Punjab PSEB Sr. Sec. NSQF Automotive (Four Wheel Service Technician) cheat sheet

Carbon monoxide is odourless and colourless; never run an engine in a closed bay - exhaust extraction or open doors with forced ventilation is mandatory.

Fuel and solvent fires are Class B; use dry powder or CO2, never water, and never water on live electrical (Class C/E) fires.

Battery charging releases hydrogen; keep flames, sparks and grinding at least 3 m away and charge only in ventilated areas.

A hydraulic jack is a lifting device only - support the vehicle on rated axle stands before any work under it.

Waste segregation: used engine oil, coolant (ethylene glycol), brake fluid, solvents and lead-acid batteries are separate hazardous streams; never mix coolant with oil or pour either into drains.

Coolant and brake fluid are toxic and absorb through skin; wear nitrile gloves, and never open a hot pressurised cooling system cap.

Eye protection is required for grinding, chipping, compressed-air blowing and battery work; compressed air must never be directed at skin.

Airbag and seat-belt pretensioner modules contain pyrotechnics - wait the manufacturer-specified time (commonly 30 s to several minutes) after disconnecting the battery before touching them.

PPE baseline: safety shoes, overalls, gloves matched to the task, and hearing protection above roughly 85 dB(A) in the workshop.

Vernier caliper least count = 1 main-scale division (1 mm) ÷ number of vernier divisions (e.g. 50 divisions → 0.02 mm); reading = main scale + (coinciding vernier line × least count).

Micrometer least count = pitch ÷ thimble divisions; a 0.5 mm pitch with 50 divisions gives 0.01 mm. Always use the ratchet stop to avoid over-tightening.

Dial bore gauge and telescopic gauge measure cylinder bore; subtract the gauge's zero (set on a micrometer) to obtain true bore diameter.

Feeler gauge measures valve clearance, ring end gap and plug gap by stacking blades; plastigauge measures bearing oil clearance by crushing a plastic strip.

Torque = force × perpendicular distance; a torque wrench must be used in its mid-range, never to loosen fasteners, and recalibrated periodically.

Torque-to-yield (stretch) bolts and many head bolts are single-use - replace, and follow the specified sequence and angle-tightening stages.

A dial indicator with magnetic base measures runout, end play and backlash; zero it against a known surface before reading.

Compression tester: remove all plugs, hold throttle wide open, crank equal strokes per cylinder; a wet test that raises pressure indicates worn rings, not valves.

Micrometers and calipers read zero error before use - a positive or negative zero error must be added or subtracted from every reading.

Four-stroke cycle order: intake → compression → power → exhaust, two crankshaft revolutions per cycle; a four-cylinder engine fires every 180° of crank rotation.

Compression ratio = (swept volume + clearance volume) ÷ clearance volume; swept volume = (π/4) × bore² × stroke × number of cylinders.

Valve clearance is set with the engine cold on most engines; too little clearance causes burnt valves and hard starting, too much causes noise and reduced lift.

Firing order (e.g. 1-3-4-2) determines both ignition sequence and valve-setting sequence; identify cylinder 1 before adjusting.

Diesel compression ignition needs compression ratios around 16:1–22:1 to raise air temperature above the fuel's auto-ignition point; petrol engines typically run 8:1–12:1.

Timing belt/chain alignment marks must match before and after replacement; on interference engines, mistiming bends valves.

Oil pressure low at idle but normal at speed suggests worn bearings or a weak pump; low at all speeds suggests a blocked pickup or failed pump - stop the engine.

Coolant thermostat stuck open gives slow warm-up and poor heater output; stuck closed causes rapid overheating.

Blue exhaust smoke = oil burning (rings, valve guides, turbo seals); white = coolant/water; black = rich mixture or over-fuelling on diesel.

Clutch free play at the pedal is typically 10–25 mm; too little causes slipping, too much causes incomplete release and gear-crunch.

A clutch that slips under load shows engine speed rising without road speed; a clutch that drags shows difficult engagement with the pedal fully down.

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 km) ÷ (overall gear ratio × 1000).

Synchronisers equalise shaft speeds before engagement; grinding on downshift indicates worn baulk rings or wrong oil grade.

CV joint clicking on full-lock turns indicates a worn outer joint; vibration under acceleration indicates inner joint or driveshaft imbalance.

Differential allows inner and outer wheels to turn at different speeds in a turn; a whining diff that changes with throttle often indicates incorrect backlash or pinion bearing wear.

Automatic transmission fluid level is checked with the engine running and fluid at operating temperature, in Park or Neutral per the manufacturer.

Universal joints and driveshaft phasing must be correct - misaligned yokes cause vibration that rises with road speed.

Hydraulic pressure is transmitted equally (Pascal's law); a spongy pedal indicates air in the system, a sinking pedal indicates an internal master-cylinder leak.

Brake fluid is hygroscopic; DOT 3/4 fluid absorbs moisture and should be replaced roughly every 2 years, and never mixed with DOT 5 silicone fluid.

Disc runout limit is typically 0.05–0.10 mm; excessive runout causes pedal pulsation and is measured with a dial indicator.

Minimum pad and disc thickness are manufacturer specifications - discard, never machine, a disc below minimum thickness.

