WBCHSE HS Vocational Automobile cheat sheet

Four-stroke cycle order: intake, compression, power, exhaust; crankshaft makes two revolutions per cycle.

Compression ratio = (swept volume + clearance volume) ÷ clearance volume; petrol engines typically 8:1 to 12:1, diesel 16:1 to 22:1.

Engine displacement = bore area × stroke × number of cylinders; bore area = πd²/4.

Fire classes: A solid, B flammable liquid, C electrical, D metal; CO2 and dry powder are used on electrical and fuel fires, never water.

Stop an engine fire with CO2 or dry powder extinguisher aimed at the base of the flame; isolate battery and fuel supply first.

Workshop ventilation rule: run exhaust extraction whenever an engine idles indoors; carbon monoxide is odourless and lethal.

PPE decision rule: eye protection for grinding/drilling, gloves for chemicals and sharp edges, safety footwear for any load above ankle height.

Jack stands must support the vehicle before any body part goes under it; a hydraulic jack alone is a lifting device, not a support.

Lockout rule: disconnect the battery earth terminal and tag it before working on the starting or charging circuit.

Valve clearance is measured with a feeler gauge when the engine is cold (or per manufacturer spec) and adjusted by the rocker screw or shim.

Cylinder taper = difference between bore diameter at top and bottom of the ring travel; ovality is measured at 90° to the thrust axis.

Piston ring end gap is checked by inserting the ring square in the bore; too small a gap causes ring butting and breakage.

Torque-to-yield head bolts are tightened in stages to a torque figure then turned a specified angle; they are replaced, not reused.

Head gasket failure signs: coolant in oil (mayonnaise), bubbles in the radiator, white exhaust smoke, compression loss between cylinders.

Timing belt alignment marks must match camshaft and crankshaft marks before tensioning; one tooth error changes valve timing and can bend valves.

Diesel fuel injection timing is set relative to TDC; retarded timing gives white smoke and poor starting, advanced timing gives knock.

Oil pressure low at idle but normal at speed suggests worn bearings or a faulty relief valve; low at all speeds suggests pump or pickup blockage.

Engine overhaul sequence: remove, strip, clean, measure, machine if out of tolerance, assemble with correct clearances, run-in.

Gear ratio = driven gear teeth ÷ driving gear teeth; overall ratio = gearbox ratio × final drive ratio.

Manual gearbox components: clutch, input shaft, layshaft/countershaft, main shaft, synchroniser rings, selector forks.

Clutch slip test: in a high gear at low speed, accelerate hard; engine speed rising without road speed means slip.

Torque converter multiplication occurs when turbine speed is much lower than impeller speed; at coupling point slip is minimal.

CV joint clicking on full lock during acceleration indicates worn outer joint; a torn boot causes grease loss and joint failure.

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

Toe-in is measured in mm or degrees; incorrect toe causes rapid shoulder wear and steering pull.

Suspension types: MacPherson strut (integrated spring and damper), double wishbone, torsion bar, leaf spring for load carrying.

Damper test: bounce a corner and release; more than one rebound overshoot indicates a worn shock absorber.

Hydraulic principle: pressure is equal throughout a closed system; force = pressure × piston area, so a larger wheel cylinder gives greater braking force.

Brake pedal sponginess indicates air in the hydraulic circuit; bleed sequence is normally the wheel farthest from the master cylinder first.

DOT 3 and DOT 4 brake fluids are hygroscopic; absorbed moisture lowers boiling point and causes fade.

Disc brake runout is measured with a dial gauge; excessive runout causes pulsation through the pedal.

Brake fade causes: overheating of pads/linings, glazing, or boiled fluid; symptoms are a long or soft pedal and reduced retardation.

ABS prevents wheel lock by modulating pressure; the ABS warning lamp staying on means the system is disabled but base braking remains.

Stopping distance = thinking distance + braking distance; braking distance rises with the square of speed (double speed, four times distance).

