° flare (air brakes). ORFS (O-ring face seal), ORB (O-ring boss), split flange (bolted clamps, O-ring on a flat face).
• Hose ends: reusable (screw-on, push-on) or permanent crimped; some hoses must be skived (outer cover removed).
• Air brake lines: nylon with compression fittings, hose where there is heat, movement or abrasion. Flares are not used on air brake steel tube (compression fittings).

Bearings

• Radial load = 90° to the shaft; axial (thrust) = along the shaft.
• Plain (friction) bearings: small, low speed and load. Crush: press fit that seats the shell and transfers heat.
• Tapered roller = radial and axial (one direction). Straight roller = radial only. Spherical roller = radial + some axial, allows shaft misalignment. Needle = tight spaces. Ball: radial (deep groove takes some thrust); thrust bearings = axial.
• Number one cause of bearing failure: contamination. Failure clues: peeling (frosted) = poor lubrication; spalling = heavy preload or poor film; brinelling (dents) = impact or lack of creep; fluting/arcing = electrical current (welding ground!); fatigue = overload, shock, heat; etching = water.
• Never use a direct flame to remove; never use brass drifts (flakes contaminate) — use soft iron. Clean in fresh solvent, air dry without spinning, oil and wrap in wax paper.
• Heating limits: open bearing 120 °C (250 °F), sealed 80 °C (180 °F).
• Pressing on a shaft: push the inner race. Into a bore: push the outer race.
• Preload (not a clearance) is set with shims, spacers or threaded rings and reduces shaft deflection.

Seals

• Seal jobs: keep fluid in, keep dirt out, hold pressure or vacuum.
• Static (gaskets, sealants) vs dynamic. Don't use silicone with paper gaskets.
• A lip seals in the direction it points. Single lip (inclusion seal), spring-loaded lip (more pressure), double lip (one spring-loaded for oil, one dry for dirt), dual lip (both spring-loaded, between two oil-filled housings). Reversed = exclusion seal (keeps dirt out).
• Clearance seals (piston rings), wind-backs (threads that pump oil back; only work turning), face seals (duo-cone): high pressure, low leakage, no wear on the shaft.

Fasteners and sealing materials

• Thread terms: crest, root, pitch (crest to crest), major and minor diameter. Imperial TPI = 1 ÷ pitch; metric is the pitch in mm. UNC, UNF, UNEF, NPT (tapered pipe).
• Imperial grade: radial lines + 2 (3 lines = grade 5, 6 lines = grade 8; blank = grade 2). Metric: number stamped (8.8, 10.9). Thread class: A external, B internal.
• Fully tightened: 2–3 threads showing. Cap screw = bolt in a tapped hole. Machine screws are under ¼ in.
• Torque in stages (⅓, ⅔, full) and in sequence. Torque-to-yield / torque-and-turn: torque, then turn a set angle (stretches the bolt). Lubricant changes the torque reading; add run-down torque (prevailing torque) to the final value. Check blind holes for fluid (hydraulic lock cracks the casting).
• Anaerobic sealant cures without air (thread locker, removed with heat); aerobic needs air. Cotter pins are sized from the bottom of the eye to the end of the short leg. Taper pins drive out from the small end.

Safety features and check-out

• Keep guards, ROPS/FOPS, seat belts, backup alarms, lights, horns, steering and brake locks, fire suppression and interlocks working. Never bypass a safety interlock (neutral start, seat switch, lockout lever).
• After any repair: operational check-out at operating temperature, check for leaks, verify the complaint is fixed, replace guards and panels, remove tools and diagnostic gear, clear codes, document.

Materials

• Tensile strength = load before it breaks; elastic limit = load it can take and still return to shape; yield strength = point it permanently bends.
• Hardening = heat and quench (hard, brittle); tempering = reheat to add toughness; annealing = heat and cool slowly (soft).
• Cast iron: high carbon, brittle, porous, coarse threads. Stainless: chromium. Heat-treated frames must not be heated or welded.

A-4 · Uses communication and mentoring techniques

• Journeypersons/supervisors: coach, mentor, supervise apprentices, fill out the record book. Employers: provide training and supervision, make sure the book is filled out. Apprentices: complete training and keep the record book (blue book) current.
• Good mentors lead by example, are approachable, willing to teach, challenge trainees, respect them and encourage independent work.
• Coaching method: Plan → Tell → Show → Do (then feedback).
• Feedback: positive (says what was good, builds confidence), empty ("good job", says nothing specific), constructive (corrects without insulting).
• A lesson has: the point, the link to the trade, the individual steps.
• Feedback gives clear direction, practice, a summary of key points, and an informal assessment.
• Communication: ask questions, listen to the operator (they know the machine best), write clear work orders. Customers are the most important business asset.

• Red Seal = mobility of skilled workers across Canada and a standard of excellence. One host jurisdiction handles each trade. Sponsored by HRSDC.
• The NOA (National Occupational Analysis) is the foundation document for the Red Seal exam. Organized into blocks → tasks → sub-tasks.
• NOA development: host holds a workshop with HRSDC → draft → sent to all jurisdictions → feedback → committee revises → HRSDC publishes.

• Pass mark: 70%. Questions written by industry practitioners.
• Question types: recall/knowledge, procedural/application (at least two steps), critical thinking (cause and effect; most common on newer exams).

B-5 · Base engines

Fundamentals

• Compression ignition (diesel): heat of compression ignites atomized fuel. Spark ignition (gas, propane, natural gas): a timed spark.
• Diesel compresses air only, speed/power controlled by amount of fuel, no intake throttle, about 18:1 compression. Gasoline compresses a fuel-air mix, throttles the air, about 9:1, can't use a starting aid.
• Bore = cylinder diameter. Stroke = TDC to BDC = 2 × crank throw. Longer stroke = more torque.
• Displacement = bore area × stroke (× cylinders). Compression ratio = (swept + clearance volume) ÷ clearance volume.
• Square: bore = stroke. Oversquare: bore > stroke. Undersquare: bore < stroke.
• Volumetric efficiency = actual air ÷ theoretical air. Naturally aspirated 75–85%, turbocharged 100%+. Lowered by intake restriction.
• Thermal efficiency: only about 33–36% of fuel heat does work (about a third each to work, exhaust and cooling). Higher compression = higher thermal efficiency.
• Brake HP = indicated HP − friction HP. Mechanical efficiency = BHP ÷ IHP. HP = torque × RPM ÷ 5252; 1 hp = 746 W.
• Four-stroke = 720° per cycle, cam turns at ½ crank speed. Two-stroke = 360°, cam = crank speed, uses scavenging. Four-stroke has a longer power stroke (more efficient).
• Valve overlap: degrees both valves are open (end of exhaust, start of intake). Fuel is injected slightly before TDC.

Block and liners

• Inline cylinders are numbered from the front (fan end). Engine rotation is viewed from the front; left/right bank and everything else from the rear (flywheel). Most engines turn clockwise from the front.
• Frost (expansion) plugs protect the block if coolant freezes and must come out to clean the block properly.
• Integral cylinders (bored in the block, can be machined oversize) or replaceable liners.
• Dry liner: no coolant contact, tight fit for heat transfer. Wet liner: coolant contact, heavier wall, O-rings at the bottom seal coolant from the crankcase. Mid-stop: coolant only around the upper (combustion) area; lower part supported by the block.
• Liner protrusion: the flange stands slightly above the deck so the head clamps it. Too low = head gasket failure. Height = flange thickness + shims − counterbore depth. Measure 4 places; within 0.001 in on a liner, 0.002–0.003 in liner to liner.
• Liner cavitation: vapour bubbles implode on the coolant side, caused by liner vibration. Prevent with correct coolant additives (SCA) and correct liner fit.
• Wet liner O-rings: lubricate with vegetable oil or soap right before install. A twisted O-ring can distort the bore (piston scuffing or seizure). Store liners vertical.
• Deck flatness: straightedge and feeler gauge, at least one lengthwise and two diagonal; typically ≤ 0.004 in. Stamp the amount removed on a non-sealing surface. Machining the deck raises compression ratio.
• Main bore alignment: rule of thumb no more than ½ the bearing oil clearance. Line boring requires the caps installed and torqued.
• Crack detection: dye penetrant (any metal, portable, not on rough surfaces); magnetic particle (ferrous only); fluorescent magnetic particle (Magnaflux) = most accurate (ferrous only); pressure testing in hot water for internal cracks.
• Hot tank (caustic) = never for aluminum.
• Crack repair: welding (preheat), stitching (overlapping drilled and tapped plugs). Thread inserts beat drill-and-tap oversize.

Cylinder wear and honing

• Most wear is at the top of ring travel, 90° to the crankshaft. Major thrust side takes the combustion (power) load; minor thrust side the compression load.
• Measure 2 places (in line with and 90° to the crank) at top and bottom of ring travel = max wear, out of round, taper. Original size = below ring travel.
• Oversize: round up to the next oversize; for taper or scoring, double the wear and go to the next size. Bore 0.002 in under then hone to size. Standard oversizes 0.010/0.020/0.030 in.
• Crosshatch holds oil so the rings seal (40–60°). Deglaze with a hone; clean with hot soapy water and a stiff brush (white rag test).
• Liner scoring you can catch a nail on = replace.

Pistons, rings and rods

• Piston: seal combustion force, transmit it to the crank, dissipate heat, support rings. Hypereutectic aluminum (high silicon); forged = strongest. Steel monotherm (one-piece), two-piece articulated (steel crown, aluminum skirt).
• Cam-ground skirt: oval, smaller across the pin bosses, so it becomes round when hot. Tapered skirt for crown expansion.
• Piston cooling: splash, spray (oil jets), circulation, shaker.
• Rings: compression rings seal using cylinder pressure, ring tension and oil film. Keystone rings resist carbon sticking (top grooves). Oil rings deposit oil on the upstroke and scrape it off on the downstroke.
• End gap allows heat expansion; check a new ring square in the bore below ring travel. Too small = ring ends butt and the piston can seize.
• Stagger the gaps; install bottom (oil) ring first; don't overlap expander ends; identification mark faces up. Use a ring expander (overstretched rings are scrap).
• Keystone ring grooves are checked with a groove gauge; rectangular with a new ring and feeler gauge. Worn grooves = blow-by and oil consumption.
• Piston-to-bore clearance: measure the skirt 90° to the pin, mid-skirt. Too much = piston slap; too little = scuffing.
• Remove the carbon/metal ridge before pushing pistons out (ridge reamer for metal). Piston at BDC to remove. Remove cooling nozzles first.
• Full-floating pin: free in rod and piston (snap rings, gap down). Semi-floating: fixed to the rod. Never hammer pins out; heat the piston if it's an interference fit.
• Rods: straight cut (90°) or bias cut (angled, lets the rod pass up the bore). Fractured caps fit only their own rod. Never mix caps. V-engines usually have 2 rods per journal (or fork-and-blade).
• Bent rods: from hydraulic lock or failure; cause uneven wear and lower compression.

Crankshaft and bearings

• Throw = main journal centre to rod journal centre; stroke = 2 × throw. Fillet = radius between journal and web (stress relief, never mark it). Counterweights offset the rod/piston mass.
• Forged cranks may have a blue/black finish.
• Firing: four-stroke cylinders fire every 720° ÷ cylinders. Running mates: 6-cylinder (153624) = 1&6, 5&2, 3&4. Two-strokes have none.
• Balance shafts turn at 2 × crank speed on four-strokes. Flywheel stores energy; fewer cylinders = bigger flywheel.
• Vibration damper controls torsional (twisting) vibration. Rubber: replace if cracked, swollen or the index lines moved (no more than 1/16 in). Viscous: replace if dented or leaking. Never hammer or pry it off.
• Torsional failure at the rod journal = loose/failed damper, loose flywheel, overspeed/overload. Bending failure at fillet/web = misaligned bores, bad bearings, loose caps, belt tension.
• Store cranks vertical. Mark main caps (#1 at front); changing positions changes bore geometry.
• Wear sleeves: remove by scoring with a chisel (stay away from flange bolt holes). Seal surfaces under 0.010 in scoring can be polished.
• Regrinding: amount stamped on a counterweight. Undersize bearings (thicker, smaller ID) fit a ground crank; oversize (bigger OD) fit a machined bore.
• Bearing properties: embeddability (absorb dirt), conformability (adapt to small flaws), fatigue resistance, corrosion resistance. More embeddable = less fatigue resistant.
• Spread (wider than the bore, snaps in), crush (heat transfer, stops spinning), crush relief, locating tang (positions the shell), dowels on thick shells.
• Number one cause of bearing failure = contamination. Clues: smeared overlay = lack of oil; deep scratches = large particles; greenish brown = fuel dilution; black = coolant; localized spot wear = something behind the shell; flaking = overload.
• Plastigage: correct size, no oil, don't turn the crank, compare the crush width to the chart.
• After torquing: crank must turn freely. End play needed for oil and expansion; check with a dial indicator, prying between a cap and counterweight. Too little = misaligned thrust surfaces (loosen caps, bump crank fore/aft, retorque).
• Flywheel housing concentricity must be checked (rear seal pressure, drivetrain vibration).
• Ring gear: heat and drive off with a blunt chisel (no cutting torch); heat to install, cool at room temperature. Never quench gears.

Camshaft and valve train

• Lobe lift = nose height above base circle. Duration = degrees the valve is open (built into the cam). Lobe separation angle.
• In-block cam (pushrods, followers) vs overhead cam (fewer parts, less friction, more accurate timing).
• Flat followers are slightly crowned and offset from the lobe so they rotate. Roller followers need an anti-rotation guide. Hydraulic followers need no lash.
• Idler gear: no change to ratio or speed, changes direction. Helical gears: smooth, but end thrust.
• Check gear backlash before removal and after assembly (all drive and idler gears). Removing material from the head (overhead cam) or deck moves gears closer = less backlash.
• Straight-body followers come out the top before the cam; mushroom followers come out after the cam (from below). Keep them in order.
• Bent cam check: V-blocks and dial indicator (journal not out of round first). Signs: binding, uneven bushing wear. Lobe lift: micrometer (height − base circle) or dial indicator; in-frame at the pushrod end of the rocker.
• Flat followers get extreme pressure lube; roller followers soaked in clean engine oil. Time the cam on install (align marks).
• Cam bushing wear is opposite the followers, except next to the drive gear (opposite gear mesh).

Cylinder heads and valves

• 4 valves per cylinder = better volumetric efficiency and turbulence. Injector sleeves (copper/brass) seal coolant from the combustion chamber.
• Valve cooling: about 75% through the face/seat, 25% through stem and guide. Sodium-filled exhaust valves run cooler.
• Seat inserts = replaceable wear surface. Removable guides wear better than integral.
• Positive valve seals (fixed on the guide, stem slides through) give the best control; umbrella seals ride with the stem.
• Rotators give a changing wear surface and keep the face clean. Progressive springs: tight coils toward the head.
• Rocker ratio = valve side length ÷ pushrod side length. Valve bridges (crossheads) let one rocker open two valves.
• Head gaskets: steel (heat resistance, poor conformability) or composite (better sealing). Studs: even clamping, less block thread stress, but need more room to remove the head.
• Remove injectors and glow plugs before removing the head. Loosen head bolts from the outside in.
• Check heads for cracks between valve seats and injector bores; pressure test in hot water with dummy injectors (some cracks only open hot).
• Flatness: straightedge and feeler gauge across and diagonally. Machining amount stamped on a non-sealing face.
• Guide wear (bellmouth) lets the valve tilt = poor sealing and oil consumption. Knurling is temporary; a sleeve (reamed after install) is permanent.
• Valves: replace if cracked (especially near the lock groove), burned or bent. Cupping = margin too thin plus heat. Margin after grinding must be at least half of new. Bent valves (piston contact) show runout.
• Face angles: 30° intake (better flow), 45° exhaust (more seating force). Interference angle: seat ½–1° steeper than the face.
• Seat contacts the middle of the face and is about ⅓ the face width. 15° stone narrows from the top; 60–70° stone narrows from the bottom. Check with Prussian blue (don't spin the valve).
• Seat too wide = low seating force, carbon, leaks. Too narrow = poor cooling, fast wear.
• Valve protrusion/recession: too high = higher compression and piston contact (grind the seat); too low = lower compression (new seat/valve).
• Springs: free length, squareness (≤ 1/16 in), tension at installed height. Replace if out of spec. Installed height shims: max 2, max 0.060 in.
• Vacuum test valve sealing: no more than 10 in Hg lost in 10 s.

Valve lash

• Lash allows for heat expansion and wear. Exhaust lash is usually larger than intake.
• Adjust with the follower on the base circle (valve closed). Running mates: when #1 is on overlap (rockers rocking), #6 is at TDC compression.
• Too much lash: noisy valve train, less power (valves open late, close early). Too little: valves may not seat when hot, burned valves and seats, valve train damage.

Engine brakes

• Diesels have no intake throttle, so no natural engine braking. Engine brakes supplement service brakes; they can't stop the vehicle alone.
• Conditions: fuel shut off, clutch engaged, transmission in gear. Most effective at high RPM. Depends on good compression.
• Compression brake (Jake): opens the exhaust valve near TDC on the compression stroke, so the energy used to compress the air is thrown away.
• Master piston is driven by the injector (or exhaust) rocker; it pumps oil to the slave piston, which opens the exhaust valves. Solenoid lets engine oil in.
• Adjust exhaust valve lash first, then slave lash. Too little slave lash: exhaust valves held open (hard start, poor running). Too much: late opening, damaged parts. Low oil pressure = weak or no braking; high oil pressure can hold valves open.
• Don't use in urban areas (noise). Excessive noise usually = exhaust leak or bad muffler.

Diagnosing the base engine

• Smoke colour: black = too much fuel or not enough air; blue = burning oil (rings, guides, turbo seals); white = unburned fuel (low compression, late timing, cold) or coolant.
• Compression and cylinder leak-down tests find rings, valves and head gaskets. Excessive crankcase pressure (blow-by) = worn rings/liners, also blows oil out of seals and the turbo.
• Knocks: piston slap (clearance), rod/main knock (bearing clearance), valve train clatter (lash).

B-6 · Lubrication systems

• Oil jobs: reduce wear, clean, cool, seal the rings.
• Most diesel engines use full-pressure lubrication: pan → pickup → pump → relief/regulator → cooler → filter → main gallery.
• Oil pump: positive displacement (gear or rotor), sized bigger than the engine needs. Wet sump (oil in the pan) or dry sump (remote tank, scavenge pumps).
• Regulating valve keeps pressure in its normal range; relief valve protects from overpressure and is set higher than the regulator.
• Oil cooler bypass (thermostatic or pressure) skips the cooler when cold. Oil-to-coolant or oil-to-air.
• Full-flow filter (all oil, about 25–60 µm) has a bypass valve so a plugged or cold filter still feeds the engine. Bypass filter (5–10% of the flow, about 10 µm) cleans finer and returns to the pan.
• Oil filter change: oil the gasket, hand tight to contact then ½–¾ turn.
• Pressure is high when oil is cold or new; low when hot, thin or diluted.

