SKM Motor Vehicle Mechanic L2 cheat sheet

Hierarchy of control, most to least effective: elimination, substitution, engineering controls, administrative controls, PPE. PPE is the last line of defence only.

Carbon monoxide from petrol engines is odourless and binds haemoglobin roughly 200-250 times more readily than oxygen; never run an engine in an enclosed bay without extraction.

Petrol vapour is heavier than air and ignites between about 1.4% and 7.6% by volume in air; keep ignition sources and hot work away from fuel handling areas.

Battery charging releases hydrogen, which is flammable from about 4% to 75% by volume; charge in a ventilated area with no flames, sparks or smoking.

Brake and clutch dust may contain asbestos on older vehicles: use dust extraction or wet methods, never blow it out with compressed air, and use a suitable respirator.

Refrigerant R-1234yf is mildly flammable (A2L class) and its decomposition product hydrogen fluoride is toxic; recover with dedicated equipment and avoid open flame near leaks.

Safe jacking sequence: chock wheels, jack at the manufacturer's lifting points, then lower onto rated axle stands before working under the vehicle; a jack alone is never a support.

Fire classes: A solids, B flammable liquids, C gases, D metals, F cooking oils; electrical fires need a non-conductive agent such as CO2, and dry powder is the general-purpose workshop choice.

Torque = force x perpendicular distance; a 200 N force on a 0.5 m wrench gives 100 Nm. Extending a wrench with an adapter changes effective length and alters applied torque.

Micrometer least count = pitch of screw thread / number of divisions on thimble; a 0.5 mm pitch with 50 thimble divisions reads to 0.01 mm.

Vernier caliper reading = main scale value + (coinciding vernier division x least count); a 0.02 mm vernier with the 7th line coinciding adds 0.14 mm.

Dial bore gauge and telescopic gauge measure internal diameter; always zero the bore gauge against a micrometer set to the nominal bore before measuring.

Feeler gauges measure clearance by trial: the correct blade is the largest that enters with slight drag and the next size up does not enter.

Plastigauge measures bearing oil clearance: place a strand across the journal, torque the cap to spec, then compare the crushed width against the scale.

Multimeter functions: volts (parallel across the load), amps (in series, never across a supply), ohms (circuit dead and isolated). A clamp meter reads current without breaking the circuit.

Compression tester readings are valid only with the throttle wide open, all plugs removed, engine at operating temperature and a fully charged battery; compare cylinders within about 10%.

Four-stroke cycle order: intake, compression, power, expansion/exhaust; the crankshaft turns twice (720 degrees) per complete cycle in a four-cylinder four-stroke engine.

Compression ratio = (swept volume + clearance volume) / clearance volume; a 500 cc swept volume with 50 cc clearance gives 11:1.

Engine displacement = bore area x stroke x number of cylinders; a 4-cylinder engine of 80 mm bore and 90 mm stroke displaces about 1810 cc.

Valve clearance is set cold on most engines and checked at the specified mileage; too little clearance risks burnt valves, too much causes noise and reduced lift.

Timing belt replacement is mileage- or time-based, whichever comes first; a failed belt on an interference engine causes piston-to-valve contact and major damage.

Turbocharged engines need a cool-down idle period and clean oil; carbonised oil in the feed line starves the bearing and causes shaft seizure.

Oil specification follows the API/ACEA service category and the SAE viscosity grade in the owner's manual; wrong viscosity affects oil pressure and cold starting.

Coolant concentration is typically 50/50 with water; too little antifreeze risks corrosion and freezing, too much reduces heat transfer.

Ohm's law: V = I x R. A 12 V supply across a 6 ohm load draws 2 A. Power P = V x I, so that load dissipates 24 W.

Series circuit: current is the same everywhere, voltages add. Parallel circuit: voltage is the same, currents add and total resistance falls below the smallest branch.

Voltage drop testing is the reliable way to find high-resistance faults: measure across the circuit under load; more than about 0.5 V on a main feed indicates a problem.

A fully charged 12 V lead-acid battery rests at about 12.6-12.8 V; 12.2 V is roughly 50% state of charge. Charging voltage from the alternator is typically 13.8-14.4 V.

Battery capacity is rated in ampere-hours (Ah) and cold cranking amps (CCA); CCA is the current a battery delivers at -18 C for 30 seconds while staying above 7.2 V.

