Engineer Motor Vehicles Maintenance cheat sheet

Preventive maintenance intervals are set by the shorter of distance, time, or engine-hour limits; whichever limit is reached first triggers the service.

Wear-rate rule: component life is estimated from measured wear divided by operating hours; when wear approaches the manufacturer's rejection limit, schedule replacement before the next service interval.

Torque wrench calibration should be verified at least every 6 months or 5,000 cycles; error must stay within ±4% of indicated value.

Fastener reuse rule: torque-to-yield bolts are single-use; standard bolts may be reused if elongation is under 0.2% and threads are undamaged.

Lubricant selection follows the OEM viscosity grade and API/ACEA service category; mixing incompatible base oils can cause seal swell or additive dropout.

Workshop layout separates dirty (strip, wash, weld) from clean (assembly, measurement) zones to control contamination and measured tolerance drift.

A job card must record vehicle ID, odometer, complaint, work done, parts used, technician, and time; it is the primary record for warranty and costing.

Run-in after overhaul: limit load to about 50% for the first 500-1,000 km, then carry out an oil and filter change.

Four-stroke cycle order: intake, compression, power, exhaust; two crankshaft revolutions per cycle. Two-stroke engines fire once per revolution and rely on crankcase or blower scavenging.

Compression ratio CR = (swept volume + clearance volume) / clearance volume; a typical petrol engine is 9:1 to 12:1, a diesel 15:1 to 22:1.

Volumetric efficiency = actual air mass inducted / theoretical air mass at ambient density; it falls with intake restriction, high speed, and hot charge.

Air-fuel ratio stoichiometry: petrol about 14.7:1 by mass; diesel runs lean overall (18:1 to 70:1) and is load-controlled by injected fuel mass.

Diesel knock is caused by ignition delay; long delay raises the pressure rise rate. Cetane number, injection timing, and intake temperature control delay.

Blue smoke indicates oil burning (worn rings, valve guides, turbo seals); black smoke indicates over-fuelling or insufficient air; white smoke indicates unburnt fuel or coolant ingress.

Cylinder leakage test: pressurise the cylinder at TDC on compression and read leakage; bubbles in the radiator indicate a head gasket or crack, air at the dipstick indicates ring or bore wear.

Valve clearance is set cold or hot per OEM data; too little clearance causes burnt valves and rough idle, too much causes noise and reduced lift.

Manual gearbox ratio = driven gear teeth / driving gear teeth; overall ratio = gearbox ratio × final drive ratio.

Road speed (km/h) = (engine rpm × 60 × rolling circumference in m) / (1000 × overall ratio).

Clutch slip occurs when torque capacity is below engine torque; free play must be set so the release bearing is unloaded when the pedal is released.

Automatic transmission fluid level is checked at operating temperature with the engine running and the selector in the specified position; incorrect level causes slip or foaming.

CV joint click on full lock indicates worn outer joint; vibration under acceleration indicates inner joint or driveshaft imbalance.

Differential ratio = ring gear teeth / pinion teeth; hypoid gear oil must meet the OEM specification and limited-slip units may need friction modifier.

Belt tension rule: deflection at the midpoint should be about 10-15 mm for a V-belt; excessive tension damages bearings, low tension causes slip and glazing.

Wheel bearing play is checked by rocking the wheel at 12 and 6 o'clock; play plus rumble that changes with load indicates bearing wear.

Ackermann geometry: inner wheel turns more than the outer wheel so all wheels roll about a common centre; incorrect geometry causes tyre scrub.

Caster, camber, toe, and kingpin inclination (KPI) are the main alignment angles; toe is set last after camber and caster.

Brake force distribution: front brakes do most of the work because of load transfer; proportioning valves or EBD limit rear pressure to prevent lock-up.

Hydraulic brake pedal feel: a spongy pedal indicates air in the system; a pedal that sinks under steady pressure indicates a leaking master cylinder or line.

Brake fluid is hygroscopic; moisture lowers the boiling point and causes vapour lock. Test with a moisture meter and replace at the OEM interval or above 3% water.

Disc runout limit is typically 0.05 mm or less; excessive runout causes pedal pulsation and thickness variation.

Suspension damping: a worn shock absorber causes excessive bounce, tyre cupping, and longer stopping distance; test by bounce test and visual leak check.

ABS operates by modulating brake pressure at each wheel; a wheel speed sensor fault disables ABS and usually illuminates the warning lamp.

Ohm's law: V = I × R; power P = V × I = I²R. A 12 V circuit drawing 5 A has 2.4 Ω resistance and dissipates 60 W.

Battery capacity is rated in ampere-hours (Ah) and cold cranking amps (CCA); a load test should not drop below about 9.6 V at 10.5 V nominal during cranking.

Charging system: alternator output should be 13.8-14.8 V at idle with loads off; low output indicates a worn brush, diode, or regulator fault.

Voltage drop test: measure across a cable or connection under load; more than 0.5 V drop on a main circuit indicates excessive resistance.

CAN bus has two wires, CAN-H and CAN-L, with about 2.5 V each at rest; a short to ground or between lines disables communication.

