RBSE Sec Voc Automotive cheat sheet

Personal protective equipment for workshop tasks: safety goggles when grinding or using compressed air, face shield when chiselling or using a bench grinder, nitrile gloves for fuels, solvents and used oil, steel-toe boots for heavy components, and ear protection above 85 dB(A).

Fire class and extinguisher matching: Class A (paper, wood) water or foam; Class B (petrol, diesel, solvent) foam, CO2 or dry powder, never water; Class C (energised electrical) CO2 or dry powder; Class D (magnesium, sodium) special dry powder only.

Lifting rule: keep the load close to the body, back straight, lift with the legs, and never exceed the individual manual handling limit; use a crane, trolley or transmission jack above that limit.

Jack stands must be rated for the load and placed under the chassis or axle at manufacturer-recommended points; a hydraulic jack alone is never a support. Chock the wheels diagonally opposite the raised corner.

Torque formula: T = F x L, where T is torque in N·m, F is force in newtons and L is lever length in metres. A 0.5 m bar with 200 N applied gives 100 N·m.

Micrometer least count = pitch of screw / number of divisions on the thimble. A 0.5 mm pitch with a 50-division thimble gives 0.01 mm; add the barrel reading to the thimble reading.

Vernier calliper least count = smallest main-scale division / number of vernier divisions. A 1 mm main division with 20 vernier divisions gives 0.05 mm.

Compressed-air safety: never point an air blowgun at skin or use it to clean clothing; limit nozzle pressure for cleaning to about 2 bar (30 psi) and fit a chip guard.

Battery and fuel hazards: disconnect the negative (earth) terminal first and reconnect it last; keep sparks, flames and mobile phones away from charging lead-acid batteries because they release explosive hydrogen.

Four-stroke cycle order: intake, compression, power (expansion), exhaust, two crankshaft revolutions per cycle in a four-stroke engine; a two-stroke engine completes the cycle in one revolution.

Compression ratio CR = (swept volume + clearance volume) / clearance volume = (Vd + Vc) / Vc. A 400 cc swept volume with 50 cc clearance gives CR = 9:1.

Engine displacement = number of cylinders x (pi/4) x bore squared x stroke. For four cylinders of 80 mm bore and 90 mm stroke, displacement is about 1,810 cc.

Valve clearance is set with the engine cold on most designs and checked at the specified feeler gauge thickness; too little clearance causes burnt valves and compression loss, too much causes noise and reduced lift.

Firing order determines valve-setting sequence: with cylinders at TDC on compression, both valves of that cylinder can be adjusted; rotate the crank in the firing order and repeat.

Torque-to-yield head bolts are tightened to a torque plus a specified angle and must be replaced, not reused, because they have already stretched plastically.

Cylinder honing creates a cross-hatch of about 45 degrees to retain oil; piston-to-bore clearance and ring end gap are measured with a feeler gauge before assembly.

Blue-grey exhaust smoke indicates burning oil (worn rings or valve guides); black smoke indicates rich mixture; white persistent smoke indicates coolant entering the combustion chamber.

Oil pressure that drops at idle but rises with engine speed suggests worn bearings or a weak relief valve; a pump pickup blocked by sludge lowers pressure at all speeds.

Manual gearbox ratio = driven gear teeth / driving gear teeth. Overall ratio = gearbox ratio x final drive ratio; lower numerical ratio means higher road speed for a given engine speed.

Road speed formula: speed (km/h) = (engine rpm x 60 x tyre circumference in km) / (overall gear ratio x 1000).

Clutch pedal free play that is too small causes slippage and release-bearing wear; too much free play causes incomplete disengagement and gear clash.

Clutch slip diagnosis: engine speed rises without matching road speed, especially under load or on gradients; check for oil contamination, worn friction lining or a weak diaphragm spring.

Torque converter multiplication occurs at high stall speed and falls to 1:1 at coupling point; the stator one-way clutch freewheels at coupling so the converter behaves like a fluid coupling.

CV joint clicking on full lock during acceleration points to a worn outer joint; vibration under acceleration with a steady-speed hum points to an inner joint or intermediate shaft.

Universal joint phasing: yokes on a propeller shaft must be in phase, and working angles at each end should be equal and opposite to cancel vibration.

Differential ratio = ring gear teeth / pinion teeth. A 41-tooth ring with an 11-tooth pinion gives 3.73:1.

Hypoid differentials require limited-slip or hypoid-grade gear oil; using the wrong additive causes chatter on turns in limited-slip units.

Hydraulic pressure P = F / A. A 500 N pedal force acting through a 5:1 pedal ratio gives 2,500 N at the master cylinder; dividing by piston area gives line pressure.

Brake fluid is hygroscopic; DOT 4 absorbs moisture and its boiling point falls, so it is replaced on the manufacturer interval, typically every 1-2 years.

Disc runout and thickness are measured with a dial gauge and micrometer; below minimum thickness the disc must be replaced, not resurfaced.

Parking brake travel, cable free play and shoe adjustment are checked after any rear brake work; a dragging brake shows as heat and pull after a road test.

Wheel alignment angles: camber is the vertical tilt of the wheel, toe is the difference in distance between front and rear of the wheels, caster is the steering-axis tilt.