Alternator output is three-phase AC rectified by a diode pack; a failed diode causes ripple, dim lights and a discharged battery despite a normal charging voltage reading.

CAN bus is a twisted-pair differential network terminated by two 120 ohm resistors in parallel, giving about 60 ohms measured across the bus with power off.

LED and HID lighting and electronic modules are polarity and voltage sensitive; disconnect the battery before welding and avoid spike-inducing tests on networked vehicles.

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

Clutch slip is diagnosed by engine speed rising without road speed under load; causes include worn friction plate, weak diaphragm spring and oil contamination.

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

CV joint click on full-lock turns points to an outer joint; vibration under acceleration points to an inner joint or a worn driveshaft.

Wheel alignment angles: camber is vertical tilt, caster is steering-axis tilt affecting self-centring, toe is the difference in track front to rear.

Tyre wear patterns: centre wear from over-inflation, both edges from under-inflation, one edge from camber error, feathered edges from toe error.

MacPherson strut combines spring, damper and location in one unit; worn top mounts cause clunking and steering memory.

Power steering fluid loss and whining usually indicate a pump or rack seal fault; electric power steering faults set codes in the steering control module.

Hydraulic pressure = force / area; a larger master cylinder piston reduces line pressure for the same pedal force but shortens pedal travel.

Brake fluid is hygroscopic; DOT 4 absorbs moisture and should be replaced at the manufacturer's interval, typically every 2 years, to avoid boiling and fade.

Minimum brake pad friction material thickness is set by the manufacturer; a common workshop limit is about 2 mm, with discs replaced at the wear limit stamped on the hub.

ABS prevents wheel lock by modulating pressure; a warning lamp means ABS is disabled but base braking remains, and the fault must be diagnosed before road use.

Parking brake adjustment and cable condition are part of inspection; electronic parking brakes require a diagnostic service mode for pad replacement.

Brake hose inspection looks for cracking, bulging and chafing; a bulging hose under pressure indicates internal failure and immediate replacement.

Brake test on a rolling road or road test should show balanced braking within the manufacturer's tolerance; pull to one side indicates a seized caliper or contaminated pad.

Airbag and pretensioner circuits are pyrotechnic: isolate the battery and wait the specified time before working, and never probe with a multimeter.

Diagnostic sequence: verify the complaint, gather information, inspect, test, isolate the cause, repair, then verify the repair and clear codes.

DTCs are symptoms, not causes; a P0171 lean code can come from a vacuum leak, low fuel pressure or a faulty sensor, so confirm with live data before replacing parts.

Freeze-frame data records the conditions when a fault set: engine speed, load, temperature and speed help reproduce intermittent faults.

Fuel trim: positive long-term trim above about +10% indicates a lean condition; negative trim indicates rich running.

Oscilloscope testing of sensors shows signal shape; a good crankshaft sensor produces a clean square wave or sine wave depending on type, with no dropouts.

Voltage drop and resistance tests must be done with the circuit loaded; an ohmmeter alone can miss a high-resistance connection that only fails under current.

Inspection order for a used vehicle: identity and documents, exterior and structure, underbonnet, underside, interior, then a road test and function check.

Record before-and-after measurements and keep replaced parts available until the customer has accepted the repair.

Service intervals follow the manufacturer's schedule for time and distance, whichever comes first; severe-use schedules shorten oil and filter intervals.

Oil change procedure: warm the engine, drain fully, replace the drain plug washer, fit a new filter, refill to spec, run and check for leaks, then reset the service indicator.

Tyre pressures are checked cold; a tyre at operating temperature reads higher and should not be bled down to the cold figure.

Tread depth legal minimum is 1.6 mm across the central three-quarters of the tread; replace earlier for wet grip.

Cooling system service includes checking concentration, pressure testing for leaks and confirming the fan and thermostat operate at the specified temperatures.

Brake, steering, suspension and driveline checks are part of every full service, with findings recorded on the job card.

Battery maintenance includes terminal cleaning, charge state check and confirming the charging system output under load.

Post-service road test confirms the repair and checks for new noises, warning lamps or fluid leaks before handover.

A job card must record vehicle details, VIN, odometer, customer complaint, work authorised, parts used, labour time and technician signature.