Sensor types: potentiometer (throttle position), Hall effect (crank/cam), inductive (ABS wheel speed), and thermistor (coolant temperature).

LED and HID lighting require correct ballast or driver; HID lamps need high voltage at start and must not be handled with bare skin.

Immobiliser and key programming require the correct PIN and diagnostic tool; loss of key data can prevent engine start.

Diagnostic sequence: verify the complaint, gather data, form a hypothesis, test, repair, and verify the repair; never replace parts without a confirmed cause.

OBD-II codes: P = powertrain, B = body, C = chassis, U = network; a code identifies a circuit or system, not necessarily the failed component.

Four-gas exhaust analysis: CO, CO₂, HC, and O₂; high CO and HC with low O₂ indicate rich mixture; high O₂ with low CO₂ indicates a lean misfire or exhaust leak.

Brake test on a roller brake tester: compare left and right brake force; imbalance above about 30% or low overall efficiency fails the inspection.

Headlamp aim is checked against a beam setter; aim must be within the manufacturer's tolerance to avoid dazzle and to meet the test standard.

Side slip test measures wheel alignment while rolling; excessive side slip indicates toe or camber error and causes tyre wear.

Diesel smoke opacity is measured in m⁻¹ or percentage; excessive opacity indicates over-fuelling, clogged air filter, or injector fault.

Freeze frame data captured at the moment a fault is stored helps reproduce the condition; compare live data against OEM specifications.

Labour charge = labour time × hourly rate; parts charge = cost + markup; total estimate = labour + parts + consumables + tax.

Standard time comes from the OEM flat-rate manual; actual time variance = actual time − standard time, used to measure technician efficiency.

Break-even point in hours = fixed costs / (hourly rate − variable cost per hour).

Stock control: reorder level = maximum usage × maximum lead time; safety stock covers demand variability.

Job costing records direct labour, direct materials, and overhead absorption; overhead is absorbed per labour hour or per job.

Invoice must show parts, labour, VAT/tax, and warranty terms; retain records for the statutory period for tax and warranty claims.

Depreciation of workshop equipment: straight-line = (cost − residual value) / useful life.

Productivity = hours sold / hours attended; efficiency = standard hours produced / hours worked.

COSHH-type rule: assess every hazardous substance, use the least hazardous option, and keep the safety data sheet accessible.

Ventilation: exhaust gas must be extracted at the tailpipe; CO exposure limit is typically 30 ppm as an 8-hour time-weighted average.

Battery safety: charge in a ventilated area, wear eye protection, and neutralise acid spills with sodium bicarbonate; hydrogen is explosive above about 4% in air.

Fuel and solvent storage requires bunded containers and fire-resistant cabinets; keep ignition sources away and use earthing when decanting.

Lifting rule: never work under a vehicle supported only by a jack; use axle stands or a lift rated above the vehicle weight.

Waste segregation: used oil, coolant, brake fluid, and solvents are separate streams; never pour them into drains.

PPE selection follows the task: safety glasses for grinding, gloves for solvents, hearing protection above 85 dB(A), and respiratory protection for paint.

Airbag and SRS handling: disconnect the battery and wait the specified time before working near airbags; store modules face up.

Steel carbon content: low carbon up to 0.3%, medium 0.3-0.6%, high carbon 0.6-1.4%; hardness and strength rise with carbon but ductility falls.

Cast iron has good damping and wear resistance; it is used for cylinder blocks and brake discs but is brittle in tension.

Aluminium alloys give a high strength-to-weight ratio and are used for cylinder heads and pistons; they require correct torque and thermal expansion allowance.

Heat treatment: annealing softens, normalising refines grain, hardening followed by tempering trades hardness for toughness.

Non-ferrous bearing materials: white metal (babbitt) for plain bearings, bronze for bushes; they embed dirt and resist seizure.

Composite and plastic components (bumpers, intake manifolds) are joined by adhesives or clips, not by welding; repair methods differ from metal.

Corrosion protection: galvanic corrosion occurs when dissimilar metals touch in the presence of an electrolyte; isolate with coatings or insulating washers.

Material identification: spark test for steel, magnet test for ferrous versus austenitic stainless, and hardness test for heat-treated parts.

Micrometer reading = main scale + thimble scale × least count; a 0-25 mm micrometer with 0.01 mm thimble reads to 0.01 mm.

Vernier caliper least count = main scale division / number of vernier divisions; typical 0.02 mm.

Cylinder bore wear is measured with a bore gauge at three heights and two directions; taper and ovality must be within OEM limits.

Torque = force × perpendicular distance; a 200 N force at 0.3 m gives 60 N·m.

Pressure = force / area; 500 N on 0.01 m² gives 50 kPa.

Thermal expansion: ΔL = α × L × ΔT; a 0.5 m aluminium part (α = 23 × 10⁻⁶/K) heated 100 K expands about 1.15 mm.

Electrical measurement: a multimeter in series measures current, in parallel measures voltage; never measure resistance on a live circuit.

Tolerance stack: total variation is the sum of individual tolerances; a clearance fit requires the minimum hole above the maximum shaft.