Assam ASSEB Higher Secondary Skill Subject Automotive cheat sheet

Fire classes: A solid combustibles, B flammable liquids, C electrical, D metals; CO2 and dry powder suit fuel/electrical fires, water must never be used on live electrical or fuel fires.

Battery charging releases hydrogen; keep chargers in ventilated areas, no sparks or open flame within 3 m, and connect positive last, disconnect negative first.

Carbon monoxide is odourless and lethal; never run an engine in a closed bay - exhaust extraction or open-door ventilation is mandatory.

Jack stands must be rated above the vehicle kerb weight and placed on chassis jacking points; a hydraulic jack alone is a lifting device, not a support.

Compressed air for cleaning is limited to about 2 bar (30 psi) and never aimed at skin - air embolism risk; use a nozzle with relief holes.

Fuel and solvent storage: closed metal containers, bonded earthing during transfer, maximum 50 L per container in most workshop rules, no smoking signage.

PPE matrix: safety glasses for grinding and battery work, face shield plus apron for solvent/acid, gloves for hot or sharp parts, hearing protection above 85 dB(A).

Lockout/tagout before working on a vehicle: chock wheels, apply parking brake, disconnect battery negative, tag keys and controls.

Torque = force × perpendicular distance; a 40 N force on a 0.5 m wrench gives 20 N·m; torque wrenches are calibrated at mid-scale and rechecked annually.

Micrometer least count = pitch ÷ thimble divisions; a 0.5 mm pitch with 50 divisions reads 0.01 mm.

Vernier caliper least count = main scale division ÷ number of vernier divisions; 1 mm ÷ 20 = 0.05 mm.

Dial gauge/plunger-type bore gauge measures ovality and taper; measure at top, middle and bottom of the bore in two planes 90° apart.

Feeler gauge is used for valve clearances, spark plug gaps and piston ring end gaps; never force a blade - drag fit indicates the true clearance.

Six-point sockets grip flats better than twelve-point and are preferred for high-torque or damaged fasteners; impact sockets are black, chrome-vanadium hand sockets are not for impact guns.

Left-hand drill bits and screw extractors remove broken studs; drill centrally, use cutting fluid, and never enlarge the hole beyond the extractor size.

Hydraulic press and pullers are for bearings and bushes; apply force only to the race being fitted (inner race on shafts, outer race in housings).

Four-stroke cycle: intake, compression, power, expansion/exhaust - two crankshaft revolutions per cycle; a two-stroke completes the cycle in one revolution.

Compression ratio = (swept volume + clearance volume) ÷ clearance volume; petrol engines typically 8:1–12:1, diesel 16:1–23:1.

Engine capacity = π/4 × bore² × stroke × number of cylinders; a 4-cylinder 80 mm × 90 mm engine displaces about 1809 cm³.

Firing order determines crankshaft layout and ignition/injection sequencing; common in-line four orders are 1-3-4-2 and 1-2-4-3.

Valve clearance is set cold for most engines; too little clearance causes burnt valves and compression loss, too much causes noise and reduced lift.

Piston ring set: top compression ring (barrel-faced or keystone), second scraper ring, oil control ring with expander; ring end gaps staggered 120° on assembly.

Cylinder head bolts on many modern engines are torque-to-yield and must be replaced; tighten in the specified sequence, usually centre-outwards.

Timing belt/chain alignment marks must match before and after rotation; interference engines suffer valve-piston contact if timing slips.

Stoichiometric air-fuel ratio is about 14.7:1 by mass for petrol and about 14.5:1 for diesel; rich mixture = excess fuel, lean = excess air.

Petrol octane number rates anti-knock quality; diesel cetane number rates ignition quality - higher cetane means shorter ignition delay.

Common-rail diesel injection pressures reach 1800–2500 bar; injector leak-off and rail pressure sensor faults cause hard starting and limp mode.

SAE viscosity grade: first number is cold (W) performance, second is hot; 5W-30 flows better cold than 15W-40. API/ACEA service category must meet or exceed the manufacturer specification.