An estimate is a price indication; work beyond the authorised scope requires fresh customer approval before proceeding.

Waste streams are segregated: used oil, filters, coolant, brake fluid, solvents, batteries and tyres each go to separate licensed disposal routes.

Used oil must never be mixed with solvents, coolant or brake fluid; contamination makes recycling impossible and may be illegal.

Refrigerant must be recovered and recycled by certified equipment; venting to atmosphere is prohibited under ozone and fluorinated gas regulations.

Spill response: contain with absorbent, prevent entry to drains, dispose of contaminated absorbent as hazardous waste, and record the incident.

Data protection applies to customer details and vehicle data; share only what is needed for the repair and keep records secure.

Explain findings in plain language, show the worn part where possible, and record the customer's decision on the job card.

Torque-to-yield head bolts are tightened to a torque then turned a specified angle; they must be replaced, not reused, because they stretch permanently.

Cylinder head gasket failure signs: coolant loss without external leak, white exhaust smoke, bubbles in the expansion tank and oil/coolant mixing.

Valve timing must be set with the engine at top dead centre on the compression stroke, confirmed by timing marks on the crank and cam.

Fuel injection pressure is far higher on common-rail diesel (often 1000+ bar) than port injection; never crack a high-pressure line while the engine runs.

Diesel engines rely on compression ignition, typically 16:1 to 22:1; petrol engines use spark ignition, typically 9:1 to 12:1.

Air filter restriction reduces power and increases fuel consumption; replace at the specified interval or when restriction is measured.

Exhaust gas recirculation reduces NOx by lowering combustion temperature; a stuck-open EGR valve causes rough idle and black smoke.

Bleeding order follows the manufacturer's sequence, usually furthest from the master cylinder first; keep the reservoir topped up to avoid air ingress.

Brake pedal feel: a spongy pedal indicates air in the system; a long pedal indicates rear brake adjustment or a leaking master cylinder.

Disc thickness variation causes pulsation through the pedal; measure with a micrometer at several points around the disc.

Wheel speed sensors are the primary ABS input; a damaged tone ring or air gap fault sets a wheel speed code.

Seat belts, airbags, horns and lights are safety inspection items; a failed safety system may make the vehicle unroadworthy.

Brake line corrosion is a common failure in salt-sprayed regions; replace steel lines rather than patching where corrosion is significant.

Differential allows wheels to turn at different speeds in a turn; a locked or noisy differential may indicate incorrect oil or worn gears.

Driveshaft universal joints cause vibration that worsens with speed; check for play and grease condition.

Steering free play is measured at the steering wheel rim with the wheels straight ahead; excessive play indicates worn tie rods or a worn rack.

Power steering system bleeding after repair requires turning the wheel lock to lock with the engine running and the reservoir topped up.

Suspension bushes wear causes vague steering and uneven tyre wear; check with a pry bar under load.

Wheel bearings are checked by spinning the wheel and rocking it; a growling noise that changes with cornering load indicates a bearing fault.

Starter circuit: battery, ignition switch, relay, solenoid and starter motor; a click with no crank points to a solenoid or earth fault.

Earth faults cause multiple unrelated symptoms; check engine-to-body and battery-to-body straps for corrosion and tightness.

Fuses protect circuits by current rating; never fit a higher-rated fuse than specified.

Relays switch high current with a low-current control; test by confirming coil supply and continuity of the contact circuit.

Sensor reference voltage is typically 5 V; a shorted signal wire pulls the reading to 0 V or 5 V and sets a code.

Immobiliser and key programming require manufacturer procedures and security access; do not attempt without the correct tool and authorisation.

Noise diagnosis: identify the condition (cold, hot, speed-related, load-related, cornering) before inspecting components.

Intermittent faults are best caught with a data logger or scope recording while the vehicle is driven under the conditions that trigger the fault.

Smoke testing the intake finds vacuum leaks that a visual inspection misses.

Fuel pressure and volume tests confirm pump and filter condition; a pump can deliver pressure but insufficient volume.

Compression and leak-down tests distinguish ring, valve and head gasket faults; leak-down shows where air escapes (intake, exhaust, crankcase, coolant).

Always confirm the repair with a road test and a scan tool check that monitors have run and no codes return.