Weight transfer under braking moves load to the front axle, so front brakes normally do the majority of the braking work.

Parking brake is a mechanical cable or electronic actuator acting on the rear wheels; it must hold the vehicle on a specified gradient.

Ohm's law: V = I × R; power P = V × I = I²R = V²/R.

Series circuit: current is common, voltages add. Parallel circuit: voltage is common, currents add.

Battery capacity is rated in ampere-hours (Ah) and cold cranking amps (CCA); a 12 V battery has six 2.1 V cells in series.

Alternator output is three-phase AC rectified by a diode pack; a failed diode causes ripple and a dim or flickering charging lamp.

Charging voltage check: about 13.8–14.5 V at the battery with the engine running and loads off.

Starter motor draws high current through a solenoid; a clicking solenoid with no crank suggests low battery or a bad earth strap.

LED and ECU circuits are polarity sensitive; never test them with a high-current test lamp, use a multimeter or logic probe.

CAN bus is a twisted-pair differential network; a shorted pair or missing 60-ohm termination stops communication across modules.

Fuse rating must not exceed the circuit's rated current; a blown fuse is replaced only after finding the cause of the overload.

Diagnostic order: verify the complaint, gather symptoms, check for fault codes, test the circuit or component, confirm the repair.

OBD-II standardised codes: P = powertrain, B = body, C = chassis, U = network; the second digit identifies manufacturer-specific or generic status.

A fault code identifies a circuit or system, not necessarily the failed part; always confirm with a live-data or resistance test.

Compression test: crank with the throttle open and all plugs removed; a wet test that raises compression indicates worn rings.

Cylinder leakage test locates leakage by listening at the intake (inlet valve), exhaust (exhaust valve), or oil filler (rings).

Service intervals are set by time or distance, whichever comes first; engine oil and filter, air filter, brake fluid, coolant are the core items.

Coolant freeze point is checked with a refractometer; a 50:50 glycol mix protects to about −37 °C and raises boiling point.

Tyre tread minimum is 1.6 mm in the major grooves; check pressures cold and adjust for load as stated on the tyre placard.

Battery test: open-circuit voltage about 12.6 V at rest means fully charged; below 12.4 V indicates a partial discharge needing recharge.

Vernier calliper least count = smallest main scale division ÷ number of vernier divisions; typical metric value is 0.02 mm.

Micrometer reading = main scale + thimble reading; a 0–25 mm micrometer with 0.01 mm thimble divisions reads to 0.01 mm.

Dial gauge (DTI) measures runout, end play and backlash; it must be zeroed against a known surface before use.

Feeler gauge measures gaps such as valve clearance and spark plug gap; use the largest blade that fits with slight drag.

Torque wrench must be set to the specified value and rechecked; never use it to loosen fasteners.

Cast iron is strong in compression and used for blocks; aluminium alloy is light and needs steel liners or coated bores.

Steel classification: low carbon (mild steel) is ductile and weldable; high carbon steel is hardenable by heat treatment.

Bearing clearance is checked with Plastigauge; the crushed width is compared to the scale to give the oil clearance.

Brinell and Rockwell tests measure hardness; hardness correlates with wear resistance and machinability.

Break-even point = fixed costs ÷ (selling price per unit − variable cost per unit).

Gross profit = sales − cost of goods sold; net profit = gross profit − operating expenses.

A workshop quotation should separate parts, labour and tax; labour is charged as hours × hourly rate.

GST/VAT is charged on the taxable value and shown separately on the invoice; keep records for the statutory period.

Job card must record customer details, complaint, work done, parts used and technician sign-off for traceability.

Employability basics: punctuality, correct PPE, safe housekeeping (5S), and clear communication with the customer.

Statutory compliance for a workshop includes trade licence, fire safety certificate, waste oil disposal record and employee insurance.

Marketing a workshop relies on service quality, repeat customers and transparent billing rather than price cutting alone.

Cash flow differs from profit: a profitable workshop can fail if receivables are not collected on time.