Oil pump is usually crankshaft- or chain-driven; low oil pressure warning at idle plus noisy tappets indicates worn pump, blocked pickup or diluted oil.

Cooling system: thermostat opens around 82–90 °C, pressurised cap raises boiling point about 3 °C per 0.1 bar; 50/50 coolant-water mix gives freeze protection near −37 °C.

Air locks after coolant refill are removed by running to temperature with the heater on and bleeding at the highest point.

Fuel filter replacement direction follows flow arrows; a restricted filter causes power loss under load and low rail pressure codes.

Clutch transmits torque by friction between flywheel, driven plate and pressure plate; free play at the pedal is typically 10–25 mm and prevents release-bearing drag.

Clutch slip symptom: engine speed rises without road speed; cause is worn friction lining, oil contamination or weak diaphragm spring.

Manual gearbox ratios multiply torque: overall ratio = gearbox ratio × final drive ratio; lower numerical ratio gives higher top speed, higher numerical gives more pulling force.

Synchronisers equalise shaft speeds before gear engagement; worn baulk rings cause crunching on downshift.

Automatic transmission uses a torque converter with a stator; stall test and line pressure test diagnose converter and pump condition.

CV joints: outer joint handles steering angle, inner joint allows plunge; a torn boot throws grease and causes clicking on turns.

Differential allows wheels to turn at different speeds in a corner; final drive ratio is ring gear teeth ÷ pinion teeth.

Propshaft universal joints must be phased; incorrect phasing causes vibration that rises with road speed.

Hydraulic pressure = force ÷ area (P = F/A); a larger caliper piston increases clamping force but increases pedal travel.

Brake fluid is hygroscopic; DOT 4 boiling point drops as water content rises - replace every 2 years or when moisture exceeds about 3%.

Disc run-out and thickness are measured with a micrometer/dial gauge; below minimum thickness the disc must be replaced, not machined.

Parking brake adjustment follows cable free play specification; dragging brakes show as heat, blue discs and poor fuel economy.

Wheel alignment: toe-in/out, camber (vertical tilt) and caster (steering axis tilt) are the primary angles; incorrect toe causes rapid shoulder tyre wear.

Tyre wear diagnosis: centre wear = over-inflation, both shoulders = under-inflation, one shoulder = camber or toe fault, cupping = worn dampers.

Power steering: hydraulic systems use a belt-driven pump and pressure relief valve; electric systems use a torque sensor and motor - both give heavy steering when failed.

Suspension inspection covers ball joints, tie-rod ends, bushes, strut mounts and dampers; check for play with the wheel loaded and unloaded.

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

Battery nominal 12.6 V at rest fully charged; below 12.4 V is partly discharged, and cranking below about 9.6 V indicates a weak battery.

Series circuit: same current, voltages add. Parallel circuit: same voltage, currents add - vehicle loads are wired in parallel with the chassis as return.

Alternator output is typically 13.8–14.5 V at idle with loads off; low or high charging voltage points to regulator, belt or wiring faults.

Starter draw test and voltage drop test: more than 0.5 V drop across a cable or connection indicates excessive resistance.

Fuses protect circuits by current rating; never fit a higher-rated fuse - trace the short instead. Relays use a small coil current to switch a large load current.

CAN bus is a twisted-pair differential network; about 2.5 V on both lines at rest, and a shorted or open line disables multiple modules.

Sensors: MAP measures manifold pressure, MAF measures air mass, lambda/O2 measures exhaust oxygen; a biased O2 sensor corrupts fuel trim.

Steel body panels are welded, bolted or bonded; high-strength steels must not be heated above about 200 °C because temper is lost.

Panel gap and flush are checked with feeler gauges and gap gauges; typical door gaps are 4–6 mm and must be even around the aperture.

Paint system layers: pretreatment, primer, base coat, clear coat; total dry film thickness on a car is commonly 80–150 µm.

Paint defect matching: orange peel from too little thinner or too much air pressure; runs from too much material; blisters from moisture in the air line.

Colour match requires checking the variant under daylight or a colour-corrected lamp, not workshop fluorescent light alone.