• Check the oil level on level ground, several minutes after shutdown.
• Crankcase ventilation removes blow-by gases, water and fuel vapour. Blocked breather = crankcase pressure, oil leaks at seals (often the turbo), dipstick or cap pushed out, sludge. Newer engines route the crankcase to the intake (closed crankcase ventilation, with a filter).
• Test pressure at operating temperature with a gauge that reads mid-range.
• Oil pump checks: gear-to-gear, gear-to-body and gear-to-cover clearance; prime it on assembly. Oil coolers: compare in and out temperature; pressure test in hot water; no caustic cleaners.
• Oil analysis: silicon = dirt (intake leak), iron = liner/ring wear, chromium = rings, lead/copper/tin = bearings, sodium/potassium/glycol = coolant leak.

B-7 · Intake systems

• Intake air is cleaned, cooled and silenced. Denser (cooler) air = more power; warm air or altitude = less.
• Air-fuel ratio (by weight): diesel about 80:1 at idle down to 18:1 at full load; gasoline about 14.7:1. The ECM controls it by changing fuel.
• Pre-cleaners use centrifugal force to remove up to 80% of large dirt; dust bowls must be emptied; dust ejector valves and exhaust aspirators (venturi) remove dust automatically.
• Dry pleated paper filters (about 99.5%): primary (outer) and secondary/safety (inner, only protects if the primary fails, don't clean it). Oil bath filters (older, 98.5%): too much or too thin oil can be sucked in and cause overspeed.
• Restriction indicator/gauge reads in inches of water after the filter. Service at about 20–25 in H₂O. Use a water manometer, never mercury, on the intake.
• Clean (only if allowed) by blowing from the inside out, nozzle 90° to the pleats.
• Any intake leak after the filter lets dirt in = fast ring and liner wear (dusting). Silicon in the oil sample confirms it. Constant-tension clamps keep hoses tight.
• Air shutdown blocks intake air; it's the surest way to stop a runaway engine (one running on outside fuel, like oil or flammable gas). Emergency use only.

Turbochargers

• Exhaust-driven centrifugal air pump. Speed depends on exhaust heat and flow (load), not directly on engine RPM.
• Turbocharged engines need lower compression, more fuel and adjusted (retarded) timing. Gains about 50% more power; compensates for altitude.
• Compressor housing (aluminum) and turbine housing (cast iron); shaft on bushings with thrust washers; piston-ring style seals keep oil out of the housings. Lubricated by engine oil.
• Roots blowers (gear driven, positive displacement) rob engine power and don't compensate for altitude.
• Always pre-lube a new turbo. Cool down 3–5 min at low idle before shutdown or the oil cokes in the bearings. Cover the stack when towing/transporting (windmilling with no oil).
• Low boost: intake restriction/leaks, exhaust leaks before the turbo, worn engine (also high crankcase pressure), fuel problems, leaking aftercooler. Check the intake and exhaust before blaming the turbo.
• Oil in the housings/blue smoke: high crankcase pressure, restricted oil drain, worn bearings, intake restriction. Wheel damage = foreign objects (leaky intake). Radial play can't be adjusted; some play is normal.
• Test boost at the intake manifold at full load. Mark housings before removing.
• Wastegate opens at set boost to bypass exhaust around the turbine. Stuck open = low power. Stuck closed or sensing line off = overboost and possible damage.
• Variable geometry turbo (VGT): moveable vanes. Closed = more boost and more back pressure (helps EGR flow and engine braking); open = less boost.
• Aftercooler/charge air cooler lowers intake temperature after the turbo = denser air. Air-to-air (in front of the radiator, needs moving air) or air-to-coolant (works without airflow). Pressure test for leaks (air-to-coolant in warm water).

B-8 · Exhaust systems

• Exhaust needs as little restriction as possible; test exhaust back pressure with a mercury manometer (in Hg).
• Manifolds (one or multi-piece), rigid pipe, flexible pipe where there is movement, mufflers (straight-through: low restriction; reverse-flow: quieter, more restriction), stacks with rain caps.
• Spark arrestor traps sparks (angled vanes, centrifugal force); critical in forestry.
• Exhaust leaks are an operator safety hazard (CO) and show as soot trails, especially at gaskets. No lock washers on exhaust (they lose tension with heat). Heat shields protect people and components.
• A plugged DPF/DOC or crushed pipe = high back pressure = low power, high exhaust temperature.
• Smoke diagnosis: black = rich (restricted intake, overfuelling); blue = oil; white = unburned fuel or coolant.

Exhaust brakes

• A valve after the engine and before the muffler (often on the turbo outlet) restricts exhaust flow; back pressure resists the pistons on the exhaust stroke. Quieter but weaker than a compression brake.
• Air or electrically closed, spring opened. Fuel must be shut off. Sliding or butterfly valve (butterfly lets some gas by).
• Use it regularly or it can seize from moisture.

B-9 · Engine management systems

ECM basics

• ECM: receives inputs, processes, controls outputs (closed loop). Inputs: operator switches, sensors (transducers), data link. Outputs: actuators (injector solenoids, relays), lamps.
• Memory: RAM temporary (volatile = lost with battery power), ROM permanent (factory), PROM programmed once, EPROM reprogrammed after removal, EEPROM reprogrammed in place with software (customer parameters).
• Sensors use a 5 V reference; signal 0.5–4.5 V is normal. Below 0.5 V = short to ground; above 4.5 V = open (or short to voltage).
• Thermistors: NTC (temp up, resistance down; most engine temp sensors) and PTC. Pressure sensors: 3 wires (5 V ref, signal, return); strain gauge in a bridge. Speed/position sensors: magnetic reluctance (AC signal) reading a toothed wheel or flywheel ring gear. TPS: 3-wire potentiometer, about 0.5 V idle, 4.5 V full.
• Three main fuelling inputs: throttle position, boost (intake manifold) pressure, engine speed/position. Also coolant, air and fuel temperature, barometric pressure.
• Fuel quantity is controlled by the pulse width (PWM duty cycle) of the injector solenoid.
• Injector calibration (trim) codes must be programmed into the ECM when an injector is replaced (compensates for manufacturing differences).
• Some ECMs are fuel-cooled (cooling plate).
• Engine protection: warning, warning + derate, warning + derate + shutdown. Mobile equipment shutdowns have a delay so the operator can get to safety. Each shutdown circuit is wired independently.

Fault codes and diagnosis

• FMI = failure mode (how it failed: short, open, out of range). SID = subsystem. PID/SPN = the parameter or component. MID = which module sent it.
• Active (hard) code = happening now. Inactive/intermittent (soft) = logged but not present.
• Order: check the sensor, then the harness, then clear codes and verify. Talk to the operator. A wiggle test (key on, engine off) finds intermittent connections.
• Testing an injector solenoid doesn't prove the injector can inject fuel.
• Harness repair: replace the harness if possible; otherwise a resistance-free crimped or soldered splice (rosin core, never acid core). Dielectric grease on connectors.
• Protect the ECM: unplug before welding, no fast charger to start, never disconnect batteries with the engine running, key off before unplugging, ground strap.
• Reprogramming parameters is done with OEM software, within factory limits.

Electronic fuel systems

• EUI (electronic unit injector): mechanically actuated by a cam lobe and rocker, electronically controlled by a solenoid poppet valve. Solenoid energized = poppet closed = injection.
• EUP (unit pump): cam-driven pump in the block, short high-pressure line to a nozzle. Lines must be the same length. Rotate so the tappet is on the base circle to remove/install.
• HEUI: hydraulically actuated by high-pressure engine oil (high-pressure oil pump), with an intensifier piston (about 6:1) raising fuel pressure. Injection actuation pressure control valve (ECM) sets oil pressure using the IAP sensor. Engine oil condition matters more.
• Common rail (HPCR): a high-pressure pump keeps fuel at injection pressure in the rail (up to about 26,000 psi). The solenoid only decides when and how long; several injections per cycle (pilot injection = quieter). Rail stays pressurized after shutdown. Pressure limiting valve; flow limiters shut off a leaking injector; replace rail lines once removed.
• Drain fuel passages before removing injectors (fuel in the cylinder = hydraulic lock; EUP fuel into the oil = dilution). New sealing washers. Bleed the fuel system before starting. Don't hit the solenoid when installing.
• Injector O-ring failure = fuel in the oil.

B-10 · Fuel delivery systems

Diesel fuel

• Cetane = ignition quality. Higher cetane = ignites easier (lower temperature). #1D (winter, cetane 45–50, thinner) and #2D (summer, more heat energy).
• Cloud point = waxes start to form. Pour point = lowest temperature it will flow. Flash point = gives off vapour that can ignite (return fuel can be hot).
• Ultra-low sulphur diesel: 15 ppm max. Sulphur makes acids and poisons aftertreatment.
• Water is heavier than diesel and settles; fuel should sit a day after a delivery. Microbes grow at the fuel-water line: slime, plugged filters, rotten egg smell; control with dry, clean fuel.
• Never add gasoline to diesel. If allowed, isopropyl alcohol only (max 0.5%).
• Fill tanks at the end of the shift (less condensation). Never galvanized tanks (zinc reacts). Tanks must be vented; pickup 1 in or more off the bottom (bulk 3–4 in); return and pickup at least 12 in apart. Dual tanks need a crossover line.

Combustion

• Needs air (density drops with altitude), atomized fuel and heat (500–700 °F). Ignition delay → flame propagation → direct combustion → after-burn.
• Combustion knock = long ignition delay (cold, advanced timing, low injection pressure, low cetane).
• Direct injection (open chamber): flat head, chamber in the piston, high pressure, good economy, easier starting. Indirect (pre-chamber): needs glow plugs, smoother, lower efficiency.

Starting aids

• Intake grid heaters; glow plugs (failure = hard or no start on pre-chamber engines); coolant heaters (circulating type heats more evenly); oil and fuel heaters.
• Fuel heaters go before the filter and must be thermostatic (too hot = power loss).
• Starting fluid (ether): only when needed; can damage engines and cause runaways; never with grid heaters or glow plugs.

Mechanical injection

• Five functions (TRAMP): Timing, Rate, Atomization, Metering, Penetration. Injection starts BTDC; timing advances as speed increases.
• Low-pressure (primary) side: tank → primary filter (suction, 25+ µm, often with water separator) → transfer pump → secondary filter (pressure, about 5 µm) → injection pump. High-pressure side: pump → lines → nozzles.
• A leak on the suction side lets air in (aeration); a leak on the pressure side leaks fuel out.
• High-pressure lines are all the same length and ID. Never bend or repair them; loosen with two flare-nut wrenches.
• Transfer pumps: diaphragm (spring sets max pressure; fuel from the vent hole = torn diaphragm), plunger, gear/vane (relief valve sets pressure; gear pump seal leak = fuel in the oil), hand primer (lock it down after use).
• Fill new filters through the inlet side with clean, filtered fuel (or let the primer do it). Bleed air before starting; air delays injection and causes rough running.

• Hard start after sitting = fuel draining back to the tank (bad check valve or suction leak).

Port and helix (inline) and rotary pumps

• Port and helix: plunger stroke is fixed by the cam; rotating the plunger (rack) changes the effective stroke (metering). Fixed beginning, variable end of injection. Vertical groove to the spill port = no fuel (shutoff).
• Delivery valve gives a fast start and sharp end of injection and keeps residual pressure in the lines.
• Aneroid / AFRC limits rack travel until there's boost = prevents black smoke on acceleration. A cracked diaphragm = low power.
• Static timing with #1 at TDC compression; turn the engine in its normal direction (takes out gear lash), never with the starter. Spill (port closing) timing is the most accurate for inline pumps. 1° pump = 2° crank.
• Rotary (opposed plunger) pump: transfer pressure rises with speed and also moves the advance piston; housing pressure cools and lubricates. A restricted return can shut the engine down. Timing by rotating the pump body in slotted holes; never loosen with the engine running. Don't spray cold water on a hot pump (seizure).

Nozzles

• Inward-opening needle opened by fuel pressure, closed by a spring (shims or screw set opening pressure). Needle and body are a matched set.
• Multi-hole nozzles (direct injection) open at the highest pressure; pintle nozzles (indirect) the lowest.
• Chatter on a test bench is normal (good atomization). Dribble (a drop at the tip after injection) is the most critical fault (smoke, carbon). Back leakage cools and lubricates; too much = worn needle-to-body clearance.
• Opening pressure too low = poor atomization and penetration. Copper sealing washers are never reused.
• On-engine tests: cylinder cut-out (crack the line or use the scan tool; a good cylinder makes RPM drop) and exhaust port temperature (a dead cylinder runs about 50 °C cooler).

Governors

• A governor controls fuel to hold speed against load: controls low idle, high idle, and on variable-speed governors, every speed in between. Diesels need one because they have no air throttle.
• Limiting speed (min/max) governor: controls only low and high idle; the operator controls everything between (on-highway). Variable speed (all-speed): holds the selected speed regardless of load (off-road).
• Flyweights try to reduce fuel as speed rises; the spring tries to add fuel; the throttle adds spring force.
• Speed droop = change from high idle (no load) to rated speed (full load). Isochronous = zero droop (generator sets). Hunting = speed surging (sticking linkage, weak spring). Sensitivity = how much speed changes before it reacts.
• Adjust idle speeds at operating temperature with no load, small changes, check linkage first. Breaking a seal can void the warranty; reseal after.
• Loss of control: load pushing the engine (downhill), overload, outside fuel source (runaway).

Tune-up and testing

• Tune up after injectors/valves are disturbed, poor running, or on schedule.
• Dynamometer measures brake horsepower: engine (flywheel), chassis (wheels, includes driveline losses) or PTO. Water brake or electric absorber. Chain and block on a chassis dyno; load only when warm; cool down after.
• Altitude derate (low air density): about 3% per 1000 ft for naturally aspirated engines.
• Diagnosis: gather information, operational test, list causes, test.

B-11 · Emission control systems

• Perfect combustion makes only CO₂, water and nitrogen. Diesels make less HC and CO but more NOx and soot.
• HC = unburned fuel. CO = not enough oxygen. NOx = high temperature and pressure. PM = soot. SOx = sulphur (acid rain). NOx + HC + sunlight = smog.
• On-road: EGR (2002), DOC + DPF (2007), SCR (2010). Off-road Tier 4 (2008–2015) reached similar limits.

DPF regeneration

• Passive: exhaust above about 260 °C (500 °F) during normal hard work. Active: fuel is dosed (doser valve or late post-injection) into the exhaust; the DOC burns it to raise temperature (about 600 °C). Started by the ECM, a dash switch, or the service tool (parked).
• Soot load is calculated from differential pressure across the DPF, fuel burned, and time since the last regen. 3 temperature sensors (DOC in, DOC out/DPF in, DPF out), 2 pressure sensors.
• Soot levels: 40% solid DPF lamp (regen soon), 60% flashing (derate soon), 80% + check engine lamp (derate, park and force a regen), above 80% + stop lamp (no operator regen; service).
• High exhaust system temperature lamp: exhaust very hot at low speed. A plugged DPF or DOC raises back pressure. DPF cleaning: special machine, flow test, thermal clean, flow test.

DEF and SCR

• DEF = 32.5% urea, 67.5% water; check concentration with a refractometer. Freezes at −11 °C. Plastic tank (some metals react), blue cap, shelf life about a year in a cool dry place.
• Coolant heats the tank; electric heaters on the lines and doser. Purge after key-off — don't cut battery power for about a minute.
• Low DEF never shuts the engine off for safety, but it derates progressively. Wrong fluid sets quality codes. NOx sensors at SCR inlet/outlet (handle carefully).
• Aftertreatment engines need ULSD and low-ash (CJ-4 or newer) oil; crankcase vapours are filtered. Low-sulphur diesel is not a substitute.

B-12 · Cooling systems

• About 33% of fuel heat goes into the cooling system. Heat moves by conduction (contact), convection (fluid movement) and radiation.
• Boiling point rises about 3 °F per 1 psi of cap pressure. Pressure also reduces cavitation and aeration.
• Air cooling: lighter, fewer parts, no coolant, but noisy and sensitive to dirt on the fins. Liquid cooling: more even temperature and control.
• Coolant: 50/50 (about −37 °C); up to 60% antifreeze. More than 60% can gel and protects worse. Corrosion inhibitors and SCA (supplemental coolant additives) protect liners. Check strength with a refractometer, pH with test strips.
• Thermostat: opens with heat, controls temperature and speeds warm-up. The stamped number is the start-to-open temperature. Test in hot water: start-to-open, full-open and lift. Never reuse the gasket; heat-sensing element faces the engine.
• Bypass passage circulates coolant through the engine while the thermostat is closed (even warm-up, no hot spots).
• Order on warm-up: thermostat opens first, then shutters, then the fan engages.
• Surge tank = pressurized with its own cap; recovery tank = vented, coolant pushed and pulled through the radiator cap. They let the system breathe and separate air.
• Radiators: down-flow, cross-flow, multi-pass (low-flow). Shrouds and baffles make the fan more efficient. Pusher fans blow through the radiator from the engine side (keeps debris off the core). Reversing fans clean the core.
• Fan clutches engage by coolant temperature (viscous bimetal, air, or electronic). Leaking viscous clutches are replaced.
• Water pump = non-positive centrifugal (high volume, low pressure). Constant-tension clamps.

Cooling system service

• Check the coolant level first. Open the cap only when cool.
• Pressure test at no more than 10% over the cap rating. Pressure drops with no visible leak = internal leak (head gasket, EGR cooler, oil cooler, liner seals). Test the cap at its rating. A plugged recovery tank vent stops coolant returning.
• Clean the radiator core with low-pressure air or water opposite to normal airflow, nozzle square to the fins.
• Scale badly reduces heat transfer. Flush with the thermostat out, in the reverse direction of normal flow, block first.
• Fill to remove air, warm up to operating temperature, cool, top up. Change coolant filters per the OEM; too-frequent changes can over-treat (high SCA).
• Overheating causes: low coolant, slipping belt, stuck thermostat, plugged core (inside or out), failed fan clutch or shroud, weak cap, aerated coolant (combustion gas leak), overloading, late timing.
• Overcooling: thermostat stuck open, shutters stuck open, fan always engaged = poor combustion, sludge, fuel dilution.
• Belts: check first on charging/cooling complaints; use a tension gauge.

C-13 · Steering systems

Wheeled equipment steering

Steering types (Part A)

• Front wheel steer. Wheel lean (motor graders): front wheels tilt left/right to offset side thrust from the blade.
• Rear wheel steer: tight spaces (forklifts).
• Crab steering: all four wheels turn the same direction (diagonal travel); gives maximum flotation (each tire on its own ground).
• Co-ordinated steering: front and rear turn opposite (tight circle).
• Combination: front or rear plus crab and co-ordinated, chosen with a selector switch.
• Skid steering: one side slows, stops or reverses.
• Articulated: machine pivots at its centre; cannot have a rigid frame (loaders, graders, scrapers, rock trucks, packers). Every other type uses a rigid frame.