Tyre wear patterns: wear on both shoulders indicates under-inflation; wear in the centre indicates over-inflation; one-sided wear indicates camber or toe error.

Steering free play is measured at the steering wheel rim with the wheels straight ahead; excessive play indicates worn tie-rod ends, steering box or rack bushes.

Shock absorber test: bounce the corner and release; more than one or two rebounds indicates a worn damper. Leaking fluid on the damper body confirms failure.

Wheel bearing play is checked by rocking the wheel at 12 and 6 o'clock; play there with the brake applied suggests a bearing, while play at 3 and 9 o'clock suggests steering linkage.

Ohm's law: V = I x R. Power: P = V x I = I squared x R = V squared / R. A 12 V circuit drawing 5 A dissipates 60 W.

Series circuit: total resistance is the sum and current is the same everywhere. Parallel circuit: 1/Rt = 1/R1 + 1/R2, and voltage is the same across each branch.

Battery capacity in ampere-hours indicates the current it can deliver for a stated time; cold cranking amps (CCA) indicate starting capability at low temperature.

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

Alternator output is checked at about 2,000 rpm with loads on; a healthy charging system holds roughly 13.8-14.5 V at the battery.

Starter circuit faults: a click with no crank suggests a low battery, bad earth or faulty solenoid contacts; slow crank suggests high resistance or a failing starter.

CAN bus is a twisted-pair differential bus; measure about 60 ohms across the two data lines with the bus powered down, which is two 120-ohm terminators in parallel.

LED and halogen lamps differ in current draw; fitting LEDs to a circuit monitored by the body control module can trigger bulb-failure warnings unless the module is configured.

Relay terminals: 85 and 86 are the coil, 30 is the supply, 87 is the normally open output and 87a is the normally closed output.

Stoichiometric air-fuel ratio for petrol is about 14.7:1 by mass; lambda = actual AFR / stoichiometric AFR, so lambda 1.0 is chemically correct.

Diesel combustion is lean overall, with excess-air ratio typically above 1.3 at full load; smoke limit is set by the air available, not by fuel alone.

Cooling system pressure raises the boiling point: a 15 psi cap raises the boiling point of water by roughly 25 degrees C compared with an open system.

Thermostat test: suspend it in water and heat; it should begin to open at its rated temperature (for example 82 degrees C) and be fully open about 10-15 degrees higher.

Coolant mixture is typically 50:50 antifreeze and water; too much antifreeze reduces heat transfer, too little reduces freeze and corrosion protection.

Three-way catalytic converter controls CO, HC and NOx only when the mixture oscillates around stoichiometric; a lazy oxygen sensor raises emissions and sets a fuel-trim fault.

EGR reduces combustion temperature and therefore NOx; a stuck-open EGR valve causes rough idle and stalling, a stuck-closed valve causes knock and high NOx.

PCV system vents crankcase blow-by into the intake; a blocked PCV valve raises crankcase pressure and causes oil seal leaks.

Fuel pressure diagnosis compares measured pressure with specification at key-on and at idle with the vacuum line connected and disconnected; a failing regulator shows wrong pressure at both.

Diagnostic sequence: verify the complaint, gather information, inspect visually, test with instruments, isolate the fault, repair, then verify the repair and clear or recheck codes.

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

Freeze-frame data records operating conditions when a fault was stored; comparing freeze frame with live data separates intermittent from continuous faults.

Tyre pressure is checked cold; pressure rises about 0.1 bar (1-2 psi) for every 10 degrees C of temperature increase, so hot readings must not be used to set pressure.

Service intervals are set by time or distance, whichever comes first; short-trip use falls under severe service and shortens oil and filter intervals.

Brake and tyre inspection limits: replace tyres at the legal tread depth (commonly 1.6 mm) and brakes when lining thickness falls below the manufacturer minimum.

Fluid checks: engine oil on a level surface with the engine off, automatic transmission fluid at operating temperature in the specified range, brake fluid at the reservoir mark.

Road test before and after repair confirms the complaint and the fix; record the result on the job card so the customer has documented evidence.

Pre-delivery and periodic inspection sheets list items by system; a defect found must be recorded even if the customer declines the repair.

Job card essentials: customer details, vehicle identification number, odometer reading, complaint in the customer's words, work authorised, parts used, labour time and signature.

Labour charge = labour hours x hourly rate; total invoice = labour + parts + consumables + applicable tax. Show each line separately.

Break-even point in units = fixed costs / (selling price per unit - variable cost per unit).

Gross profit margin = (revenue - cost of goods sold) / revenue x 100; a workshop with low margin but high volume can still be profitable.

GST/VAT on a tax invoice must show the tax rate, tax amount, taxable value and the registration number of the business.

Written estimate before work protects both parties; additional work found during repair requires fresh authorisation, preferably in writing or by recorded message.

Spare parts inventory turnover = cost of parts used / average inventory value; slow turnover ties up working capital and risks obsolete stock.

Workplace communication: report hazards immediately, use the chain of command for escalations, and record verbal instructions in writing to avoid disputes.

Business registration, insurance, waste-oil disposal records and technician certification are compliance items that a new workshop must arrange before trading.