Body filler is applied over bare, abraded metal in thin layers; filler over rust or paint traps moisture and fails.

MIG welding of thin panels uses low amperage, short stitch welds and copper backing to avoid distortion; plug welds replace factory spot welds.

Trim clips and fasteners are single-use on many modern vehicles; replace damaged clips rather than forcing panels back into place.

Service intervals follow the manufacturer schedule by distance or time, whichever comes first; severe-duty use shortens oil and filter intervals.

Daily/pre-start checks: engine oil level, coolant, brake fluid, washer fluid, tyre pressures and visible leaks.

Tyre pressure is checked cold; pressure rises about 0.1 bar per 10 °C temperature rise, so hot readings must not be corrected downward.

Tread depth minimum is 1.6 mm in the central three-quarters; replace before this for wet grip. Check for uneven wear, cuts and sidewall damage.

Battery maintenance: clean terminals, check electrolyte level in serviceable types, and verify charging voltage after every service.

Brake inspection includes pad thickness, disc condition, fluid level, hose cracks and parking brake travel.

Torque every wheel nut to specification with a calibrated wrench and recheck after 50–100 km on alloy wheels.

Road test confirms repairs: listen for noises, check steering pull, braking balance, gearshift quality and warning lamps.

Diagnostic sequence: verify the complaint, gather symptoms, check for technical service bulletins, scan for codes, test the circuit, repair, then verify the fix.

DTCs identify the circuit or system, not always the failed part; a P0171 lean code can come from a vacuum leak, low fuel pressure or a dirty MAF.

Fuel trim: short-term plus long-term beyond ±10% indicates a mixture fault; positive trim means the ECU is adding fuel (lean condition).

Voltage drop testing is more reliable than resistance testing for high-current circuits; load the circuit and measure across the connection.

Compression test: crank with throttle open and all plugs removed; a wet test that raises compression indicates worn rings, not valves.

Cylinder leakage test locates the leak path - into the intake (intake valve), exhaust (exhaust valve), crankcase (rings) or coolant (head gasket).

Intermittent faults are captured with a data logger or by wiggle-testing harnesses while monitoring live data.

After repair, clear codes, perform a readiness drive cycle and confirm no pending codes return before releasing the vehicle.

Job card must record customer complaint, work done, parts used, labour time and technician signature; it is the legal and billing record.

Estimate versus invoice: an estimate is an approximate quotation, an invoice is the demand for payment; any extra work needs customer approval before starting.

5S workplace organisation: sort, set in order, shine, standardise, sustain - reduces search time and lost tools.

Labour rate × standard hours plus parts cost plus applicable tax gives the invoice total; track parts margin separately from labour.

Warranty on repairs must state duration and coverage; keep replaced parts for a period in case of dispute.

Customer communication: explain the fault in plain language, show the worn part, and give a written recommendation with cost before work proceeds.

Entrepreneurship basics: fixed costs (rent, salaries), variable costs (parts, consumables), break-even volume = fixed costs ÷ (price − variable cost per job).

Environmental compliance: used oil, coolant, batteries and tyres go to licensed recyclers; workshop waste manifests must be retained.

Fire triangle: fuel, oxygen, heat. Remove any one to extinguish.

CO2 extinguishers are for electrical and fuel fires; water is not.

Hydrogen from charging batteries ignites easily; ventilate and avoid sparks.

Jack stands, not jacks, support a raised vehicle.

Air line pressure for cleaning is limited to about 2 bar.

Safety glasses are mandatory when grinding or working near batteries.

Disconnect the battery negative terminal before major electrical work.

Store fuels and solvents in closed, earthed metal containers.

Torque = force × distance; 50 N at 0.4 m = 20 N·m.

Micrometer least count = pitch ÷ thimble divisions.

Vernier caliper least count = 1 mm ÷ vernier divisions.

Feeler gauge measures valve clearance and plug gap.

Six-point sockets grip flats better than twelve-point sockets.

Impact sockets are black and rated for impact guns.

Bore gauge measures ovality and taper in two planes.