Power steering

• Part-time = on-highway power assist. Full-time = off-road hydraulic steering: the steering wheel is not physically connected to the wheels.
• Follower valve: the wheels turn a fixed amount for a given steering wheel movement. Without it, holding the wheel turned keeps the wheels turning until released.
• Steering usually has priority over other hydraulic functions.

Components

• Reservoir: stores, cools, settles air and dirt; baffles; may have a clean-out cover for servicing the bottom.
• Pump: fixed or variable, gear/vane/piston, generally engine driven; may have an electric or ground-driven backup.
• Cavitation: inadequate inlet pressure (plugged inlet, low oil). Vacuum bubbles implode. High-pitched squeal under load; "chewing marbles" at no load.
• Most pump failures = contaminated oil.
• Flow controls; a flow divider can work with a steering shuttle valve to control an emergency steering pump.
• Hand metering unit (orbital): a rotary DCV plus a gerotor pump. If the pump fails, the operator can turn the gerotor by hand to steer. Meters oil while steering.
• Spool type steering valves are used with a joystick instead of a steering wheel.
• Cylinders: often two double acting cylinders; displaced oil from one may feed the other.
• Non-differential cylinder: one cylinder, rod out both ends, equal pressure/area both sides. Single-rod double acting = differential cylinder.
• Cushion stops limit articulation. Neutralizer valve: relieves steering cylinder pressure just as full lock is reached (prevents hammering the stops; often adjustable).
• Filters: depth (through the media) or surface. Depth filters are typical. Filters rated in microns, strainers in mesh. Usually in the return line to tank. Magnets with fire-resistant oil.

Machine steering behaviour

Diagnosis and service

• 70%+ of hydraulic problems relate to the oil. Cleanliness (cap and plug every opening).
• Safety: lower everything, let oil cool and relieve pressure before opening, cardboard (never a hand) for leaks, lock bars on the articulation joint.
• Know the system, talk to the operator, visual inspection, test. Infrared heat gun checks components without opening them.
• Don't hammer pivot pins or caps (deforms the knuckle). Tie rods set toe; thread fully engaged. Articulation joint repair = split the machine, depressurize, disconnect cylinders, stands and hoists, protect chrome, new O-rings.
• Pressure too low = sluggish; too high = jerky and stresses parts.

Skid steers (Part B)

• Small reservoir (so the oil cooler is critical). Closed loop: one or two reversible variable piston pumps (hydrostatic) to two motors.
• A separate gear pump supplies charge oil. Each motor drives one side (wheels or tracks). Case drains return leakage.
• Wheeled: belts, chains, sprockets or gears let one motor drive both wheels on a side. Tracked: final drive + hydraulic motor.

Steering geometry (on-highway and wheeled machines)

• Caster: tilt of the steering axis forward/back. Positive caster (top tilted back) gives straight-line stability and return to centre. Too little = wander; uneven side to side = pull.
• Camber: tilt of the wheel in/out at the top. Wrong camber wears one shoulder of the tire.
• Toe: front of the tires closer (toe-in) or farther (toe-out) than the rear. Wrong toe = feathered tread wear and is the most common alignment fault; set with the tie rod.
• Kingpin inclination (steering axis inclination) helps return to centre and reduces steering effort.
• Ackerman (toe-out on turns): the inside wheel turns sharper than the outside so both roll around the same centre. Bent steering arms change it and scrub tires in turns.
• Check order: tires and pressures, wheel bearings, kingpins, tie rod ends and drag link, steering gear lash, then alignment angles.
• Integral power steering gears (recirculating ball with a rotary valve and power piston) use relief and poppet (unloading) valves that stop pressure near the axle stops.

Tracked equipment steering

Clutch and brake steering (Part A)

• Gradual turn: clutch disconnects power to that side. Sharp turn: brake also applied on that side (pivot).
• Levers work clutch and brake per side; brake pedals brake without releasing clutches; park brake is mechanical.
• Steering clutches deliver power to the final drives. Bevel pinion → bevel gear on a cross shaft; clutches on each end.
• Conventional clutch: spring applied, hydraulically released, multi-disc.
• Band brake: contracting band over the drum; the clutch sits inside the brake drum. Service brake applied hydraulically with a booster.

• Clutch engaged = no input. Lever partly pulled = clutch released. Lever fully pulled, pedal, or park = brake on.
• Modular: clutch and brake both spring applied (Belleville springs), released by oil; wet multi-disc; one housing; oil circuits in series. Clutch engaged → brake released; clutch released → brake applied. Clutch on input, brake on output.
• Priority valve guarantees steering and brakes before the transmission. Steering valve gives proportional control.

• Diagnosis: oil, leaks, linkage, track condition and adjustment, filters and screens.
• Stall test: warm, park on, in gear; brake slip = creep; open area; test all ranges.
• Clutch pressure = 0 until the lever is pulled, then rises smoothly. Brake pressure = 0 until the clutch is released, then rises smoothly. Also: steering relief, cooler restriction and bypass.
• Removal: modular = remove track and final drive; conventional = also the cab and fuel tank.
• Beware spring pressure when disassembling. Mineral spirits only for bearings and friction discs. Warpage/cone with a feeler gauge (drag). Disc clearance with a feeler gauge; too little = drag, overheating, failure. Adjust linkages.

Hydrostatic steering (Part B)

• Two variable, reversible pumps, each feeding its own motor and final drive. (A big skid steer.)
• Power at both tracks while turning; infinite speed. Counter-rotation (spot turn): one track forward, the other reverse.
• Charge pump replaces oil. Flushing valve (3-position shuttle) cools the closed loop by sending the low side to the charge relief.
• Electronics: engine speed, steer, decelerator, speed, FNR; track motor speed sensors (tracking); pump speed sensor lets the ECM prevent engine kill under heavy load.

Differential steering

• Transmission input = speed and direction. Hydraulic steering motor = left/right turns (drives the steering ring gears).
• Brakes: modular multi-disc wet, spring applied, hydraulically released; used to stop or slow, not to steer.
• Both tracks always powered, even cornering; what one loses in speed the other gains.
• Straight: steering motor holds (stationary). Turn: power from both motor and transmission. Counter-rotation: motor supplies all power, transmission in neutral.
• 2-planetary: transmission drives both suns, carriers output to finals, motor controls both rings. 3-planetary: steering, drive, equalizing; suns tied together; equalizing ring held.
• Motor doesn't turn = machine won't turn. Motor/ring won't hold = won't track straight.
• Checks: turn diameter (360° left and right, compare) and sprocket speed difference (tracks off, count sprocket revs in one minute at full steer).

C-14 · Suspension systems

Frames and suspension basics

• Sprung weight is above the springs; unsprung below. More unsprung weight = more shock to absorb.
• Jounce = wheel moving up (compressing); rebound = moving back down.
• Torque rods: transverse rods for side-to-side stability (cornering); longitudinal rods for braking and acceleration. They are also the axle alignment adjustment point.
• Shock absorbers dampen spring oscillation by forcing fluid through orifices.
• Equalizer (walking) beams share load between axles and cut the shock to the frame about 50%; used with solid mounts, leaf springs or rubber cushions. Check centre bushings by lifting the frame; end bushings with a jack under the beam end.
• Leaf spring rate goes up with more leaves, thicker or wider leaves, or a shorter effective length. Constant rate vs progressive (contact points move in as load increases) vs auxiliary (helper springs for loaded operation). A broken centre bolt can let the axle shift (misalignment). Never replace single leaves; replace the pack. Shackles let the spring length change.
• Rubber cushion suspension: walking beam with rubber blocks.
• Air springs (rolling lobe or double convoluted) always use shocks. Pressure protection valve holds air from the springs until system pressure reaches about 85 psi (brakes first). Height control (levelling) valve keeps the axle-to-frame distance by supplying, holding or exhausting air; it has a small dead band so it doesn't react to bumps. Dump valves let the driver exhaust the bags.
• Suspension faults: wandering (pulls, won't track straight), bottoming (can't absorb the load).

Off-road suspensions

• Suspension: absorbs and transmits torque to the frame, responds to terrain, cushions, positions axles, lets axles move independently.

Scraper cushion hitch

• Located between the front tractor and the rear bowl. Allows articulation and slight side tilt.
• Basic hitch (no vertical movement) = "stiffneck".
• Cushion hitch: two links and a load cylinder with accumulators.
  • Tractor bracket: mounted on the tractor, holds the head end of the load cylinder.
  • Scraper hitch assembly: on the scraper, holds the rod end.
• Parts: 1–2 accumulators, leaf spring linked to the leveling valve, control lever, bleed valve, variable pump, load cylinder.
• Operator controlled: suspension or rigid. Suspension stops "loping".
• Pump and reservoir are the only parts shared with the rest of the hydraulic system. Axial piston pump driven by the transmission.
• Push-pull (twin engine) scrapers use a bail (pull loop) and hook. Options: bail down lockdown, bail up lockdown, cushion ride.
• Cushion ride: hitch lifted off its rest by the load cylinder; used travelling (loaded or empty).
• Lockdown: hitch sits on the tractor bracket, held by oil pressure keeping the cylinder retracted; used when loading.
• Leveling valve (moved by the leaf spring) maintains ride height (hitch-to-bracket clearance). Bleeds down at shutdown.
• Oil from the load cylinder moves into the accumulator and compresses nitrogen → cushion.
• Pump servo valve moves the control piston to set swashplate angle by pressure.

Cushion hitch service

• Level ground, engine off, park brake, lower equipment, visual check, cycle controls to bleed pressure, check oil.
• Ops test: warm oil, time the load cylinder extension in cushion ride, compare to spec. Leaf spring to leveling valve linkage may need adjusting.
• % flow loss = (GPM @ 100 psi − GPM @ 1000 psi) ÷ GPM @ 100 psi × 100; more than 5% loss is bad.

• Oil leakage test: check case drain output (OEM spec, or 4.2 L in 30 s is bad) = internal wear.
• Accumulator charging: dry nitrogen only (99.8%). Oxygen = explosion. Too high a charge = stiff ride. Engine off, pressure discharged. Correct the nitrogen pressure for ambient temperature (charge cold = overfill; charge hot = underfill). When installing, purge air by putting a little oil on the nitrogen side and expelling it.

Haul trucks

• Rubber blocks on smaller trucks, hydropneumatic on larger. Rubber blocks usually sit above the axle housing, carry vertical loads only, stiffen when loaded.
• Panhard rod keeps the axle centred left to right under the frame. Worn panhard bushings cause repeated tearing of the rear rubber blocks.
• Walking beam lets tandem axles see-saw. A-frames locate the axles front to rear (longitudinal support).
• Hydrair struts: control vertical movement; front struts are part of the steering (attached to the steering knuckle). Nitrogen above the piston, oil below; orifices and check valve damp movement. No shock absorbers needed.
• Struts need regular greasing, often from a central point. Charging valve (keep the cover on). Payload valve converts strut pressure to an electrical signal to estimate load. Seals seal the piston to the wall; wiper keeps dirt out (and wears the chrome over time).
• Service: lower or secure everything, empty the box. Soapy water finds nitrogen leaks. Don't check oil until all nitrogen is out; don't remove fittings until the rod is fully retracted and gas released.
• Service if: fault code, missing chrome, oil/gas leaks, harsh ride, rod fully extended when empty.
• Charge both sides (both fronts or both rears) at the same time so ride height is level.

C-15 · Brake systems

Brake basics

• Brakes turn kinetic energy into heat by friction, faster than it can leave, so drums and rotors are heat sinks.
• Coefficient of friction = force to slide ÷ weight. Static friction is higher than kinetic. Brake fade = linings too hot, friction drops.
• Double the weight = double the braking energy; double the speed = 4×; both = 8×.
• Stopping distance = reaction time + lag time (air to reach the chambers) + braking time.

Hydraulic brakes

• Pascal's law: pressure on a confined liquid is transmitted equally in all directions. Bigger output piston = more force, less travel. Every piston in the system gets the same pressure.
• Brake fluid's most important property: boiling point. Glycol fluids (DOT 3, 4, 5.1) are hygroscopic (absorb water, which lowers the boiling point), so change them on schedule. DOT 5 is silicone, not hygroscopic, never mixed with others and not used with ABS.
• Any petroleum product in the system = replace all rubber parts. Clean with brake fluid, alcohol or soap and water, never solvent.
• Master cylinder: dual (split) circuit for safety; the bigger reservoir feeds the disc brakes. Primary piston is pushed by the pedal; the secondary by trapped fluid. Primary failure = the primary piston mechanically pushes the secondary; secondary failure = the secondary bottoms out.
• Ports: the larger inlet/replenishing port and the smaller compensating (vent) port; the primary cup sits between them at rest. The diaphragm in the cap seals the reservoir.
• Self-energizing: drum rotation drags a shoe tighter into the drum. Servo: one shoe applies the other (duo-servo both ways). Secondary (rear) shoe has the longer lining and stronger spring. Self-adjusters work in reverse.
• Disc caliper square-cut seal seals and retracts the piston (no return spring). Floating calipers need clean, lubed slides. Discs cool better and fade less.
• Valves: residual check holds 12–15 psi in drum circuits (keeps cups sealed); pressure differential (warning) valve lights the lamp when one circuit fails; metering valve holds off the front discs until the rear drums apply; proportioning valve limits rear pressure on hard stops (prevents rear lockup); combination valve = all three.
• Service: drums and rotors have max/min sizes cast in. Drums on an axle within 0.010 in. Measure the rotor in about 12 places for thickness variation (parallelism); remove bearing play before checking runout (excess = pulsing pedal). Finish machined surfaces with 240 grit (non-directional). Replace shoes = machine or replace drums.
• Hone only light scores; never hone aluminum bores. Bleed from the farthest wheel to the nearest (RR, LR, RF, LF); hold the metering valve open; bench bleed a new master cylinder.
• Brake booster types are named by the boost source: hydro-boost (power steering pump) with an accumulator for reserve (spring: about 3 stops, gas: about 2) and an electric backup pump; vacuum booster (gas engines, vacuum suspended); air-over-hydraulic. Vacuum booster test: pump the pedal engine off, hold it, start: the pedal should drop. Diesels don't make vacuum.
• Electric trailer brakes: an electromagnet grabs the drum face and swings an actuating lever. A breakaway switch with its own battery applies them if the trailer separates.

Parking brakes

• Parking brakes hold the vehicle on a grade; they are not the service brake.
• Types: cable to rear shoes (adjust the cable last), driveline drum or disc, spring applied, hydraulic released (SAHR) wet or dry discs (off-road), electric-hydraulic.
• A driveline park brake can't hold the machine if the driveshaft is broken or disconnected.

Off-road service brakes

• Wet multi-disc brakes (oil-cooled discs in the axle or final drive) last much longer than dry brakes. Service brakes are usually hydraulically applied; parking brakes SAHR. Accumulators store pressure so the machine can still stop if the engine dies (charging valve + accumulators + pressure switch).
• Check accumulator precharge (nitrogen only) and brake hold tests per OEM. Overheating wet brakes = dragging, wrong oil, low cooling flow.

Air brake foundation brakes

• Brake chambers turn air pressure into linear force: F = P × A. Chamber type = diaphragm area in square inches.
• Slack adjusters turn linear force into torque (T = F × L), multiply force and set lining clearance. At full application the pushrod-to-slack angle should be 90° (no more than 95°). Automatic slacks adjust on the release stroke.
• Number one cause of air brake failure: improper adjustment.
• S-cam = quicker application (most foundation brakes). Flat cam = slower (some steer axles). Left and right cams are different.
• Shoe: heel at the anchor pin, toe at the cam. Return springs at the toe, retaining springs at the heel. The forward (leading) shoe is the energized shoe. Lining edge codes show the maker and friction rating; match linings to the axle rating (GAWR) or they overheat and fade.
• Reline at 5/16 in or less at the centre of the lining. Drum thickness controls how much heat it can take.
• Pushrod travel at a 100 psi application: CVIP allows about ¾–1½ in, no more than ¼ in difference across an axle. Cam bushing radial play ≤ 0.085 in; cam end play 0.005–0.030 in.
• Air disc brakes fade less; caliper slides on pins; automatic adjuster inside.

Air brake circuits and valves

• Five circuits: supply, primary service (green lines, usually drive axles), secondary service (red lines, steer axle and trailer), parking/emergency, trailer control. The supply side (compressor, governor, dryer, tanks) is in G-36.
• Valve rule: released = inlet closed, exhaust open; applied = inlet open, exhaust closed; balanced (holding) = both closed.
• Dual brake (foot) valve: graduated (rubber spring gives feel). If the primary fails, the primary piston mechanically opens the secondary. Exhausts through the centre.
• Relay valve: a small control signal applies a big air supply close to the chambers = less lag (faster apply and release). Cracking pressure about 2.5–8 psi (standard 2–4); relays on a combination within ±1 psi.
• Quick release valve speeds up release only.
• Spring brake chambers: spring applied, air released. Must hold a loaded vehicle on a 20% grade (CMVSS 121). Cage the spring before removal.
• Supply double check valve blends primary and secondary air to keep parking brakes released if one circuit fails.
• Anti-compounding valve stops service + spring brakes adding together (would overload the foundation brakes).
• Spring brake (modulating) valve: if primary air is lost, the foot valve modulates the spring brakes on the drive axles (about 3 applications).
• Park control (yellow, diamond) and trailer supply (red, octagon): trailer supply pops out between 60 and 20 psi; park control between 45 and 35 psi. Pulling the yellow also pops the red.
• Tractor protection valve closes the trailer service line when there's no trailer supply pressure (lets you bobtail); about 45 psi to open. Glad hands: blue = service, red = supply (emergency).
• Trailer hand valve applies trailer brakes only, through a double check valve.
• Brake light switch closes at about 5 psi. Ratio valve (steer axle): reduces pressure on light applications (2:1 under 40 psi, 1:1 over 60 psi). Bobtail proportioning relay reduces drive-axle pressure without a trailer.

Trailer air brakes

• Pre-121 trailers: service chambers only and a relay emergency valve that dumps full reservoir air into the chambers if supply is lost ("dynamiting"). They release if the air leaks off, so block the wheels. Reservoir ≥ 8 × chamber volume.
• CMVSS 121 trailers have spring brakes and a spring brake control valve (SR-5 type): applies the springs if supply is lost, fills the spring chambers first, opens to the reservoir at about 85 psi, closes at about 70 psi, and keeps the springs released if only the reservoir leaks.
• Pilot relay on a towing trailer has 0 psi crack pressure (passes the signal undiminished). Converter dolly pressure protection opens at 70, closes at 60.

Testing (CVIP style)

• Low air warning comes on at or above 60 psi.
• Leak-down over 2 minutes: supply circuit (engine off, no application) 2 psi × n; service circuit (full application at 100 psi held) 2 psi + 2 psi × n, where n = axles/axle groups behind the steer axle. The service limit is higher because it's checked applied.
• Compressor build-up and governor tests are in G-36.