Pre-delivery and periodic inspection checklists cover lights, fluids, tyres, brakes, belts, hoses and warning lamps.

Belt condition: cracks, glazing and fraying mean replacement; check tension and alignment of pulleys.

Hose inspection: squeeze to check for hardening, cracking or swelling; replace hoses showing any of these.

Battery terminal service: clean corrosion, apply protective coating, check hold-down and confirm charge.

Wiper and washer service: check blades, fluid level and jet aim as part of the visibility check.

Service indicator reset is part of the service and must be recorded on the job card.

Parts returns and warranty claims need the original invoice, part number and fault description.

Vehicle history records help trace recurring faults and support a fair diagnosis.

Customer handover should include what was done, what was found, what may need attention next and the warranty terms.

Environmental duty of care includes correct storage of chemicals in labelled, bunded containers and keeping safety data sheets accessible.

Energy and water use in the workshop are part of environmental management; switch off equipment and use parts washers with appropriate solvents.

Complaints should be logged, investigated and answered in writing, with corrective action recorded.

Risk assessment: identify the hazard, decide who may be harmed and how, evaluate the risk, record the findings and review.

Noise exposure above about 85 dB(A) over an 8-hour day requires hearing protection and hearing checks.

Manual handling injuries are the most common workshop injury; use lifting aids and team lifts for heavy components.

Hand-arm vibration from impact tools is limited by tool choice and exposure time; use low-vibration tools and rotate tasks.

Electrical safety: use RCD-protected outlets, inspect leads for damage and never use equipment with a damaged plug or cable.

First aid and emergency contacts must be posted, and spill kits and fire extinguishers must be accessible and in date.

Torque wrenches are calibrated periodically; store them at the lowest setting to protect the spring.

Impact sockets are hardened for use with impact tools; chrome sockets can shatter and must not be used on impact wrenches.

Two-post lifts must be rated for the vehicle, with arms positioned at the manufacturer's lift points and the vehicle balanced.

Tyre pressure gauges and inflation equipment should be checked against a known reference.

Diagnostic scan tools need current software and a stable power supply during programming to avoid module corruption.

Measuring tools should be checked against a standard and handled carefully to preserve accuracy.

Engine oil pressure is checked with a mechanical gauge; low pressure can come from a worn pump, blocked pickup or excessive bearing clearance.

Cooling system pressure test: pressurise to the cap rating and watch for a pressure drop or external leak.

Thermostat testing: heat in water and check opening temperature against specification.

Timing chain wear shows as rattle on start-up; check tensioner and guides before replacement.

Turbo wastegate and variable geometry faults cause low boost and overboost codes; check actuator movement and vacuum supply.

Engine mounts wear causes vibration and clunking; check for separation and fluid leaks on hydraulic mounts.

Lighting circuits: check bulb, fuse, relay, switch and earth in that order before condemning a component.

Charging system test: measure battery voltage at idle and at about 2000 rpm with loads on; output should stay within specification.

Parasitic drain test: measure current with everything off and modules asleep; a reading above the manufacturer's limit indicates a fault.

Wiring repair should use correct gauge, sealed connectors and heat-shrink to prevent corrosion and future faults.

Network faults: check termination resistance, then look for chafed wiring and water ingress at connectors.

Battery registration on vehicles with smart charging is required after replacement to keep charging correct.

Clutch hydraulic system bleeding is required after replacement; a spongy pedal indicates air.

Gearbox oil level and condition are checked at service; metallic debris on the drain plug magnet indicates wear.

CV boot damage allows grease loss and dirt ingress; replace the boot before the joint fails.

Wheel alignment should be checked after suspension or steering repair and after kerb impacts.

Tyre rotation evens wear; follow the manufacturer's pattern and check pressures after.

Steering column and universal joint play causes vague steering; check for movement with the wheels straight.

Brake pad replacement requires cleaning and lubricating the caliper slides and checking piston retraction.

Brake disc runout is measured with a dial gauge; excessive runout causes pulsation and uneven pad wear.

Handbrake cable adjustment should allow the specified number of clicks and hold the vehicle on a slope.

Brake fluid level drop without a leak can indicate worn pads as the caliper pistons extend.

ABS warning lamp diagnosis starts with a scan tool code read and a check of wheel speed sensor signals.

Road test braking should be straight and progressive; any pull or noise must be investigated before handover.