Apply press force to the race being fitted, never through the balls.

Four-stroke cycle uses two crankshaft revolutions.

Compression ratio = (swept + clearance) ÷ clearance.

Engine capacity = π/4 × bore² × stroke × cylinders.

Firing order sequences ignition and injection events.

Too little valve clearance burns valves; too much causes noise.

Stagger piston ring end gaps about 120° at assembly.

Torque-to-yield head bolts are usually single-use.

Interference engines suffer valve damage if timing slips.

Petrol stoichiometric ratio is about 14.7:1 by mass.

Octane rates petrol anti-knock; cetane rates diesel ignition quality.

SAE 5W-30 flows better cold than 15W-40.

Thermostat opens around 82–90 °C on most engines.

A 50/50 coolant mix freezes near −37 °C.

A pressurised cap raises boiling point about 3 °C per 0.1 bar.

Low oil pressure plus noisy tappets suggests pump or pickup faults.

A restricted fuel filter causes power loss under load.

Clutch pedal free play is typically 10–25 mm.

Clutch slip: engine speed rises without road speed.

Overall ratio = gearbox ratio × final drive ratio.

Synchronisers equalise shaft speeds before engagement.

A torn CV boot throws grease and causes clicking on turns.

Final drive ratio = ring gear teeth ÷ pinion teeth.

Torque converter stall test checks converter and engine power.

Propshaft universal joints must be correctly phased to avoid vibration.

Hydraulic pressure P = F/A.

DOT 4 brake fluid is hygroscopic; replace every 2 years.

A disc below minimum thickness must be replaced.

Toe, camber and caster are the primary alignment angles.

Centre tyre wear indicates over-inflation.

Both-shoulder tyre wear indicates under-inflation.

Cupped tyre wear points to worn dampers.

Electric power steering uses a torque sensor and motor.

V = I × R and P = V × I.

A fully charged 12 V battery rests at about 12.6 V.

Cranking voltage below about 9.6 V indicates a weak battery.

Alternator charging voltage is typically 13.8–14.5 V.

Voltage drop above 0.5 V across a cable indicates excess resistance.

Never fit a fuse with a higher current rating than specified.

CAN bus is a twisted-pair differential network at about 2.5 V rest.

O2 sensor bias corrupts fuel trim and emissions.

High-strength steel must not be heated above about 200 °C.

Typical door panel gaps are 4–6 mm and must be even.

Paint layers: pretreatment, primer, base coat, clear coat.

Total paint dry film thickness is commonly 80–150 µm.

Orange peel comes from too little thinner or excess air pressure.

Apply body filler in thin layers over bare, abraded metal.

MIG stitch welds with copper backing reduce panel distortion.

Replace single-use trim clips rather than forcing panels.

Service by distance or time, whichever comes first.

Check tyre pressure cold; pressure rises about 0.1 bar per 10 °C.

Minimum tread depth is 1.6 mm in the central three-quarters.

Torque wheel nuts and recheck after 50–100 km.

Check brake pad thickness, disc condition and hose cracks.

Verify charging voltage at every service.

A road test confirms braking balance and steering pull.

Daily checks cover oil, coolant, brake fluid and leaks.

Sequence: verify, gather symptoms, scan codes, test, repair, verify.

A DTC identifies a circuit or system, not always the failed part.

Fuel trim beyond ±10% indicates a mixture fault.

Positive fuel trim means the ECU is adding fuel for a lean condition.

Voltage drop testing beats resistance testing on loaded circuits.

A wet compression test that raises pressure indicates worn rings.

Cylinder leakage test locates the leak path by sound.

Clear codes and run a readiness drive cycle after repair.

A job card records complaint, work done, parts and labour.

Extra work requires customer approval before starting.

5S: sort, set in order, shine, standardise, sustain.

Invoice = labour rate × hours + parts cost + tax.

Break-even volume = fixed costs ÷ (price − variable cost per job).

Keep replaced parts for a period in case of dispute.

Used oil, coolant and batteries go to licensed recyclers.

State warranty duration and coverage on every repair.