Air ABS

• ABS keeps the wheel turning (just short of lockup) so you can still steer, modulating up to about 100 times per second.
• Wheel speed sensor (magnetic) reads a tone ring; push it in until it touches the ring (the ring knocks it back to the gap). Don't wiggle the hub side to side on removal. ECU, modulator valve with two solenoids (hold and exhaust). If ABS fails, normal braking remains.
• ABS lamp comes on at key on and goes out if the system is OK. 4S/4M = 4 sensors and 4 modulators.
• Keep tire sizes matched (within about ¾ in circumference) so wheel speeds compare properly.
• Trailer ABS power comes over the trailer cord (PLC4Trucks talks over the power line).

C-16 · Undercarriage systems

Design (Part A)

• Narrow pads = more traction and less undercarriage wear. Wide pads = more load carrying (flotation) but they bend and wear the chain. Use the narrowest pad that works.
• Stability matters most on excavators and cranes. Tracked cranes lift best with loads close to the machine.
• Fixed track frames: bolted front and rear, more stable. Oscillating (swing frame): one central pivot, frames move up and down.
• Equalizer bar (with pivot shaft): supports the front of the machine and lets the frames oscillate.
• Track links: bottom is a wear surface (rail). Bushing pressed into the big bore, pin into the small bore. Sprocket picks up the bushing end first.
• One-piece links; split master link (bolted through the pad); master pin (removable): slightly recessed and smaller diameter; needs a press.
• Conventional track: seals/spacers (low-travel machines like excavators). Lubricated track (SALT) reduces wear.
• Flush link type: master pin only. Interlocking: bushing extends into the next link (less dirt).
• Pads: bolts with anti-seize, self-locking nuts. Grouser = raised bar for traction. Standard (single grouser, best traction). Open centre: for soils that pack between sprocket and chain. Multi-grouser: better flotation, less traction, less surface damage. Rubber: paved and ag. Extreme service: heat-treated, thicker.
• Sprocket/final drive = last reduction. Low drive: sprocket in line with the front idler; more stability, low centre of gravity. High drive: reduces shock loads on sprocket and final drive; needs a rear idler; more pin/bushing wear.
• Change sprockets with the track, both sides together. Segmented sprocket: replace without removing the track (change all segments).
• Carrier (pedestal) rollers: top, support the chain. Track rollers: bottom, support the machine's weight. Bogie rollers spread weight.
• Roller flanges keep the chain aligned (cornering and slopes). Double vs single flange (single next to the sprocket).
• Roller shaft is fixed to the frame; the shell turns around it.
• Duo-cone seal = two metal rings; install with an alcohol-based lubricant, never oil. Rollers are lubed for life at manufacture.
• Idlers: centre flange; part of the tension and recoil system; replace in pairs.
• Tension: loosen in packing soils; tighten to make up for chain wear (too tight wears everything; too loose throws track). Hydraulic adjuster: add grease = tighter (nitrogen systems are opposite). Threaded rod on small/old machines.
• Recoil spring absorbs shock and is always under tension; large machines may use nitrogen.

Service (Part B)

• Undercarriage can be ~50% of a machine's maintenance cost. Causes: speed, wrong components, misalignment, poor maintenance.
• Sag check: drive toward the front idler and coast to a stop; string or straightedge; measure between the front idler and the next direction change (carrier roller, or sprocket on high drive).
• Leaking rollers/idlers: grease/oil as a stop-gap; will need replacement.
• Pin and bushing wear: stretch the track tight, measure pin centre to pin centre over several links (avoid the master).
• Sprockets, pins and bushings wear faster in reverse (bushing scrubbing).
• Track pitch (pin to pin) increases with wear; measure across several pins.
• Link height, grouser height (straightedge), roller/idler diameters, sprocket: tooth side to opposite side across teeth.
• Alignment: diagonal measurements must match. Idler must line up with sprocket and rollers (guide plates). Idler high for pulling, low for pushing.
• Lifting gear rated for the load; stands or hardwood blocking. Grade 8+ fasteners, anti-seize, mating surfaces clean of rust, dirt, paint, torque and retorque.
• Remanufacture only if enough material remains (pull before total failure). Weld up links, turn pins and bushings, replace grousers. Ice lugs (cleats) welded on grousers add traction (1-2-1 pattern). Some idlers need holes cut before welding.
• Recoil springs must be caged (a broken retaining bolt releases huge force). Never fully remove nitrogen valves; release grease slowly.

C-17 · Wheel assemblies

Wheels and hubs

• Spoke wheels use inboard brakes; rims clamp on with wedge clamps (tighten evenly); a spacer band separates duals.
• Stud-piloted disc wheels centre on the studs with ball-seat (acorn) nuts. Hub-piloted wheels centre on the hub bore with flange nuts that only clamp. Never mix the parts. Aluminum wheels need longer studs.
• Torque nuts in sequence and stages, then re-torque after 100–150 km (or 1–2 hours off-road). A drop or two of oil between the nut and flange (hub-piloted) if specified.
• A broken stud usually means over-torque; replace it and the studs on each side. Studs may be left- or right-hand thread.
• Never weld or braze a wheel.

Tires

• Bias ply: plies cross at angles; sidewall and tread work together; carries more load, less comfort, low speed. Radial: plies 90° to the bead with belts; sidewall flexes independently; better comfort and traction.
• Rib tread for steer/trailer, block or lug for drive, flotation for soft ground, rock/traction for off-road.
• Sizes: 10.00R20 = 10 in wide, radial, 20 in rim. 750/65R25 = 750 mm wide, 65% aspect ratio, radial, 25 in rim.
• Duals: no more than ¾ in (19 mm) circumference or ¼ in (6 mm) diameter difference. Measure with a tape or string, not the sidewall. The slightly bigger tire goes on the outside (road crown). Tandem axles: same size all round.
• Wear: under-inflation = both shoulders (and the most heat, worst for tires); over-inflation = centre; misalignment = cupping or feathering. Min tread: 3 mm steer, 2 mm drive (typical). No recaps on steer axles.
• Static imbalance (heavy spot on the centreline) = tramp, hop or bounce. Dynamic imbalance (heavy spot to one side) = shimmy or wobble. Split correction weights between the inside and outside of the rim.

Tire safety

• Never re-inflate a tire run at less than 80% of its pressure without checking for damage.
• Deflate multi-piece rims before removing the wheel (the clamps may be holding a cracked rim together).
• Inflate in a cage or restraint; matched rim parts only.
• Seat beads with no more than 40 psi on-road, 20 psi off-road.
• Any damage that cuts the plies = replace the tire. Rim chording (flat spot on the flange) = replace.
• Use nitrogen in large off-road tires (less fire and explosion risk).

Hubs and wheel bearings

• Dead axle: spindle fixed, hub turns on it. Live axle: hub on the housing, axle shaft drives it.
• Wheel bearing adjustment: torque to seat while turning, back off, set end play (typically 0.001–0.005 in) per the OEM. Too tight = heat and spalling; too loose = wobble, seal and brake problems.

D-18 · Charging systems

Alternator construction

• Generator principle: moving a conductor through a magnetic field (or a field past a conductor) induces voltage. Amount depends on field strength, speed, angle and number of conductors.
• The rotor carries the magnetic field (field winding between two claw pole pieces, always an even number of poles). Current reaches it through brushes on slip rings. The soft iron core doesn't stay magnetized.
• The stator is three windings on a laminated core (laminations cut eddy currents and heat); it's where output is induced.
• Wye (Y) stator: high voltage even at low speed (common on 24 V). Delta (Δ): high current, less voltage at low speed (common on 12 V).
• Peak voltage when the rotor field is at 90° to the stator winding.
• Rectifier bridge: 6 diodes (3 positive, 3 negative) give full-wave rectification of three-phase AC to DC. The diodes also stop the battery discharging back through the alternator.
• Diode trio feeds the field from the alternator's own output once it's running.
• Voltage regulator controls output by switching field current on and off (thousands of times a second). It may be internal, external, or in the ECM. A-circuit: regulator after the field (ground side). B-circuit: regulator before the field (positive side).
• A capacitor across the output (in parallel with BAT) absorbs spikes and radio interference.
• R / STA terminal: half-wave AC tap, reads about half the output voltage; drives tachometers, hour meters and relays.
• Brushless alternators (stationary field) are used where there are explosive gases (less output).
• Alternators can be turned either way, but the fan must suit the rotation. A smaller pulley speeds the alternator; an oversize pulley = less output.
• Self-limiting: at max output the stator's inductive reactance limits current. "Full fielding" bypasses the regulator for a max output test (can damage ECMs).

Charging system testing

• Check the belt first (tension gauge, glazing, cracks, oil). Loose = slipping, squeal, glazing, undercharging. Too tight = bearing damage.
• Then test the batteries (fully charged and tested individually); one bad battery can ruin the others.
• Check connections and voltage drop: max 0.5 V (12 V) / 1 V (24 V) on each side of the circuit with current flowing.
• Regulated voltage: 13.8–14.8 V (12 V) or 27.5–29.6 V (24 V), checked with low electrical load. Over 14.8 V = faulty regulator (overcharge: bright, failing bulbs, boiling batteries).
• Max output test: carbon pile (rated for the system voltage) loads it until output peaks; must be within 10% of rating. Keep carbon pile time short.
• Undercharging causes: belt, worn brushes or dirty slip rings, open diode, shorted/open stator, regulator, high resistance.

• Bench tests: diodes forward biased read about 0.5 V on diode check, reverse = OL. Rotor: ohmmeter slip ring to slip ring (resistance/open); slip ring to shaft must be OL (no ground). Stator: lead to core must be OL.
• Clean slip rings with 400 grit or finer; replace brushes under ½ of new length. Sandpaper, never emery cloth, on electrical parts. Hold the rotor nut with a strap wrench.
• Use needle-nose pliers as a heat sink when soldering diodes.
• Disconnect the battery ground before servicing. Coat terminals with dielectric grease. Never use acid-core solder.
• Alignment: no more than about 1 mm offset per 20 cm between pulleys.

D-19 · Starting systems

Cranking motor

• A DC motor that turns electrical energy into rotary motion. Diesels need about 250 RPM; gas about 150.
• Crank no more than 30 s, then cool for 2 min (it only cools by convection).
• Armature (rotating, windings + commutator bars) and field coils on pole shoes (laminated) make the stationary field. Brushes come in pairs; same number of brushes as field coils.
• Motor principle: a current-carrying conductor in a field moves from the strong side to the weak side.
• CEMF (counter-EMF) rises with speed and limits top speed and current. Low CEMF (slow or stalled motor) = high current.
• Series-wound: highest starting torque and current. Compound (shunt) winding limits top speed.
• Pinion to ring gear about 20:1 (torque multiplication). Overrunning (roller) clutch, sprag clutch or Posi-Tork stop the engine spinning the armature after start. A brake washer stops the armature when released. Sintered bronze bushings can't be honed.

Solenoid and controls

• Solenoid jobs: shift the pinion into mesh and close the high-current contacts (B to M).
• Terminals: B battery, M motor, S switch (signal), G ground.
• Pull-in winding (heavy, low resistance) goes S to M (grounded through the motor until the disc closes, then it's shorted out). Hold-in winding (fine wire) goes S to G and holds the plunger.
• Chatter (clicking in and out) = open hold-in winding or a weak battery / high resistance in the control circuit.
• Test hold-in at S and G; pull-in at S and M (max 8 s, then cool 2 min). Can't ohm-check the pull-in properly. Magnetic cancellation test.
• Magnetic switch (relay): small current controls big current.
• Safety switches in series in the control circuit: neutral start (NC in neutral, automatics/powershifts), clutch switch (closes with the clutch down), PTO switch (opens when engaged). Over-crank protection (thermal switch, NC, opens when hot) is in series with the ground. ECMs can block cranking (theft, gear, low oil, low voltage).

Starting system testing

• Test the batteries first. A weak battery makes the motor draw more current (damages contacts and the motor).
• Voltage drop (cranking): max 0.5 V (12 V) or 1 V (24 V) per side; 0.2 V across solenoid contacts; 0.1 V across a switch or relay. Measuring voltage drop across each part is the best whole-system check.
• Before replacing a starter, bar the engine over 2 full turns (it may be seized or hydraulically locked). Check the pinion and ring gear teeth.

• Burnt commutator bars = open armature winding. Growler: shorts (hacksaw blade vibrates), opens, grounds. Field coils can't be checked for shorts with an ohmmeter.

Air and hydraulic starting

• Air starters: vane type (high pressure, needs lubrication, single reduction) or turbine (lower pressure, no lube, planetary reduction). Inertia or pre-engaged drive.
• Air system priority: a pressure protection valve fills the starter tank only after the brakes have about 75–90 psi; a check valve holds the starter tank; tank sized for about 2 cranks; safety valve about 150 psi. Starter valve (solenoid) → relay valve → motor.
• Hydraulic starters: axial piston motor fed by an accumulator (nitrogen precharge, never oxygen or air), enough for about 6 s of cranking; a hand pump recharges it. Unloader valve with a check valve instead of a plain relief.

D-20 · Battery systems

• Battery jobs: store energy, crank, supply loads with the engine off, absorb voltage spikes.
• Lead-acid: positive plates lead peroxide (PbO₂), negative sponge lead (Pb), electrolyte about 36% sulphuric acid, 64% water. Each cell is 2.1 V; 6 cells = 12.6 V.
• Discharging turns both plates to lead sulphate, electrolyte becomes mostly water (so a dead battery freezes easily). Charging reverses it. Overcharging boils off hydrogen and oxygen (explosive).
• Plate area = high current fast (CCA); plate mass = current over time.
• Types: maintenance (refillable), low maintenance, maintenance-free (sealed), deep cycle (repeated deep discharge), AGM (glass mat, pressurized, more CCA, don't mix types). Use distilled water; keep electrolyte above the plates (about 10 mm).
• Ratings: CCA = amps for 30 s at −18 °C while holding about 7.2 V. CA = same at 0 °C. Reserve capacity = minutes at 25 A. Amp-hour = 20-hour rate.
• Battery efficiency falls with cold (about 65% at 0 °C, 20% at −30 °C).
• Series: voltage adds, CCA stays the same. Parallel: voltage stays the same, CCA (and capacity) add. Series-parallel for 24 V start, 12 V run.
• Replace batteries in a bank as a set: same age, size, brand, type.
• Sulphation = hardened lead sulphate crystals from sitting discharged; raises resistance and lowers capacity.

Testing and charging

• Visual first (case, terminals, level). Dirt and acid on top can make a discharge path. Clean with baking soda and water.
• Remove surface charge before testing open-circuit voltage (load about 300 A for 15 s, or wait).
• Full charge: 12.6 V (12 V) / 25.2 V; specific gravity 1.265–1.280 at 27 °C, cells within 0.050. Built-in hydrometer: green = OK, dark = charge it, clear/yellow = low electrolyte (replace).
• Load test: battery at least 75% charged; load to ½ CCA for 15 s; must stay above 9.6 V (at about 21 °C).
• Slow charging is the only way to 100% (about 1% of CCA or 7% of Ah). Fast charging reaches about 75% and can damage it. Trickle = 2 A or less. Don't boost above about 15.5 V (12 V) / 31 V (24 V).
• Store batteries cool, dry and fully charged.
• Booster cables: dead +, good +, good −, then ground on the dead machine (away from the battery). Remove in reverse order.
• Remove the ground cable first, install it last.
• Battery disconnect switches are in series with the battery. Fast discharge = parasitic draw, internal short, charging fault.

D-21 · Electrical components

Theory

• Conductors have 1–3 valence electrons, insulators 5+, semiconductors 4.
• Voltage = electrical pressure (potential difference). Current = electron flow. Resistance = opposition (ohms). Conventional current flows + to − (the theory AIT uses); electrons actually go − to +.
• Resistance goes up with length and temperature (PTC metals) and down with bigger cross-section. Gold conducts worse than copper but doesn't corrode.
• Ohm's law: E = I × R. Watts = volts × amps.
• Series: resistances add, same current everywhere, voltage drops add up to source voltage.
• Parallel: same voltage across each branch, branch currents add, total resistance is less than the smallest branch; adding a branch lowers total resistance.
• Kirchhoff: current in = current out; voltage drops = source voltage.

Magnetism and induction

• Like poles repel, unlike attract; flux goes north to south outside the magnet. Permeability = how easily a material carries flux; reluctance = the opposite.
• Electromagnet strength = turns of wire × current (ampere-turns) and the core. Right-hand rule: thumb = conventional current, fingers = flux.
• Self-induction: a collapsing field makes a voltage spike when a coil is switched off. Suppress it with a clamping diode or resistor in parallel with the coil.
• Mutual induction: one coil's changing field induces voltage in another (transformers, turns ratio).

Test equipment

• Voltmeter in parallel; ammeter in series (shunt) or inductive clamp; ohmmeter only on de-energized circuits.
• Use a high-impedance DMM; low-impedance meters and test lights load the circuit and can damage electronics.
• Ohmmeter OL = open (too high to read); 0 = continuity. Voltage drop must be measured with current flowing.
• Open-circuit voltage = no load connected.

Wiring and protection

• Wire size: AWG — bigger number = smaller wire. Never replace with a smaller gauge.
• Coaxial cable shields low-level signals (radio, GPS); twisted pair cancels interference (keep the twists); shielded twisted pair adds a foil and drain wire.
• Solder: 60/40 rosin-core, never acid core. Heat the joint, not the solder; dull, balled solder = not hot enough. Tin wire ends and trailer connectors. Dielectric grease on connections. Never pierce insulation to test.
• Fuses: same amp rating only. Fusible links: same gauge and length.
• Breakers: Type I cycling (auto reset; headlights), Type II stays open while power is on (resets when power is removed), Type III manual reset.
• Relay: small current switches a big one. Terminals: 85 and 86 coil, 30 common feed, 87 normally open, 87a normally closed. About 0.2 V drop across good contacts.
• Solenoid = electrical energy into linear motion.

Electronic components

• Semiconductors: silicon doped to make N type (extra electron, e.g. phosphorus/antimony) and P type (holes, e.g. boron).
• Diode = electrical check valve. Forward bias (P/anode to +) conducts; reverse bias blocks. Diode test: a reading one way, OL the other. Shorted = reads both ways; open = OL both ways.
• Zener diode conducts in reverse at a set voltage (regulators, protection). LED = light-emitting diode (forward biased).
• Resistors: fixed, tapped, variable. Rheostat = 2 terminals (controls current, e.g. dash dimmer). Potentiometer = 3 terminals (voltage divider, e.g. throttle sensor).
• Transistors switch or amplify (small base current controls a big collector current).
• Protect electronics from heat, vibration, voltage spikes and static (wrist strap). Before welding: disconnect the battery and unplug ECMs.