Customer complaint wording guides the diagnosis; ask when, where and how often the fault occurs.

Visual inspection before testing saves time: look for leaks, damage, loose connectors and recent repairs.

Scan tool live data confirms sensor behaviour under load; compare with expected values from the manufacturer.

Wiggle tests on wiring harnesses can reveal intermittent faults while monitoring live data.

Record the diagnosis path and the evidence for the repair on the job card.

Confirm the repair with the same conditions that produced the original complaint.

Fluid checks: engine oil, coolant, brake fluid, power steering, transmission and washer fluid, each against the correct level mark.

Tyre inspection includes tread depth, pressure, sidewall condition and uneven wear patterns.

Brake inspection includes pad thickness, disc condition, hose condition and fluid level.

Lighting check covers all exterior and interior lamps, including indicators and brake lights.

Underbonnet check covers belts, hoses, battery, fluid leaks and air filter condition.

Record all findings and advise the customer of any items needing attention before the next service.

Estimates and invoices must itemise parts, labour and VAT clearly.

Warranty terms should be explained at handover and recorded on the invoice.

Customer complaints about workmanship should be handled promptly with an inspection and a written response.

Hazardous waste records must be kept for the required retention period.

Safety data sheets for all chemicals must be available in the workshop.

Environmental incidents such as spills must be reported and recorded.

Personal protective equipment for workshop tasks includes safety glasses, gloves, safety footwear and hearing protection where required.

Loose clothing and jewellery must be removed before working near rotating parts.

Compressed air must never be used to clean skin or clothing; it can force air into the body.

Vehicle lifting equipment must be inspected and rated; never work under an unsupported vehicle.

Fire extinguishers must match the fire class and be checked regularly.

Emergency stops and isolation points must be known and accessible.

Torque specification must be applied with the correct thread condition (dry or lubricated) as stated by the manufacturer.

Angle torque gauges are used for torque-to-yield fasteners.

Bearing pullers and presses must be used with correct adapters to avoid damaging components.

Diagnostic pressure gauges must be rated for the system pressure being tested.

Micrometers should be zeroed before use and checked against a standard.

Vernier and digital calipers should be checked for zero error before measuring.

Air-fuel ratio for petrol stoichiometric combustion is about 14.7:1 by mass.

Diesel combustion is lean overall; excess fuel causes black smoke, excess air lowers power.

Ignition timing too advanced causes pinking; too retarded causes overheating and poor power.

Spark plug condition indicates engine condition: oil fouling, carbon fouling and electrode wear each point to different causes.

Fuel filter replacement interval depends on fuel quality and manufacturer schedule.

Engine oil consumption above the manufacturer's limit indicates wear or a PCV fault.

Battery testing should include state of charge, cranking test and charging test.

Jump starting must follow the correct sequence to avoid sparks near the battery and protect electronics.

Alternator belt condition and tension affect charging; a slipping belt causes low output and noise.

Fuses and relays should be tested with a meter or substitution, not by guesswork.

Module programming requires a stable power supply and correct software version.

Electrical faults should be traced with a wiring diagram and systematic testing.

Transmission fluid condition indicates wear; burnt smell and dark colour suggest overheating.

Driveline vibration can come from worn joints, unbalanced shafts or incorrect pinion angle.

Steering rack boots protect seals; split boots allow dirt in and cause leaks.

Suspension springs can sag or break; check ride height and spring condition.

Shock absorbers are checked by bounce test and visual inspection for leaks.

Wheel alignment should be checked after any suspension or steering work.

Brake system repairs must use correct fluid and clean components; contamination causes seal failure.

Brake pedal free play should be within specification; excessive free play indicates wear or air.

Brake lights must operate correctly; a faulty switch can affect cruise control and starting.

Parking brake performance is part of the safety inspection.

ABS sensors and tone rings must be clean and undamaged for correct operation.

Brake repairs must be road tested before handover.

Diagnosis should follow a logical path: symptom, system, component, test, conclusion.

Use the manufacturer's diagnostic flow charts where available.

Confirm the fault before replacing parts; parts swapping is not diagnosis.

Keep a record of tests and results for future reference.

Intermittent faults may require the vehicle to be driven under the conditions that trigger them.

Verify the repair and clear codes only after the fault is fixed.