Wheel alignment angles: camber is the inward/outward tilt in front view; caster is the steering-axis tilt in side view; toe is the difference in distance between front and rear of the wheels.

Incorrect toe causes rapid, feathered tyre wear; incorrect camber causes one-sided shoulder wear; incorrect caster causes poor straight-line stability and return.

Total toe = left toe + right toe; toe-in means the front of the wheels is closer together than the rear.

Brake bleeding order is normally farthest from the master cylinder first (rear right → rear left → front right → front left on a typical layout).

Parking brake travel and rear brake adjustment must be set before a roller brake test; imbalance above roughly 30% between sides fails inspection.

Tyre size code 195/65 R15: 195 = section width mm, 65 = aspect ratio %, R = radial, 15 = rim diameter inches.

Section height = width × aspect ratio; overall diameter = rim diameter × 25.4 + 2 × section height (mm).

Load index and speed rating must meet or exceed the vehicle manufacturer's specification; mixing radial and cross-ply on one axle is unsafe.

Tread wear indicator bars sit at 1.6 mm; legal minimum in most jurisdictions is 1.6 mm across the central three-quarters of the tread.

Cold tyre pressure is the correct reference; pressure rises with heat, so never bleed a hot tyre down to the cold figure.

Symptoms: cupping/scalloping = worn shock absorbers or imbalance; centre wear = over-inflation; both shoulders worn = under-inflation; diagonal wear = misalignment or worn suspension.

Wheel balance is corrected with clip-on or adhesive weights; static imbalance causes bounce, dynamic imbalance causes shimmy.

Steering free play beyond about 25 mm at the rim, or a wandering vehicle, indicates worn tie-rod ends, idler arm or steering box.

MacPherson strut combines the shock absorber and spring in one unit and provides the upper suspension pivot; it must be realigned after replacement.

Ohm's law: V = I × R; power P = V × I = I²R = V²/R. A 12 V, 55 W bulb draws about 4.6 A.

Battery capacity is rated in Ah and cold cranking amps (CCA); a fully charged 12 V lead-acid battery rests at about 12.6–12.8 V, 12.2 V is roughly half-charged.

Charging system: alternator output should be about 13.8–14.5 V at the battery with the engine running and loads on.

Starter current draw is very high (hundreds of amps); a voltage drop test (max 0.5 V per cable) is more reliable than resistance testing on high-current circuits.

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

Fuse rating protects the wiring, not the component - replace with the same rating; a blown fuse indicates a short or overload to be found, not just replaced.

LED and HID lamps need correct polarity and ballast/driver modules; never test an HID with a conventional test lamp.

CAN bus is a twisted-pair differential network; a shorted or open bus can disable many modules, so check bus resistance (commonly about 60 Ω across the pair) before condemning a module.

Disconnecting the battery on a modern vehicle can erase adaptive learning and radio codes; use a memory saver or follow the relearn procedure.

Stoichiometric air-fuel ratio is about 14.7:1 by mass for petrol and about 14.5:1 for diesel; lambda = actual AFR ÷ stoichiometric AFR.

Three-way catalytic converters reduce CO, HC and NOx only within a narrow lambda window (roughly 0.99–1.01); a faulty oxygen sensor drives emissions up.

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

PCV valve recirculates crankcase blow-by gases; a blocked PCV causes oil leaks and sludge; a stuck-open PCV causes lean running.

Diesel smoke diagnosis: black = over-fuelling/restricted air; blue = lube oil; white = unburnt fuel or coolant, often late injection timing.

Common-rail diesel injection pressure can exceed 2000 bar; never crack a high-pressure line while cranking - the jet can penetrate skin.

CNG/LPG vehicles use a reducer/vaporiser and a separate ECU; cylinders require periodic hydrostatic testing and leak checks at every service.

Petrol direct injection (GDI) builds carbon on intake valves because fuel no longer washes the port; port injection does not.

OBD-II codes P0xxx are powertrain; a fuel trim beyond about ±10% long-term indicates a vacuum leak (positive) or restricted fuel supply (negative).

Periodic service intervals are set by time or distance, whichever comes first; typical first service is at 1000–5000 km or 1 month for oil and inspection.

Pre-delivery/pre-service checklist order: fluids and leaks → tyres and pressures → brakes → lights and horn → steering and suspension → battery and charging → road test.

Diagnosis sequence: verify the complaint → gather information → inspect visually → test → isolate the fault → repair → verify and clear codes.

A diagnostic trouble code identifies a circuit or system, not a failed part; always perform the pinpoint test before replacing components.

Fluid checks: engine oil on level ground with the engine off; automatic transmission fluid with the engine running at operating temperature; brake fluid at the MAX mark with the cap clean.

Battery test sequence: visual inspection → state of charge → load/capacity test (e.g. conductance tester) → charging system test.

Road test should include low speed, highway speed, braking, turning both directions and a check for vibration, noise and pull.

Tyre rotation patterns depend on drive layout; directional tyres must stay on the same side, and radial tyres should not be cross-switched front-to-rear on some vehicles.

A vehicle fails inspection on brake imbalance, excessive free play, tyre below 1.6 mm, faulty lights, or emissions beyond the statutory limit.