Lights, gauges and motors

• Battery disconnect switch: in series with the positive or negative of the battery group; isolates batteries.
• Breakers are in series:
  • Type I: cycling, auto reset fast; safety circuits (headlights).
  • Type II: a heat-producing resistor holds it open while overloaded; must cool (power removed) to reset.
  • Type III: non-cycling, manual reset button.
• P = I × E (sizing; fuse slightly larger than the load).
• Lights: head, flood/work, tail (red), clearance (amber). Panel lights dim with a rheostat or PWM; may use fibre optics.
• Turn signal flasher is in series with the lamps. LED retrofit needs an electronic/solid-state flasher (LEDs don't draw enough current for a thermal flasher).
• DC motors: permanent magnet or wound field. Armature = rotating part. Commutator = sliding contact surface for brushes, made of bars that feed specific windings.
• CEMF: the faster the motor, the more CEMF; less CEMF = more current draw; CEMF limits top speed.
• Right-hand rule: thumb = current (+ to −), fingers = flux. Like poles repel, unlike attract. Field goes north to south. Armature moves from the strong field side to the weak side.
• Gauges: direct (air gauge) or indirect (sensor). Air core (ferrite core). Balancing coil: pointer moves by electromagnetism. Bimetal: heating coil heats the strip according to voltage.
• Closed loop sensor: power and ground return to the ECM. Open loop: ground does not return to the ECM.
• Changing tire size changes the speedometer reading. Hourmeters use EEPROM.
• Thermistor = temperature-sensitive resistor. PTC: temp up, resistance up. NTC: temp up, resistance down (most engine temp sensors).
• Pressure transducer: three wires (5 V ref, signal, return). Fixed resistor in series with a strain gauge, voltage drop compared with two fixed resistors (a bridge), with an amplifier.
• Ammeter in series (or clamp style); voltmeter in parallel.

Off-road electrical service

• Schematics are drawn de-energized, components in their normal position.
• Circuit = source, load, conductors, control (+ protection).
• Series: R total = sum; voltage drops add to source voltage; current is the same everywhere.
• Parallel: total current = sum of branches; same voltage on each branch; adding branches lowers total resistance; total is less than the smallest branch.
• E = I × R. Resistance turns electrical energy mostly into heat.
• Kirchhoff current law: current in = current out. Voltage law: voltage drops = source voltage.

• Isolating = eliminating components that aren't the cause. OEM flow charts.
• Voltage drop must be tested with current flowing. Max 0.5 V (12 V) or 1 V (24 V).
• Repairs: electrically sound, resistance-free splice (crimped or soldered); replacing the harness is the ideal repair. Western Union (solid wire, soldered), crimp splice.
• Replace: oil-soaked or melted insulation, faulty switches/relays/breakers, lamps with opens or shorts.
• Specific gravity 1.265–1.280 @ 27 °C = full charge; cells within 0.05.
• Bimetal gauges are affected by voltage, so a small voltage regulator keeps them accurate. LCD = no light emitted; LED = forward biased; VFD = glowing pixels.
• Precautions: high impedance DMM, never pierce insulation, wrist strap (or touch ground), disconnect power/ground and ECMs before welding, don't force connectors.

D-22 · Equipment management systems and electronic components

Control modules

• An ECM takes inputs, processes them, and drives outputs (closed loop when it reads the result back). Usually switches the ground side.
• Memory: RAM (temporary; volatile keep-alive memory is lost if the battery is disconnected), ROM (permanent, factory), PROM (programmed once), EPROM (erase by removing), EEPROM (reprogrammed in the machine with software).
• The CPU (microprocessor) is the brain; a clock sets the timing of each bit.
• Inputs: switches, 5 V reference sensors (signal 0.5–4.5 V; below 0.5 = short, above 4.5 = open), voltage-generating sensors (magnetic pickups), thermistors.
• Analog = continuously variable; digital = only fixed levels (on/off). The ECM can only process digital, so inputs go through an A/D converter; outputs needing analog go through a D/A converter.
• Bit = one binary digit (0 or 1); byte = 8 bits; bytes are packed into message frames.
• PWM: on-time vs off-time over a set period = duty cycle (75% duty = on 75% of the time). Used for solenoids, dimmers, fan clutches, proportional valves.
• On-board diagnostics show codes on the display or by flashing lamps; off-board means a scan tool or laptop.
• Most faults are wiring and connectors, not the module.

Networks

• Multiplexing = sending and receiving digital signals on a data link (fewer wires, shared data).
• Baud rate = data speed. J1587/J1708 = 9.6 kb/s; J1939 = 250 kb/s; ISO 11783 = 250 kb/s.
• Backbone (data bus) = the physical connection, max 40 m. Stub = branch to a node, max 1 m. Node = module with an address.
• Terminating resistor at each end reduces standing waves (reflections) and interference.
• Protocol = common language. Twisted pair resists magnetic fields; shielding (foil + drain wire) adds resistance to interference.
• Topology: ring (series, one fails = all fail), star (master module), linear (modern: independent modules on a backbone).
• CAN is serial (one bit after another), no master; a module must wait until the bus is idle to transmit.
• J1587/J1708: MID (source), PID (parameter), FMI (fault type), checksum (end of message). Unshielded orange/white twisted pair, 40 m, 20 nodes. Connectors: 6-pin Deutsch 1996–2001, 9-pin from 2001.
• J1939: PGN (message), SPN (where the fault is; replaces PID), FMI. Frame: arbitration (priority, sender), control/data, CRC, ACK, end of frame.
• CAN-H yellow, CAN-L green. J1939-15 "lite" = unshielded; J1939-11 "heavy" = shielded. Drain wire goes to battery ground.
• 120 Ω at each end; 60 Ω measured between CAN-H and CAN-L. Less than 60 = short; more than 60 = open. CAN lines ~2.5 V idle. CAN-H/CAN-L to ground: no continuity (1–4 MΩ or OL).
• ISO 11783: any make of machine connects to any make of implement (ag/forestry); unshielded twisted quad (CAN-H, CAN-L, power, ground for the terminating bias circuit).

Diagnosing electronic systems

• Codes: MID (module), PID/SPN (component or parameter), FMI (failure mode: short, open, out of range). J1939 uses SPN + FMI.
• Order: talk to the operator, check codes, check the sensor, then the harness, then clear codes and verify. Use the OEM troubleshooting tree.
• Wiggle test with the key on, engine off. Breakout boxes/harnesses let you read signals without piercing wires.
• Never pierce insulation, never use a test light on electronic circuits, never unplug modules with the key on.
• Replace a damaged harness if you can; otherwise use a sealed crimp or rosin-core solder splice. Program the new module (or transfer parameters) after replacement.

E-23 · Clutches

• Overcentre clutch couples a drive and driven unit; intermittent use, not for shifting gears. Doesn't use spring force to apply (the linkage locks overcentre); may be hydraulic or air operated. Slips if overloaded. Stationary uses (pumps, generators). On/off only; doesn't control speed.
• Parts: flywheel, drive ring (internal teeth, drives the disc), clutch disc (driven by the drive ring), pressure plate assembly (back plate and hub near the flywheel, floating plate clamps toward the flywheel).
• Flywheel, drive ring and disc always turn at engine speed (drive members). Plates, hub and output shaft turn at flywheel speed only when engaged (driven members).
• Overcentre linkage: no springs; lever "snaps" the clutch in; forks. Adjusting ring: toward the pressure plates = more force; away = less. Release pin must be pulled to adjust. Linkage is under 90° when engaged.
• Path: engine → drive ring → clutch disc → pressure plate assembly → output shaft.
• Bearings need regular greasing. Heat spots (blue/dark). Pull-in force (checked with a pull scale) sets clamping force and torque capacity.
• Engine seal leak glazes the disc → slipping; discs must be replaced. Disengaging at high RPM can damage the engine.
• Removal checks: crankshaft endplay before and after (dial indicator), flywheel face runout (push crank fully one way to remove endplay; add all readings ignoring + and −), bore runout, housing concentricity and face runout.

Special clutches

• Overrunning: drives one way, freewheels the other; output can turn faster than input. Roller (spring and roller in a ramped race), sprag, dog/jaw (ratchet-like).
• Cone and bevel: cone into a bevel; slips under overload (transmissions, rear axles).
• Electromagnetic: current applied = clutch applied (A/C compressors, fan hubs, mowers; easy remote control).
• Wet (oil) clutches: hydraulic piston clamps discs and plates; oil cools and cleans.

E-24 · Torque converters, fluid couplers and hydraulic retarders

Fluid couplings

• A fluid coupling has only an impeller (pump, engine driven) and a turbine (output). It transmits torque but can't multiply it (no stator); output is always a little slower than input (slip).
• Used to cushion shock loads (some fans, conveyors, older drives). Adding a stator turns it into a torque converter.

Torque converters

• Hydrodynamic drive: transfers energy through fluid motion. Energy transfer depends on volume (size, vanes), velocity (impeller speed) and viscosity.
• Fluid clutch: replaces a mechanical clutch; slippage prevents engine stall with the machine stopped.
• Impeller = engine side (input). Turbine = driveline/transmission side (output).
• Low engine speed = high slippage; high speed = low slippage.
• Fluid clutch pros: no mechanical link, shock absorption, gradual start. Cons: slip losses, heat, only transmits torque; cannot multiply it.
• Torque converter = a fluid coupling that multiplies torque.

Flow and phases

• Lockup phase: turbine locked to impeller, 1:1 (only with a lockup clutch).
• Stages = number of turbines (times the oil hits a turbine before returning). Usually 1.
• Stall: turbine stopped, impeller at max engine speed = max torque, max vortex flow, max heat.

Parts

• Impeller (pump): bolted to the flywheel; vanes turn mechanical energy into fluid kinetic energy. Turbine: to the transmission; kinetic → mechanical.
• Stator (reaction member): redirects oil back to the impeller; multiplies torque only when stationary.
• Split guide ring: reduces turbulence (between impeller and turbine).
• Torque multiplication needs: impeller faster than turbine and stator stopped.
• Fixed stator: never reaches coupling, constant multiplication, very hot. Freewheeling stator (overrunning clutch): held in converter phase, freewheels in coupling phase.
• Lockup clutch: physically connects impeller and turbine, removes slip, raises efficiency, used in rotary flow, 1:1, no multiplication. Wet multi-disc.
• Variable capacity: changes torque to suit demand (dual impeller with one clutched, dual turbine, variable pitch stator).
• Temperature sender in the converter outlet; max 250 °F (121 °C).
• Oil: light, usually shared with the transmission. Too heavy = transmits torque in the cold (start in neutral).
• Housings: dry, or wet (lubricates gears, can drive a PTO) with low oil level to avoid drag. Scavenging by gravity, or a scavenge pump when the converter is level with the transmission.

Torque divider

• Torque converter + planetary set; torque is split; more multiplication than a plain converter.
• Impeller and sun at engine speed; turbine and ring gear together; carrier = output. Fixed stator.
• With the output stalled, the turbine turns in reverse.

Mounting, diagnosis, service

• Engine mount (integral, driven by flywheel) or remote mount (driveline between engine and converter).
• Use gauges rated above expected readings.
• Stall test in the highest gear (low gears risk driveline damage), level open ground, park brake on, keep oil below 250 °F.

• Overheating: gear too high, operator misuse, plugged cooler, bad charge pump, low oil, high oil (drag), hot ambient.
• Removal: note leaks and damage; converter may need to be secured to the transmission. Mark parts, leak test under water, aluminum-safe cleaner, pre-lube, plug every hole for storage.
• Mounting checks: crankshaft endplay (always after install), flywheel housing bore and face, crank hub runout and face, flexplate flatness, converter axial location, pilot diameter.
• Shift linkage must be adjusted or two gears can engage.

Hydraulic retarders

• A supplementary brake that absorbs kinetic energy by fluid friction. Designed to slow, not stop. Extends brake life.
• Input retarder: integral with the transmission (between converter and transmission); works with the transmission (downshift after applying) to be more effective.
• Output retarder: on the output shaft; independent of engine RPM and gear; most effective at high vehicle speed.
• Remote / driveline mount.
• Parts: rotor (input side) with vanes inside the housing (stator, stationary), pump and supply (own or shared), cooler (air or coolant), control (manual, air over hydraulic, electronic), accumulator (fast fill), sometimes wet brakes for faster braking.
• Max oil temp 250 °F (121 °C); damage above 275 °F (135 °C).
• When off, all oil is drained from the housing. Kinetic energy → heat by friction between rotor and oil.
• Retarders are most effective at high vehicle speed. Converter lockup is required when the retarder is between the crank and flywheel.
• Electronic: inputs (throttle, retarder temp, output speed, brake applied), outputs (solenoid, warning light, brake lights, temp light).
• Troubleshooting: low temps, clean oil, right level, rotor clearance, check for leaks.

• Overheating speeds oxidation and kills lubrication: low oil, retarder too small for the job.

E-25 · Manual transmissions and transfer cases

Gearing principles

Terminology

• Diameters: root, pitch (pitch line to pitch line), outside. Circular pitch must match for two gears to mesh.
• Clearance: tip of one tooth to root of the other (needed for lubrication).
• Backlash: gap between faces/flanks in mesh (heat expansion and lube).
• Timed gears: certain teeth must mesh together. Non-hunting = must be timed; whole-number ratio (2:1); distinct wear patterns.
• Partial non-hunting: ratio with 1–2 decimals (4.5:1); must be timed.
• Hunting: every pinion tooth meets every crown tooth; no timing needed (uneven ratio such as 3.333:1).

Ratios

• Ratio = driven teeth ÷ drive teeth, written ?:1. Multiply torque, divide RPM. Total ratio = all ratios multiplied together.

• Two external gears turn opposite ways. An idler changes output direction but does not change torque or RPM.

Gear types

• Spur (straight cut): basic, no end thrust, one tooth in contact (lower torque).
• Internal gear: teeth on the inside of a ring.
• Helical: handles more torque and runs quieter than spur (several teeth in contact) but creates axial (end) thrust.
• Double helical (herringbone): axial loads cancel; very high load and torque.
• Rack and pinion: rotary to linear.
• Angle drives: pinion = small drive gear; crown = large driven gear. Straight bevel = noisy, low torque. Spiral = curved teeth, quieter, better contact.
• Spiral bevel: pinion in line with crown centreline.
• Amboid: pinion above crown centreline ("A for above"); maximum ground clearance (driveshaft higher).
• Hypoid: pinion below crown centreline; lots of sliding contact, needs extreme-pressure (EP) oil.
• Worm and wheel: high torque, big reduction.

Simple planetary

• Input and output shafts stay aligned. Sun (centre), planet pinions on a carrier, ring (internal) gear.
• Each set needs a drive, a driven and a held (reaction) member.
• Ratio trick: don't count planet pinion teeth. Carrier "teeth" = sun + ring. Size order: sun (fewest) < ring < carrier (most).
• Direct drive = two members locked. Neutral = nothing held. Reverse = carrier held. Reduction = carrier driven (output). Overdrive = carrier drive (input).

• Idler gear: no effect on ratio or speed; it changes the direction of the output compared with two gears meshing directly, so the output turns the same way as the input.
• Underdrive (small drive gear turning a large driven gear) = reduction: speed down, torque up. Overdrive (large drive, small driven) = speed up, torque down. Direct drive = 1:1.
• Bevel gears transmit power between shafts at an angle; drive = pinion, driven = bevel (crown) gear.

Manual transmissions

• Countershaft (constant mesh) design: input (main drive) gear drives the countershaft; mainshaft gears float on the mainshaft and are locked to it by sliding clutches (collars) or synchronizers.
• Synchronizers match the speed of the gear and shaft (brass blocker rings) before the collar engages; worn synchros = grinding on shifts. Non-synchronized truck transmissions need double clutching or matching RPM.
• Twin countershaft transmissions split the load between two countershafts (they must be timed on assembly).
• Range section (planetary or gear) doubles the ratios (low/high range); splitter gears split each ratio in half. Range shifts are air-assisted and should be preselected; never range shift going down a hill in neutral.
• Interlocks stop two shift rails moving at once (two gears = locked transmission). Detents hold the rail in gear.
• Jumping out of gear: worn clutch teeth (collars and gears), weak detent springs, worn bearings, misalignment (engine/trans), bent forks.
• Noise in all gears = input/countershaft bearings or low oil. Noise in one gear only = that gear set. Hard shifting = clutch not fully releasing, linkage, low oil (cold), worn synchros.
• Lubrication: splash from the countershaft gears; some use a pump and cooler. Use the specified oil (often synthetic or engine oil, not always gear lube). Overfilling causes heat and foaming; check level at the fill plug with the vehicle level.
• Clutch brake (on-highway, non-synchro): stops the countershaft so you can shift into first or reverse from a stop. Only applies in the last inch of pedal travel; using it while moving wears it out.

Transfer cases and drop boxes

• A transfer case splits power to the front and rear axles (all-wheel drive) and may add a low range. Part-time cases lock the front and rear together (don't use on dry pavement: wind-up). Full-time cases use an inter-axle differential (often lockable) between front and rear outputs.
• A drop box moves power down or sideways to line up with the driveline; it may hold the input to a PTO.
• Power take-offs (PTO) drive accessories (hydraulic pumps, winches) from the transmission or flywheel. Gear-type PTOs must be engaged with the clutch released (no rotation) or with a hydraulically/air-engaged clutch. Check backlash with the gasket shims.

E-26 · Automatic and powershift transmissions

Components and planetaries

• Transmission jobs: modify power, multiply torque, reverse, provide a PTO, change speed/direction without interrupting power.
• Powershift: operator shifts speed and direction, can shift under full load; cycling work (forward/reverse).
• Automatic: shifts happen without operator action (operator still picks F/R); hauling work.
• Planetary advantages: constant mesh, compact, thrust loads offset, very versatile. Needle bearings on pinion shafts, thrust washers; more planets = more torque capacity.
• Neutral: input but no output.

Planetary rules

• Two external gears: opposite rotation. External + internal: same direction.
• Carrier output = always reduction. Carrier input = always overdrive. Carrier held = always reverse.
• Neutral = input but nothing held. Direct = any two members locked (or two driven at the same speed).

• Dual input: two inputs, one output, nothing held. Inputs same direction = more speed; opposite = less. Equal and opposite inputs = zero output.
• Dual output: two sets share two rotating members.
• Tooth speed = teeth passing a point per second; not the same as RPM.

Components

• Clutches: hydraulically applied, (usually) spring released. Rotating clutch locks two rotating members; stationary clutch locks a ring gear to the housing.
• Friction discs: grooves allow lube when released and let oil escape when squeezed (no delayed engagement). Steel plates go between every pair of friction discs (reaction surface; tangs/teeth on outside).
• Pistons are hydraulically actuated; return springs; splines stop rotation.
• Countershaft transmission: spur gears on parallel shafts; rotating wet multi-disc clutches lock gears to the countershafts. Not as compact (big bearings, thrust not cancelled) but cheaper; good for shuttling; lowers the path of power and can feed front and rear axles with no transfer case.
• High/low unit: no idler (both turn the same way). Reversing unit: idler between input and countershaft.

Control and shifting

• Three circuits in priority order: 1. clutch application (highest pressure), 2. torque converter charging, 3. lubrication (last).
• Oil path: reservoir → charge pump → selector/clutch pressure regulator → converter (with relief) → filter → cooler → lube (with relief).
• Charge pump provides flow for transmission control. Towing can damage the transmission (no lube): disconnect the driveshaft or lift the drive wheels.
• Pumps: transmission scavenge (leakage from main housing to reservoir), converter scavenge (from converter housing), charging pump (feeds the circuits), steering clutch/brake lube.
• Filter bypass valve passes unfiltered oil if plugged. Priority valve (normally closed) guarantees steering and brake pressure.
• Shift selector valve: speed and direction (speed spool, direction spool).
• Modulating relief valve: max clutch pressure. Load piston: rate of pressure rise in the clutch (prevents shock and slipping). Ratio valve = converter relief. Pressure differential valve = holds pressure to the speed clutch.
• Converter outlet relief: max working pressure in the converter (cold start protection) and keeps it full at shutdown.
• Clutch modulation: gradual pressure rise = controlled slip, no shift shock.

Automatic (hydraulic) control

• Shift points depend on ground speed and throttle (operator demand).
• Governor: turns output shaft speed into hydraulic pressure; spool balances centrifugal force against governor pressure. Low speed = low pressure; high speed = high pressure.
• Shift signal valves apply/release clutches through a relay valve; lower gears' signal valves have weaker springs.
• Orifice in the governor line prevents a rapid downshift if pressure drops (hard braking or governor failure).
• Modulator: changes shift points with throttle; opposes governor pressure. Full throttle = later (delayed) shifts (more HP and momentum per gear).

• Orifices control release rate. Trimmer valve controls application rate. Trimmer modulator boost stops trimming under heavy load (prevents slip).
• Automatic shifting can be prevented under heavy work (loading, pushing).

Electronic control

• Inputs: power, shift selector, input and output speed sensors (replace the governor, set shift points; not needed on powershift, only automatics), throttle position (replaces the modulator), pressure and temp sensors, data link.
• ECM: shifting, modulation, diagnostics.
• Outputs: shift solenoids (speed, direction, converter lockup), warnings, backup alarm.

Testing and service

• Steps: information → operator (most important source) → visual (oil level and condition always first; water/glycol is very damaging; check filter) → operational tests.
• Warm oil to 150–180 °F before testing (leakage rises with heat; problems may hide when cold).
• Stall test: brakes on, highest gear, full throttle, open area, watch temperature; the machine may move. Max 30 s, then neutral 30 s to cool. Don't overheat.

Pressure tests

• Gauge rated for double system pressure.
• Charging pump inlet: below 12 psia = inlet restriction (the pump is pulling too hard, so cavitation). Higher than normal = air being drawn in through a leak, which fills in the normal slight vacuum.
• Clutch pressure (set by clutch pressure regulator): check at low and high idle; should be the same. Low at idle but normal at high idle = internal leakage.
• Converter charge pressure: resistance in the converter; max set by the converter relief on the inlet side. Low = poor pump flow, clutch or converter circuit leakage.
• Lube pressure: last circuit to get oil. Low = poor pump flow, clutch/converter leakage, low in one range = a leaking clutch piston, lube leaks.
• Flowmeter (in series) accurately finds internal leakage upstream of it.
• Shift points are adjusted against governor pressure, output shaft speed, odometer (usually by shift signal valve spring tension; check governor pressure first).
• Electronic: fault codes, follow the manufacturer manual; protect the ECM.

Rebuild

• Safety, relieve pressure, follow the service manual. Clean, closed containers; lint-free towels.
• Nicks and scratches on valve parts = sticking valves.
• Clean bearings and friction discs only with mineral spirits; dry with dry air; soft wire for passages.
• Disc wear: micrometer. Warpage and cone: on a flat surface with a feeler gauge (cause drag).
• Clutch clearance: gap with the clutch released. Too little = drag (heat, wear). Too much = delayed engagement. Adjust with different reaction disc or piston thickness. Measure with feeler/go-no-go or stack height.
• Wrong control valve torque = spool binding.
• Install: new oil and filters; flush the cooler and lines. Misalignment = vibration. Check crankshaft endplay before removal and after installation. Adjust linkages.

E-27 · Driveline systems

• A driveline connects components that are not in line and that move relative to each other. Parts: yokes, universal joints (cross and bearing / Cardan), slip joint (splined yoke for length changes), tubes, centre (hanger) bearings on long drivelines.
• A single Cardan U-joint running at an angle makes the output speed up and slow down twice per revolution (non-uniform velocity). Two joints cancel this only if:
  • the yokes on each end of a shaft are in phase (in line with each other), and
  • the working angles at each end are equal (and opposite).
• Out-of-phase yokes or unequal angles cause vibration that gets worse with speed and load, and wear out U-joints, transmission and axle bearings.
• Working angles: keep them small but not zero. A joint at exactly 0° doesn't move its needle rollers and brinells (dents) the races; a little angle (often about ½° minimum) lets the needles roll. Too large an angle = heat, vibration and short life. Measure with an inclinometer on the yokes and shafts.
• Slip yoke splines must be lubricated and the slip joint assembled with the yoke arrows/marks lined up (phasing). Seal the slip joint.
• U-joint lubrication: grease until fresh grease purges from all four bearing seals. If one won't purge, loosen that cap or replace the joint.
• U-joint failure clues: true brinelling (rollers dented into the races) from too small an angle, overload or worn joints; false brinelling (polished marks, no dents) from vibration with no rotation; burned or rusty caps from lack of grease.
• Driveshafts are balanced; replace missing weights and keep tubes straight. Vibration that changes with road speed (not engine speed) points to the driveline or tires.
• Centre bearings: rubber cushion cracks and dry bearings cause vibration and noise; shims set the angle.
• Constant velocity (CV) joints (double Cardan, ball-type) keep output speed constant at larger angles (front steer drive axles).
• Safety: block the wheels and release the park brake only when safe before removing a driveshaft. A driveline park brake doesn't hold once the shaft is out. Support the shaft so the joints don't fall and get damaged; strap the caps on.

E-28 · Drive axles and differentials

Basics (Part A)

• Axles carry weight, turn power 90°, and reduce speed. GAWR = weight an axle can safely carry (housing is the main factor).
• Full-floating: shaft carries no load; can be pulled with weight on the tires. Semi-floating: shaft carries load; wheels must be raised to pull the shaft.
• Carrier holds the gear components (may be integral). Pinion depth set with shims at the pinion cage.
• Drive pinion delivers torque to the bevel gear (from the input yoke). Overhung: no support at the pinion end. Straddle: bearing on the pinion nose (high torque; short stub).
• Bevel (ring) gear delivers torque to the differential case; bolted to the flanged case half. Heel = outer edge, toe = inside.
• Thrust screw (opposite the pinion, behind the bevel gear) prevents bevel gear deflection under heavy load.
• Adjusting rings both sides set side bearing preload/position. Cross shaft (spider) sits between the case halves and carries the differential pinions. Side gears pass torque to the axle shafts. Thrust washers throughout. Output shaft for tandem axles.
• Path of power: yoke → pinion → bevel gear → case → cross shaft → diff pinions → side gears → axle shafts → wheels.
• Differential lets driven wheels turn at different speeds (essential cornering; outer wheel faster). In a straight line the diff pinions don't spin on the cross shaft.
• Most carriers are single speed, single reduction; ratio = pinion to bevel gear.
• Torque is equal at each wheel regardless of RPM; the good wheel only gets as much as the slipping wheel. Wheel torque = bevel gear torque ÷ 2.
• RPM can't be lost: total wheel RPM = bevel gear RPM × 2.
• Transfer case (drop box): splits power front/rear and drops it to driveline height. Inter-axle differential (optional) splits torque front/rear at varying amounts.
• Traction devices send more torque to the wheel with the most traction (0 traction one side = 100% to the other). Lock = wheels at the same RPM; locks one axle shaft to the case.
• Locking diffs eliminate differential action (operator controlled). Jaw/dog style: engage at low engine speed, travelling straight. Disc style: electric-hydraulic, more forgiving with wheel spin.
• Limited slip: friction discs, automatic; normal diff in corners. No-spin: replaces the cross shaft, pinions and side gears; acts locked but allows cornering (ratchets).

Service (Part B)

• Oil level: level with the filler opening. Wrong viscosity or low level = premature wear. Splash or pressure lube (internal pump, often overdriven for low-speed lube).
• Leaks: seals, housing joints (O-ring or RTV), breathers, housing fatigue. Check endplay on input and output shafts.
• Diagnose: suspension, driveline, wheels/tires/hubs, road test only when it won't cause more damage.

• Overheating: low oil, high oil (foaming), dirty housing, wrong oil.
• If axles share oil with final drives, check them too.
• Before teardown: backlash (dial indicator 90° to tooth, 4 places), runout, rolling torque, tooth pattern.
• Mark side caps and all matched parts. No-spin case halves are spring loaded. Keep pinion shims. Don't spin bearings with air.
• Bevel gear and pinion are replaced as a matched set.

Assembly

• Pinion bearing preload: shims between the two pinion bearings (more shim = less preload). Press the bearings, measure rolling torque while turning, not breakaway. Torque = spring scale pull × radius. Final preload comes from yoke nut torque.
• Nominal dimension: pinion end to bevel gear centreline; etched on the pinion end. 0 or blank = standard. + = move pinion away from the bevel gear. − = move it closer.
• Shims at the bearing cage: thicker = pinion further away. Shims between inner bearing and pinion gear: thicker = closer. Old gear set = old shims; new set = adjust to the etched number.
• Side bearing preload and backlash: adjusting rings. Remove endplay, add a few notches for preload.
• Backlash: turn both rings the same amount (keeps preload). Reusing old gears = keep the old backlash. More backlash: move the gear away from the pinion (loosen flanged side, tighten plain side). Less: move toward the pinion.
• Runout: radial (side) and back face.
• Tooth pattern: paint the convex (drive) side; keep drag on the pinion.

E-29 · Final drive systems

• Final drive = last reduction (multiplies torque); takes shock loads.
• Types: spur (single/double), planetary (single, double, triple), combination, chain.
• Inboard planetary: driveshaft between it and the wheel; needs a very thick shaft. Outboard: wheel/sprocket mounts directly; carrier often acts as cover and mount; small axles; much easier to service.
• Typical path: sun = input, carrier = output, ring = held. Double: first set drives the second; total = ratio 1 × ratio 2.
• Mechanically driven on wheeled machines, hydraulically on tracked (motor may spin opposite the output).
• Chain final drives: skid steers and motor graders. Grader tandem housing works like a walking beam (ground clearance). Chain pros: one input, several outputs, no slip over distance, cheap, compact; housing often holds brakes.
• Spur: bevel set turns 90°. Small spur = drive pinion (bull pinion); large = driven (bull gear). Big gears, radial loads, big housing.
• Maintenance: splash lube, right oil, clean. Oil level: level ground, level line parallel to ground, remove plug and check. Small centre cover gives access to the sun gear and axle; not vented, so pressure builds when warm. Pressed-on sprockets: caution with presses.

F-30 · Heating systems

• Most cab heat comes from engine coolant pumped through a heater core (a small radiator) in the HVAC box. A blower pushes cab or outside air through it.
• Temperature is controlled by a heater control valve (coolant flow: manual, cable, vacuum, electric or PWM) and/or a blend door that mixes heated and unheated air.
• Blower speeds: often a resistor pack in series with the motor; high speed bypasses the resistors. If the blower only works on high, suspect the resistor pack (or its thermal fuse). Newer systems use a PWM speed module.
• Heater coolant hoses come off the engine's hot side (before the thermostat outlet on many engines) and return to the pump inlet so coolant circulates whenever the engine runs.
• Auxiliary (fuel-fired) heaters heat the coolant or the cab directly with the engine off; they need a clean fuel supply, combustion air and a clear exhaust (carbon monoxide risk).

Heating diagnosis

• After any coolant service, purge air from the heater core (heater valve open, bleed points) or the core can air-lock.
• Hoses to the heater core feel hot in and cooler out when it's working; both cold = no flow (valve, air lock, plug).

F-31 · Ventilation and filtration systems

• Cab ventilation brings in fresh air through a fresh air filter and recirculates cab air through a recirculation filter.
• Cab pressurization: the blower keeps the cab at a slight positive pressure so dust and fumes leak out, not in. Low pressurization = dust in the cab. Causes: plugged filters, bad door or window seals, missing grommets, damaged ducts, weak blower. Check with a cab pressure gauge (manometer) per the OEM.
• Filters: pleated paper for dust; activated carbon for odours and chemical vapours (spraying, landfill); HEPA for very fine particles (silica, asbestos). Hazardous work may need a certified sealed cab.
• Plugged cab filters = low airflow, poor heating and cooling, and the A/C evaporator can freeze up (not enough warm air across it).
• Service: replace filters on schedule or sooner in dust. Many cab filters must not be cleaned with compressed air or washed (it damages the media); follow the OEM. Wear a respirator and dispose of filters from hazardous sites properly.
• Check door seals, window seals and cab floor plugs during service; replace damaged seals.
• Clean the evaporator drain; a plugged drain lets water collect, grow mould and drip into the cab.

F-32 · Air conditioning systems

How it works

• A/C moves heat out of the cab by making refrigerant absorb heat as it boils (evaporates) inside the cab and give off heat as it condenses outside. Changing state moves a lot of heat (latent heat).
• The cycle, in order:
  • Compressor: low-pressure vapour in → high-pressure, hot vapour out. It separates the high side from the low side.
  • Condenser (in front of the radiator): hot high-pressure vapour gives off heat and becomes high-pressure liquid.
  • Receiver-drier (high side, with expansion valve systems): stores liquid, removes moisture (desiccant) and filters. Some have a sight glass.
  • Metering device: the thermostatic expansion valve (TXV) or a fixed orifice tube. High-pressure liquid becomes a low-pressure, cold liquid spray. This is the dividing line between the high and low sides.
  • Evaporator (in the cab): the refrigerant boils and absorbs heat from cab air. It also dehumidifies (water condenses on the fins and drains out).
  • Accumulator (low side, with orifice tube systems): stops liquid getting back to the compressor and holds the desiccant.
• Refrigerants: R-134a and the newer R-1234yf (mildly flammable). Never mix refrigerants or oils; each has its own fittings. Compressor oil is usually PAG (or POE) and is hygroscopic, so keep it capped.
• Compressor clutch: an electromagnetic coil pulls the hub to the pulley. Check the air gap with a feeler gauge, coil resistance and current, and voltage at the connector.
• Controls: low-pressure switch (stops the compressor when charge is low, protecting it), high-pressure switch (stops it if pressure is too high), evaporator temperature (thermostatic) switch or sensor that cycles the clutch to stop the evaporator from icing. Automatic climate control uses sensors and actuators run by an HVAC module.

Service and safety

• Federal and provincial rules: you need the environmental (ozone depletion prevention) certification to service A/C. Refrigerant must be recovered, never vented to the atmosphere.
• Wear eye protection and gloves: liquid refrigerant causes frostbite. Don't let refrigerant contact flame or hot surfaces (toxic gases). Work in a ventilated area (it displaces oxygen).
• Manifold gauge set: blue = low side, red = high side, yellow = service hose. Use the right fittings for the refrigerant.
• Any time the system is opened: recover, repair, replace the receiver-drier or accumulator if it was open for long or the system was contaminated, then evacuate with a vacuum pump (removes air and moisture; moisture boils off in a deep vacuum). Hold the vacuum to check for leaks. Charge by weight to the OEM spec, then performance test.
• Leak detection: electronic detector (refrigerant is heavier than air, check below fittings), UV dye and a black light, soap bubbles on a charged system.
• Add oil to match what was lost with a replaced component (per OEM).
• Clean the condenser and radiator fins (low pressure, opposite to airflow); a dirty condenser raises high-side pressure and cuts cooling.

Reading the gauges

• Test at a steady engine speed (often around 1500 RPM), doors closed, max cooling, blower on high, after the system stabilizes.

F-33 · Sound suppression systems

• Noise in the cab is controlled by insulation and sound-absorbing panels (headliner, floor mats, firewall), isolation (rubber) cab mounts, door and window seals, engine enclosures, and the muffler.
• Prolonged noise causes permanent hearing loss. Most Canadian jurisdictions set an occupational limit around 85 dBA averaged over 8 hours; above that, hearing protection and noise controls are required.
• A sudden increase in cab noise points to: missing or damaged insulation or panels, failed cab mounts (metal-to-metal contact), damaged door seals, exhaust leaks or a failed muffler, loose sheet metal (rattles).
• Repair with OEM (fire-resistant) insulation. Oil- or fuel-soaked insulation is a fire hazard and must be replaced.
• After cab work, reinstall every panel, seal and grommet; open holes let in both noise and dust (and spoil cab pressurization).
• Sound level meters measure noise in decibels (dBA); some operator stations have a posted noise level on a decal.

G-34 · Hydraulic systems

Basics

• Pascal's law: pressure on a confined liquid is transmitted equally and undiminished in all directions. Liquids barely compress; gases do.
• Pumps make flow, not pressure. Pressure is caused by resistance to flow (the load). Flow sets speed; pressure sets force.
• Bernoulli: fluid flows from high pressure to low. Pressure drop across any restriction = heat.
• P = F ÷ A. Volume = Area × Distance.
• Parallel circuits: the actuator with the lightest load moves first. Series: the first actuator must overcome the back pressure from the second.
• Head pressure = pressure from the weight of the fluid; reservoirs sit above the pump to flood the inlet. Atmospheric pressure = 14.7 psi (101.3 kPa).
• Gauge pressure (psig) ignores atmosphere (reads 0 open to air). Absolute (psia) includes it (0 only in a perfect vacuum). Vacuum is read in inches of mercury (in Hg). Bourdon tube gauges are the common type. 1 bar ≈ 14.5 psi = 100 kPa.
• Positive displacement pumps have a mechanical seal between inlet and outlet: flow barely drops as pressure rises. Non-positive (water pumps) don't.
• Displacement = volume per revolution. GPM = in³/rev × RPM ÷ 231; LPM = cm³/rev × RPM ÷ 1000.
• Volumetric efficiency = actual ÷ theoretical flow (losses are internal leakage, "slip"; some slip lubricates the pump).
• External gear pumps carry oil around the outside between the teeth and housing; they're unbalanced; pressure plates limit slip. Pump inlets are bigger than outlets (keeps the inlet flooded). Rotation is viewed from the shaft end.
• Cavitation: inlet pressure too low, so oil vaporizes and the bubbles implode (inlet restriction, plugged strainer, cold oil, overspeed, wrong inlet hose). Signs: pitting, metal in the oil, rattle/whine. Max inlet vacuum about 5 in Hg.
• Aeration: air gets into the oil (low oil level, inlet-side leak, return line above the oil level). Signs: foam, spongy/erratic actuators, noise, heat.
• Cylinders: single acting (one direction, returned by load or spring; telescopic stages extend largest first) and double acting. Double acting cylinders extend slower with more force and retract faster with less force (rod area).
• Hydraulic oil jobs: transmit power, lubricate, seal, cool.
• Reservoir: about 2–3 × pump flow (less on mobile equipment); cools, settles dirt, separates air. Baffles separate return from suction. Pressurized reservoirs keep the inlet flooded. Depressurize before opening.
• Contamination is the main cause of hydraulic failure. Ingressed (from you, during service), internally generated (wear), built-in (manufacturing).
• Strainers on the pump inlet (about 100 mesh). Filters rated in microns. Return line filters are most common; pressure filters protect sensitive parts. Beta ratio = particles upstream ÷ downstream (higher = better). Filter bypass valve opens when cold oil or a plugged element raises the pressure drop.
• Coolers usually sit in the return line. Heaters near the pump inlet in the tank.

Hydraulic symbols

The notes give symbols as pictures. These are the standard ISO 1219 / ANSI forms.

• Actuation symbols on the end of a valve: push button, lever, pedal, spring, solenoid (box with diagonal line), hydraulic pilot (triangle on a dashed line), electro-hydraulic (solenoid plus pilot).
• Pressure valves show where they sense pressure: internal upstream, internal downstream or external.

Principles, fluids and contamination

Pros and cons

• Advantages: compact, flexible installation, easy cooling and lube, operator controls several functions at once, precise smooth positioning, easily protected from overload.
• Disadvantages: pressurized fluid can injure or kill, noise, leaks (contamination or fire), temperature affects performance, trapped air causes problems.

Energy and work

• Work = force acting through a distance. Energy = the ability to do work.
• Energy cannot be created or destroyed, only transformed. Engine: chemical → heat → mechanical. Pump: mechanical → hydraulic. Actuator: hydraulic → mechanical.
• Kinetic = movement / motion (oil flow, supplied by the pump). Potential = stored energy, creates force (pressure) (example: accumulator).
• Bernoulli: total energy stays constant, so pressure + velocity stays constant. High kinetic = low potential; low kinetic = high potential. This is how an orifice works: it slows flow and raises pressure before the orifice.

Power, pressure and flow

• HP = (GPM × PSI) ÷ 1714. Pressure = Force ÷ Area. Pressure is created by resistance to flow (psi or kPa).
• Flow rate = Cylinder volume ÷ Time.
• Efficiency = output power compared with input power (%).

Losses

• A difference in potential energy creates flow (high to low). That pressure drop does no useful work and becomes heat (from fluid friction). More pressure loss = more heat.
• Laminar flow: smooth, straight parallel lines, no turbulence, low friction. Turbulence creates pressure losses. Caused by lines too small, sharp changes in cross-section, sharp changes in direction.
• Backpressure = resistance to flow on the return side. Raises internal resistance, lowers efficiency.
• Flow losses = excessive internal leakage. Complaint will be slower actuator operation.

Series vs parallel circuits

• Series-parallel: motors in series switch to parallel as load rises (more torque).

Hydraulic fluids

• Petroleum: most common, flammable. Dispose of where it cannot reach ground or water.
• Fire resistant: water-oil emulsion (60% water, 40% oil), invert emulsion (40% water, 60% oil). Water lubricates poorly, grows bacteria, freezes, foams. Water glycol: like coolant (60% glycol / 40% water), better in cold and fire, less lubricating.
• Phosphate ester (synthetic): most fire resistant, most expensive, not compatible with nitrile rubber (use Teflon-type seals), forms acids with water.
• Biodegradable: 70–80% broken down by microorganisms, harmless to organisms, poor temperature range.
• Viscosity = resistance to flow (sets lubricating ability; higher viscosity = thicker film).
• Viscosity index = how much viscosity changes with temperature. High VI = small change; low VI = large change. Most OEMs want 90 minimum.
• Oxidation = combining with oxygen; forms soaps and weak acids; heat speeds it up. Oxidized oil is darker and smells burnt.
• Demulsification = ability to separate water. 0.5% water = dull/smoky; 1%+ = milky.

Contamination

• Built-in: present on new or rebuilt machines (casting sand, wear-in). Reason initial service is short (~100 h).
• Ingressed (from outside): faulty breathers, leaking pump inlet, cylinder wiper seals (fine dirt pulled in as rods retract), unclean maintenance (the most common cause).
• Internally generated: wear, fatigue, cavitation, oxidation.
• Failures: catastrophic (sudden total failure, "grenading"), intermittent (comes and goes; transient, can move from component to component, may self-correct), degradation (gradual wear).

Pumps

• A pump converts mechanical energy into hydraulic energy; it takes power from the prime mover and generates flow (it does not make pressure).
• Displacement = internal volume per revolution (in³/rev or cc/rev).
• Positive displacement: sealed between inlet and outlet; delivers a nearly constant volume regardless of resistance. All hydraulic pumps are positive displacement. Non-positive (engine water pump): inlet and outlet connected, high flow, low pressure.
• Max flow is directly proportional to speed. Flow = displacement × RPM. Too much speed can cause cavitation.

Efficiency

• Mechanical: theoretical vs actual torque; power needed to overcome internal friction.
• Volumetric: theoretical vs actual flow; set by internal leakage; poor volumetric efficiency = longer cycle times. VE % = actual × 100 ÷ theoretical (or no-load vs full-load flow).
• Overall: input power vs delivered power (mechanical + flow losses). Peaks at normal operating pressure, drops at higher pressure because internal leakage increases.

Pump basics

• Rotation is always viewed from the driveshaft end.
• Uni-directional: built to rotate one way (inlet usually larger than outlet; may need internal changes to reverse).
• Bi-directional: inlet and outlet the same size, runs either rotation.
• Reversible: reverses oil flow direction while the driveshaft keeps turning the same way.

• Fixed displacement: same volume every revolution (gear, vane or piston). Variable: chamber volume can change (vane or piston only).
• Balanced: inlet/outlet thrust loads cancel. Unbalanced: inlet and outlet forces create a thrust (side) load.
• Pressure compensated: flow adjusted automatically by outlet pressure.
• Internal leakage: some is needed for lubrication; it reduces flow for work (slower motors). Slip = leakage from the high to low pressure side. Higher pressure = more leakage.
• Case drain returns internal leakage to the reservoir (often gravity fed). Port goes on the top so the case stays flooded. Required on piston pumps; gear and vane pumps route leakage internally to the inlet.

Gear pumps (external)

• Simple, cheap, rugged, tolerate dirt; low max pressure; fixed displacement only.
• Drive gear (from power source) meshes with the driven gear. Pressure plate limits leakage at the gear faces.
• Oil is carried around the outside of the housing in the tooth pockets, inlet to outlet.
• Unbalanced: inlet and outlet forces oppose on all but one tooth.
• Outlet = gears coming into mesh. Inlet = gears coming out of mesh.

Vane pumps

• More efficient and higher pressure than gear pumps; more expensive and more sensitive to dirt.
• Rotor = slotted cylinder (driven); vanes = sliding plates in the slots; vane pocket = space between two vanes that carries oil.
• Vanes held out by centrifugal force plus outlet pressure ported under the vane (some have small springs for start-up).
• Bevelled vane: high point is the leading edge; used at high speed and pressure; uni-directional only (cannot be used in bi-directional pumps). Rounded tips = bi-directional.
• Intra-vane: adjusts vane loading as it passes inlet and outlet.
• Cam ring = riding surface for vanes (elliptical or circular). Pressure plates control rotor-face clearance. Cartridge = cam ring, rotor, vanes and plates as one unit.
• Unbalanced: circular cam ring, rotor offset. Variable displacement is always unbalanced: the cam ring moves to change eccentricity (centred = zero flow); may be pressure compensated.
• Balanced: elliptical cam ring, two opposing inlets and two opposing outlets → higher pressure.

Piston pumps

• Reciprocating motion; most efficient and highest pressure; very sensitive to contamination; used when variable displacement is needed. Charge pumps often used to prevent cavitation.
• Radial: pistons perpendicular (90°) to the driveshaft. Check valves on each piston's inlet and outlet. Rotating cam (fixed pistons, rotating lobe) or rotating piston (pistons in a rotating block against a cam ring).
• Axial: most common in mobile equipment; pistons parallel to the driveshaft; barrel turns, pistons slide on a swash plate. Swash plate angle sets stroke length, so more angle = more displacement. Drilled piston passages balance the piston and lubricate the shoes.
• Pressure compensated axial pump: system pressure acts on the compensator. Bias piston + spring pushes toward maximum displacement. Stroking piston (large, pressure controlled) pushes toward zero displacement (swash plate flat). No relief valve needed because the pump destrokes itself.
• Reversible: swash plate angle is reversed (manual or hydraulic).
• Bent axis: housing angled to driveshaft; pistons held by ball joints, not sliding on a swash plate; more reliable; variable by changing the barrel-to-shaft angle.

Pump service

• Start-up: back off pressure controls (prevents overload), idle, load to warm oil, check max engine speed, test.
• After a component failure: change oil and filters, clean the reservoir, flush the system as needed. Check the shaft slides freely in the coupling, check rotation, mark all mated parts and housings, pre-lubricate the pump before start-up (fill and turn by hand).

Failure analysis

• Gear pump: mark housings and gears. Inlet = out of mesh, outlet = into mesh. The pressure plate trap (relief groove) is on the outlet side. Aluminum housings are designed to be cut by the gears during break-in (tiny clearance). Trap groove gives trapped oil an escape (less noise, better performance).
• Vane pump: cam ring inlet and outlet must be in the correct position; backwards = low or no flow and cavitation.
• Piston pump: pistons are mated to their original bores.

Actuators

Cylinders

• Convert hydraulic energy into linear mechanical energy.
• Parts: barrel (honed, maybe chromed), rod end cap/gland (seals and wiper), piston and rod, seals, bearings. Wiper seal keeps contaminants out.
• Base side force = piston area × pressure. Rod side force = (piston area − rod area) × pressure. Smaller cylinders extend faster (less volume to fill).
• Single acting: powered one way, external force returns it. Ram type: oil passes the piston, sealed by packing at the end. Telescoping: stages fitted inside each other, each stage less force and more speed; the largest (first) stage moves first.
• Double acting: powered both ways. Single rod (differential): extension = more force, slower; retraction = less force, faster.
• Internal leakage reduces speed (longer cycle times). Pressure equalizes both sides so only the rod area produces force.
• Drift: a leaking cylinder drifts toward extension. Pressure intensification: the cylinder cannot drift to retract, and pressure inside can rise higher than system pressure (excavators especially).
• Double rod (non-differential): equal area and volume both sides → same force and speed both ways.
• Double-acting telescoping (haul trucks): last stage out is first back in.
• Piston relief valve: lets oil bypass when the piston bottoms (a double check valve in the piston; used where the reservoir is small).
• Cushions slow the cylinder at end of stroke (plunger restricts return oil, or drillings get blocked progressively).
• Quick drop valve: faster lowering by regeneration (rod-end oil fed to the base end in full lower; in parallel with the actuator).

Motors

• Convert hydraulic energy into rotary mechanical energy.
• Speed (RPM) depends on inlet flow and displacement. More flow = faster; more displacement = slower.
• Torque depends on inlet pressure and displacement. More pressure or displacement = more torque.
• Inlet pressure is set by the resistance on the motor shaft.
• Same port sizes = bi-directional; different = uni-directional. Motors need a case drain.

Actuator service

• Cylinder problems: external leaks, internal leaks, structural damage. Internal leakage makes heat during movement.
• Internal leak test: stall the cylinder and remove the return line; very little oil should come out.
• Remove: depressurize, follow OEM, use lifting gear. Never use compressed air to force the piston out.
• Inspection: hone the barrel, compare ID to spec, re-chrome rod if needed. Check rod straightness: straightedge (quick) or V-blocks and a dial indicator (accurate).
• Assembly: soften seals in warm water or oil, light coat of oil, torque the piston retaining fastener to spec (a loose piston = catastrophic failure). Reversed lines = backwards operation.
• Testing: use pressurized oil, never compressed air. Final test = cycle times.
• Motors: release pressure, keep clean, component failure contaminates the system (oil, filters, reservoir, flush). Mark housings and mated parts.
• Motor cavitation: pitting, happens when the load overruns the motor (motor spins faster than the pump supplies).
• Install: pre-lubricate the motor (flood it). Start-up: back off controls, idle, warm, slowly apply pressure and check for leaks. Don't run with leaks or odd noises.

Valves and accumulators

Pressure control valve basics (Part A)

• Normal position = state with no pressure in the system (spring returns it). NO = flow unrestricted in normal position; NC = flow blocked.
• PCV jobs: limit/regulate pressure, create a pressure condition, sequence actuators.
• Balances spring force against oil pressure. Spring cavity must drain to the reservoir (or it hydraulically locks). Externally drained = outlet is under pressure; internally drained = outlet goes to tank.
• Modulation = constant opening and closing. Spring constant = stiffness. Cracking pressure = pressure where oil starts to flow. Full flow = fully open. Pressure override = full flow pressure − cracking pressure.
• Upstream = before the valve; downstream = after.
• Direct acting: pressure directly against the spring (low flow uses).
• Pilot operated: two poppets. Pilot poppet = low flow, high pressure (strong spring, small poppet). Main poppet = high flow, low pressure (weak spring, large poppet). Main spring sets the override. Main advantage: minimizes pressure override; more accurate, less chatter, higher range.

• Relief service: gauge in parallel, warm oil, stall, compare, adjust, retest. Main relief adjusted at high idle with gauge at the pump outlet.
• Backpressure valves restrict return oil to control actuators (a safety valve). Be careful in the rod end: pressure intensification.

Flow controls

• Control actuator speed by controlling input flow. An orifice restricts flow and causes a pressure drop. Pressure after the orifice = the load after it.
• Orifice flow depends on size and shape, pressure drop, and oil viscosity. May be temperature compensated.

Accumulators

• Functions: energy storage, react to pressure fluctuations, shock absorption, gradual pressure build-up, constant pressure maintenance.
• Always discharge before service.
• Weighted (most basic, constant force), spring (force not constant), gas charged (most common) with nitrogen: piston or bladder type.
• Never use oxygen or compressed air (can explode with oil).

Directional control valves (Part B)

• Classified by flow paths (ways), positions, actuation and detents. Types: poppet (simple check valve), rotary spool (high leakage, low pressure and flow), sliding spool (most common, linear). Spool and housing are a matched set.
• Lands have grooves (lube, sealing, centring). Metering notches = "U" cuts at land edges for feathering. Centering spring returns the spool to neutral.
• Positions = number of envelopes. Hold = work ports blocked (hydrostatic lock). Float = work ports to tank. Diverter = 2-position 3-way, picks which of two circuits gets flow.
• Ways = how many lines connect to the valve.

• Actuation: mechanical (lever, pedal, button), pilot (hydraulic or air) for remote valves (pilot pump, pilot relief, pressure regulating controller), electric solenoid: electrical = on/off (no feathering); electronic = infinitely variable; combinations (electric-over-pilot gives feathering and force).
• Detents hold the spool in position (ball and spring, or electromagnet).
• Load check: stops a suspended load dropping momentarily when going from hold to raise. Anti-cavitation check: prevents a void when the load drives the actuator faster than the pump supplies (tank oil fills in). Flow compensator: constant pressure drop across the spool so several valves work at once.
• Banks: unibody or stacks. Power beyond extends the series passage to another DCV (main has priority). Closed centre banks are in parallel.
• Series passageway: first valve has priority; downstream valves lose flow. Tandem: return oil feeds the next valve (actuators in series). Series-parallel: series in neutral, parallel when operated together, lightest load has priority. Diverter valve: DCVs cannot work at the same time.

System types

• System = all interconnected components; circuit = a group within a system for one function.
• Variable pumps vary flow with demand; pump HP must be limited so the engine doesn't stall (reduce pressure or flow).
• Pump flow returns to tank with the DCV in neutral. Usually fixed displacement. Constant flow: delivers flow even with no work.
• Pump pressure = load resistance. Neutral DCV = low resistance.
• Best for systems not used for a high % of the duty cycle (wastes power at low demand and high RPM).
• Constant horsepower: regulates pump flow by load, keeps max HP, high displacement at low pressure and destrokes as pressure rises. Keeps engine speed constant; fast at light loads. Often side-by-side pumps. Any rise in the three regulator pressures destrokes the pump: NFC (monitors flow through each DCV bank), PRV/power shift (ECM controlled, set by engine speed dial), cross-sensing (the other pump's outlet).
• Blocks flow with the DCV in neutral; holds the system at max pressure. Fixed pump + unloader, or variable pump. Relief is a backup, set higher than the compensator.
• Demand flow: flow only when needed for work. Pressure compensated: limits flow, not pressure.
• Load sensing: exact flow for the load. Margin pressure: pump is kept slightly above load pressure.
  • Standby: margin pressure only at the pump; no pressure at the load.
  • Moving: pump pressure = load + margin.
  • Stalled: pump at max (set by the pressure compensator), same as load.
• LSPC parts: pump, stroking piston (destrokes), pressure compensator (sets max system pressure, opens on stall), flow compensator (keeps margin), load sense line, DCV, actuator, bias spring/piston (upstrokes).

Testing and service

• Troubleshooting: information, operator, visual (oil level and condition, leaks, damage, wear), check filter for contamination.
• Warm oil to operating temperature first (open centre: stall an actuator at full throttle).
• Cycle time at full throttle = quick check for flow loss. If slow, always check engine RPM first. Slow cycles = internal leakage.
• Stall tests: engine speed at max load, several actuators and directions (system vs circuit problem).
  • Excessive RPM drop: weak engine, or relief set too high.
  • No RPM drop: engine power increased, or relief set too low.
  • Closed centre: engine should not labour at stall (if it does: unloader or compensator fault).
• Flow problems affect speed; pressure problems affect force.
• Safety: lower loads, relieve tank pressure, discharge accumulators.

Gauges

• Bourdon tube straightens under pressure. Protect with snubber (inlet orifice) or glycerine fill (damps needle vibration).
• Choose a gauge twice the expected pressure (accurate at mid scale). Install in parallel. Compound = vacuum + pressure. Absolute = psia.

What readings mean

Leakage tests

• Cylinder drift = movement with the unit stopped (tape on rod; excavator bucket tip to ground measures all cylinders).
• A leaking component gets hotter faster (infrared/digital thermometer).
• Motor: remove drain line, stall at full throttle, measure drain flow for 10 s. Cylinder: stall and remove the opposite line.
• Flowmeter: in a test unit with a load valve. Downsides: slow, system opened, contamination risk.
• Pump efficiency: flowmeter in series; below 80% = repair or replace. High case drain flow = wear.
• Circuit leakage (tee test): flowmeter in parallel between pump outlet and return; leakage = full-load pump flow − circuit flow. Can also find relief cracking and full flow.
• Faults: learn the system, easy checks first, speed = flow, force = pressure, fix the root cause or the new part fails too.

Electrohydraulics

• Electrical = on/off; electronic = variable.
• Electrical control: battery, protection, key switch, relays, controls, solenoids (discrete: on or off), clamping diodes (absorb induced voltage, stop relay arcing), harness. Proximity switch closes the circuit when a metal plate passes.
• Electronic systems use less than system voltage (5 V). Faster, more accurate, less maintenance, better over distance, self-diagnostics.
• Inputs: reference-voltage sensors (5 V out, return measured), variable (resistor type: temperature, joystick potentiometer), discrete (on/off switches), voltage generating (speed sensors).
• Outputs: a typical solenoid is discrete; a proportional solenoid moves in proportion to the signal (PWM).
• Open loop: no feedback. Closed loop: feedback sensors for accurate position, speed, acceleration. Servo valve mimics operator input; torque motor gives limited rotation.
• Much less tolerant of contamination (tight tolerances).
• PWM = digital on/off signal; duty cycle = % on time; reduces heat (less average current).
• Hysteresis: spool needs more current one way than the other. Stiction = more effort to start a spool moving; cure = dither (constant small oscillation).
• Talk to the operator: most important information source.
• Protect the ECM: unplug when welding, no fast charging with ECM connected, never disconnect batteries with engine running, don't unplug while operating, keep it cool.

G-35 · Hydrostatic systems

Hydrostatic transmission basics

• Pump + motor; usually propel. Highest speed = min motor + max pump displacement. Lowest = max motor + min pump. More motor displacement = more torque.
• Open loop: pump from tank, motor back to tank (DCV reverses). Closed loop: motor outlet to pump inlet, no path to tank (reversible pump reverses).
• Most common: closed loop, variable pump, fixed motor.
• Advantages: infinite speed control, quick response, hydrostatic lock at zero, automatic braking (load can't overrun).
• Flushing valve: pulls hot oil from the low side through the motor case to cool; only works when the motor turns. Oil is hottest at the motor outlet.
• Crossover relief: dumps high side to low side; set wrong = drift under load.
• Isolation checks let charge oil into the low side only. Charge pump (gear) replaces leakage, prevents HST pump cavitation.
• Flushing/hot oil relief is set lower than the charge relief. Joysticks must be calibrated.

Closed loop components

• A closed loop hydrostatic drive is a variable displacement, reversible piston pump driving one or more motors; oil goes out one loop line and returns through the other straight back to the pump inlet (not through the tank).
• Charge pump (usually a gear pump on the same shaft): replaces internal leakage, keeps the low side of the loop pressurized (prevents cavitation), supplies cool, filtered oil, and often feeds the servo that strokes the pump. Low charge pressure = sluggish or no drive and a hot, noisy unit.
• Charge relief sets charge pressure. Make-up (check) valves let charge oil into whichever loop line is low.
• High-pressure (crossover) relief valves protect each loop line; pressure override (POR) destrokes the pump before the reliefs open (less heat).
• Loop flushing (hot oil shuttle) valve: shifts to the low-pressure side and bleeds a little hot loop oil to the case and cooler; the charge pump replaces it with cool oil.
• Case drains carry internal leakage (and cooling flow) back to the tank through the cooler. Case drain must be filled with oil before start-up and its flow is a key wear test: high case drain flow = worn pump or motor.
• Swashplate angle sets displacement and direction: centred (neutral) = no flow. If the machine creeps in neutral, the pump isn't centred (adjust neutral per OEM).
• Variable motors give a high-speed range (smaller angle = faster, less torque).
• Towing: most hydrostatic machines need the loop bypassed (tow valve) and brakes released per the OEM, and only for a short distance at low speed (no lubrication flow).

Hydrostatic diagnosis

• Order: oil level and condition, filters, then charge pressure first (nothing works right without it), then loop pressures, case drain flow, controls.
• Slow in both directions = low charge pressure, worn pump, low engine speed, control problem. Slow in one direction only = that direction's relief or make-up valve, or the control.
• Overheating = high case drain (internal wear), relief valves blowing off, plugged cooler, low charge flow.
• After replacing a pump or motor: fill the case, flush the loop, change filters, bring it up slowly; contamination from the failed part destroys the new one.

Hydrostatic steering and skid steers

• Many crawlers use hydrostatic (dual path) steering: one pump/motor per track. See C-13. Skid steers use two closed loops with a gear charge pump; see C-13 (wheeled steering, Part B).

G-36 · Pneumatic systems

Supply system

• Compressed air: tolerates some leakage, no bleeding needed, but has some lag and is sensitive to moisture and contamination.
• Compressor (gear or belt driven, engine or own oil, air and/or coolant cooled). Loaded = compressing; unloaded = the governor sends air to the unloader pistons, which hold the inlet valves open, so it pumps air in and out with no compression (and stays cooler).
• Governor sets cut-out (max) and cut-in (min). Ports: RES (reservoir signal), UNL (to unloaders), EXH. The piston's upper area is bigger, so it stays up until pressure drops about 25 psi (cut-in to cut-out spread no more than 25 psi). Typical system 120–130 psi.
• Air dryer (desiccant): dries air during the loaded cycle; when the governor unloads, a purge valve opens and dry air from a purge volume flows back through the desiccant to regenerate it. Alcohol injectors/evaporators prevent freeze-up on systems without dryers.
• Each reservoir needs a drain valve; drain daily (or automatic drain valves). Water in the tanks freezes and corrodes.
• Reservoir volume at least 12 × the total brake chamber volume (trailer reservoirs 8 ×). Tanks are hydrostatically tested to 5 × working pressure.
• Safety valve about 150 psi protects the system. One-way check valves between the supply tank and the service tanks hold air if supply is lost.
• Pressure protection valves isolate accessory circuits (air suspension at about 85 psi, starter tanks at about 75–90 psi) so brakes fill first and an accessory leak can't drain the brakes.
• Primary and secondary tanks each have a gauge; the low-air warning (visible and audible) comes on at 60 psi or higher (typically 68–74).
• Nylon tube with compression fittings (no flares); hose where there's heat, movement or abrasion; stainless braided hose for the compressor discharge (hot).

Supply system tests

• Build-up: 50 to 90 psi within 3 minutes at the specified engine speed. Slow build-up: worn compressor, leaking unloaders, clogged intake, leaks, dryer purge stuck open.
• Governor: check cut-out and cut-in; spread no more than 25 psi.
• Leak-down (supply, engine off, brakes released): no more than 2 psi × n in 2 minutes (n = axles/axle groups after the steer axle).
• Low air warning at or above 60 psi; park and trailer supply valves pop out in their ranges (see C-15).
• Oil in the tanks/dryer: worn compressor rings, restricted compressor intake, or excessive compressor oil supply. Some oil carry-over is normal.
• Belt deflection about ½–¾ in on belt-driven compressors.

Other pneumatic systems

• Air suspension (height control valves, pressure protection), air-operated transmission shift and range valves, air-applied clutches and fan clutches, air horns, wipers, seats.
• Air starting systems (vane or turbine motors) with their own tank and pressure protection: see D-19.
• Pneumatic actuators use the same valve rules as air brakes (inlet/exhaust, relay valves for long lines, quick releases for fast exhaust).

H-37 · Structural components

Frames

• Frames carry the weight of the machine and load, plus shock and torsional (twisting) loads. Most truck frames are a ladder design: two rails and crossmembers.
• C-channel rail: the top and bottom are flanges, the upright part is the web. Drill only through the web, at least about 1½ in from the flange (never through flanges, where the stress is).
• Gussets stop the frame going out of square (diamond). Alligator crossmembers.
• Yield strength = stress before it permanently bends. Section modulus = strength from the shape and size of the rail. Resisting bending moment (RBM) = yield strength × section modulus (overall strength). Maximum bending moment = where force is concentrated.
• Frame damage: sidesway (bent sideways), diamond (one rail pushed ahead; often from towing with one hook), twist, sag (bent down), bow (bent up), corrosion, cracks (fatigue).
• Repairs: don't scribe or nick the frame (use chalk or soapstone); hardened washers under bolts; Huck bolts don't allow flex. Heat-treated frames must be cold-worked (no heat, no welding unless the OEM allows). Dissimilar metals: use a barrier compound to stop corrosion.
• Always keep the frame square and the axles square to it.

Protective structures

• FOPS = falling object protective structure (an umbrella for rocks; on haul trucks may be built into the box).
• ROPS = roll over protective structure (usually part of the cab).
• Screens and guards protect from branches and logs; required on units with winches; may keep the operator's arms inside (skid steers).
• Only use OEM-approved fasteners. Avoid cutting or welding on structures. Contact the OEM with questions. Attach only in ways approved by an engineer.

Off-road structures

• Booms, sticks, loader arms, frames and blades take repeated loads. Cracks start at welds, holes and sharp corners (stress risers). Clean and inspect with dye penetrant or magnetic particle.
• Structural welding follows the OEM procedure (preheat, approved filler, grinding direction). On ROPS/FOPS the OEM must approve any repair; the certification label must stay legible.
• Pins and bushings: worn pins/bushings let joints knock and wear the bores out of round. Check with the bucket on the ground and a pry bar or by cycling the joint. Replace pins and bushings together; worn bores are line bored and bushed or built up and machined.
• Keep greasing on schedule; grease purging from both sides of the joint shows it's getting through.
• Articulation joints: pins, bearings and shims; lock the joint before working near it (see C-13).

H-38 · Operator station components

• The operator station must keep the operator safe (ROPS/FOPS cab, seat belt, guards, emergency exit, fire extinguisher), in control (steering, joysticks, pedals, interlocks) and informed (gauges, warning lamps, displays, cameras, alarms).
• Seat belts: inspect for cuts, fraying, UV damage and working buckles and retractors. Replace after any rollover or accident. The ROPS only protects a belted operator.
• Seats: mechanical or air suspension; check the height and damping adjustment, mounts and the seat switch. The seat switch (operator presence) can disable hydraulics or the drive if the operator gets up.
• Hydraulic lockout lever / control lock: blocks pilot oil to the controls when raised so the machine can't move when someone climbs in or out. Never bypass it.
• Controls: mechanical linkages, pilot joysticks (low-pressure pilot oil to the main valve), and electronic joysticks (potentiometers or Hall effect sensors read by an ECM). Electronic controls usually need calibration after replacement (set neutral and travel limits with the service tool).
• Gauges and displays: direct reading (Bourdon tube) or electrical (sensor + gauge or ECM display). Warning lamps go on at key on as a bulb check. Many machines show active codes on the display. See D-21 and D-22.
• Glass and wipers: cracked windows on a ROPS cab may weaken the structure's seal or the operator's protection (polycarbonate guards on some machines). Wiper motors, washers and blades are checked in the daily walk-around.
• Cab mounts isolate vibration and noise; a failed mount lets the cab move and adds noise (see F-33).
• Mirrors, cameras and object detection systems must be clean and aimed. Backup alarm, horn and lights must work before the machine is released.
• Doors and emergency exits: latches, hold-opens, and the emergency exit (often a rear window with a hammer or pull tab) must work.
• After any cab repair: reinstall all panels, seals and grommets, test every control and interlock, and check for codes.

H-39 · Attachments and accessories

Ground engaging tools and winches

• Cutting edges are cheap, replaceable wear parts that protect the softer blade or shank and help penetration.
• Dozers: edges and corner bits; not designed for backblading (unprotected bolts wear fast). Tilt up = carry; tilt down = breakout. C-frame mounts the blade to the track frame.
• Rippers: spikes at the rear, break hard ground (drawbar, shank, point). Most versatile ripper = adjustable parallelogram (moves straight up and down, angle adjustable).
• Graders: move material that is already loose. Blade = moldboard. Edges can be turned over to double their life. Serrated edges score ice. Scarifiers break up the top layer. Snow wing, V-plow.
• Loaders: edges, bits, teeth/shanks for hard digging. Excavators: teeth (wider for soft material), flat edges for clean-up; teeth fall off from poor install, wrong fit, worn adapter.
• PPE for edges; get help lifting; never use fingers to align bolt holes; block or stand the blade.
• Clean all rust, dirt and paint off the mounting surface or the bolts will loosen.
• Change edge or tooth type for different material; weld-on edges for hard ground. Tighten blade bolts from the centre outward. Minimum heat when removing weld-on parts.
• Hard surfacing (crosshatch weld) protects against abrasive soils.
• Winches: hydraulic, PTO or gear-driven; pipelayers use two (boom and pipe). Brake usually spring applied, hydraulically released. Drum-and-band in PTO winches; multi-disc clutch packs in hydraulic winches. Mount solidly (torque many times machine weight); align drives.
• Wire rope: gloves, stay clear under tension, underwound rope is most stable, use only rope recommended by the winch maker, don't kink. Ferrule ends (zinc or brass spike).

Fire suppression

• Actuators (manual, with a safety release; reachable by operator and ground crew). Sensors provide automatic operation: linear detection wire (two wires that melt together; protect it from welding heat), spot sensors.
• Dry chemical: monoammonium phosphate for A, B and C fires (A = ordinary combustibles, B = flammable liquids, C = electrical). Knocks the fire down. Poor underground (toxic dust).
• Liquid: AFFF foam for A and B fires; flows into places powder can't; prevents flare-up; used instead of powder underground.
• Expellant gas is often nitrogen, sometimes CO2.
• Automatic sequence: alarm, engine shutdown, hydraulic tank vents, system discharges.
• Inspect regularly (often every 6 months); weigh cylinders against spec. When working on it: safety pins in, system disabled.

Coupling devices and trailers

• Fifth wheel: top plate supports the trailer front, throat guides the kingpin in, jaws lock it. Stationary or sliding (moves fore and aft). Semi-oscillating (tilts front to rear; most common), fully oscillating (also side to side), compensating (for high centre-of-gravity loads), rigid (no tilt).
• Coupling: connect air lines and electrical before coupling the fifth wheel (or as specified), back under so the fifth wheel lifts the trailer, then tug test. Uncouple on level ground.
• Lubricate the top plate with water-resistant lithium grease; light oil on the jaws and cam track.
• Kingpin: no more than 1° from vertical; replace at ⅛ in (3 mm) wear; never weld a kingpin or the top plate. Check oscillating pins and bushings with a pry bar against the top plate. Lock testers check the jaws.
• Pintle hooks: replace at ⅛ in wear. No-slack pintles use an air chamber to take up slack. Fixed drawbars: pony trailers and C-dollies; hinged drawbars: full trailers and A-dollies. Safety chains or cables required.
• Landing gear: use low gear when lifting or lowering a trailer; high gear only when the legs are off the ground. Lubricate the gearbox with grease.
• Trailer combinations: A-train (converter dolly on a pintle, 3 articulation points), B-train (lead trailer has a fifth wheel, 2 points, most stable), C-train (dolly with 2 drawbars, 2 points). Lift axles reduce wear when empty. Tank trailers need their own inspection on top of CVIP.

Quick couplers and attachments (off-road)

• Quick couplers (hydraulic or mechanical) must be visibly locked: check the indicator, then do a test (curl and press the attachment against the ground) before lifting. Many have a secondary lock; never bypass it.
• Install attachments with the machine on level ground, everything lowered, pressure relieved; match the attachment's hydraulic flow and pressure to the machine (auxiliary circuit settings). Wrong settings overheat or damage the tool.
• Hydraulic hammers, augers, compactors and mulchers need the correct case drain or return-to-tank routing; some must not return through the main valve.
• Accessories (lights, cameras, heaters, beacons, payload systems, GPS grade control) are installed per the OEM: fused at the source, routed away from heat and pinch points, and with proper grounds.

I-40 · Hybrid equipment

What makes it a hybrid

• A hybrid combines an engine with an electric (or hydraulic) drive and usually some energy storage. A diesel-electric drive (engine → generator → electric traction motors) is the most common heavy equipment design (dozers, wheel loaders, mining trucks).
• Series hybrid: the engine only turns a generator; electric motors drive the machine. Parallel: engine and motor can both drive the wheels mechanically.
• Benefits: the engine can run at its most efficient speed, fewer mechanical drivetrain parts, smooth stepless drive, and regenerative / dynamic braking. Some store braking energy in batteries or ultracapacitors (or hydraulic accumulators on hydraulic hybrids) and reuse it.

Diesel-electric drive

• Needs specialized training. Usually AC motors (longer life, more efficient than DC; DC brushes can arc).
• Pros: long life, easy service, retards well downhill, fuel efficient, less oil.
• Power flow: engine → generator/alternator (AC) → main rectifier (AC to DC) → filtering capacitors → DC bus (stores DC) → inverters (DC back to AC) → traction motors.
• Generator: mechanical → electrical; stator = fixed field connected to a power supply, rotor turns.
• Dynamic braking: traction motors become generators (dynamic brake pedal); inverter phase module → DC → retarding grid ("giant toaster"); braking choppers switch the grid; grid blower cools it. Supplements the mechanical brakes.
• Self-load test holds the engine under load while stationary.
• DC bus capacitors are a serious hazard; normally discharged by firing the choppers; verify discharged before work (>50 V). Systems can be in kilovolts.
• PPE: face shield, glasses, high-voltage rubber gloves under leather protectors. Safe distance: 12 in with PPE, 2.5 ft without.

Hybrid safety and service

• Treat every orange cable and component as live until proven otherwise. High voltage DC is dangerous above about 50 V.
• Follow the OEM high-voltage isolation (lock-out) procedure: shut down, remove the key, open the service disconnect, lock and tag, wait the specified time for capacitors to discharge, then verify zero voltage with a properly rated meter.
• PPE: insulated (rubber) gloves of the right voltage class with leather protectors, inspected (air/roll test) before each use; face shield and glasses; no metal jewellery.
• Use a meter and leads rated for the job (CAT III or CAT IV, 1000 V). Test the meter on a known source before and after measuring (live-dead-live).
• Insulation faults are found with an insulation resistance tester (megohmmeter), not an ordinary ohmmeter. Many systems have an isolation (ground fault) monitor that sets a code if the high-voltage circuit leaks to the frame.
• Only trained and authorized people work on high-voltage parts.

I-41 · All-electric equipment

Battery electric equipment

• A battery electric vehicle (BEV) or machine has no engine: a high-voltage battery pack (usually lithium-ion) powers inverters, which drive AC traction and pump motors. A DC-DC converter charges the 12/24 V system that runs lights and controls.
• The battery management system (BMS) monitors cell voltage and temperature, balances cells, limits charge and discharge current, and opens the main contactors if something is wrong.
• High-voltage interlock loop (HVIL): a low-voltage circuit through every high-voltage connector; if a connector or cover is removed, the contactors open.
• Battery packs are liquid cooled or heated to stay in their temperature window. Cold reduces range and charging speed; heat shortens battery life.
• Regenerative braking: the motors act as generators when slowing, charging the battery. The friction brakes still exist for stopping and parking.
• Charging: AC chargers (on-board charger converts to DC) and DC fast chargers (straight to the pack). Inspect cables and connectors; never charge a damaged pack.
• Hydraulic functions on electric machines are driven by electric motors (often only running on demand), so hydraulic diagnostics still apply (G-34).

Service and safety

• Same high-voltage isolation as hybrids: service disconnect, lock-out, wait, verify zero voltage, insulated tools and gloves, rated meter.
• Lithium-ion hazards: damaged, overheated or water-soaked packs can go into thermal runaway (fire that can re-ignite hours or days later and gives off toxic gases). Quarantine damaged packs outdoors away from buildings, follow the OEM and fire department guidance, and use lots of water (not a small extinguisher) on a battery fire.
• Lift and support packs with the OEM fixtures (they are very heavy). Never puncture, crush or short a pack.
• Store and ship lithium batteries under TDG (transportation of dangerous goods) rules.
• Codes and data come from the BMS and inverters through the service tool; many faults are low-voltage (12/24 V) problems, so check the low-voltage battery first.