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

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Carbon monoxide hazard

CO is an odourless, colourless gas produced by incomplete combustion. It binds haemoglobin far more strongly than oxygen, so engine running indoors requires exhaust extraction or forced ventilation.

Class B fire

A fire involving flammable liquids such as petrol, diesel, oil or solvents. It is extinguished with dry powder, foam or CO2, never with a water jet that spreads the burning liquid.

Axle stand rule

A hydraulic jack lifts but does not support. Once the vehicle is raised, place rated axle stands under the chassis or axle before any part of the body goes beneath it.

Vernier least count

Least count equals one main-scale division divided by the number of vernier divisions. A 1 mm main scale with 50 vernier divisions gives a least count of 0.02 mm.

Micrometer least count

Least count equals screw pitch divided by thimble divisions. A 0.5 mm pitch with a 50-division thimble reads to 0.01 mm; the ratchet stop prevents over-tightening.

Plastigauge use

A crushable plastic strip placed across a bearing journal. After torquing the cap, the flattened width is compared with a scale to read oil clearance without a bore gauge.

Torque-to-yield bolt

A fastener tightened past its elastic limit into plastic deformation and therefore used only once. It is replaced, not reused, and tightened in stages with a torque angle gauge.

Compression ratio formula

Compression ratio is swept volume plus clearance volume, divided by clearance volume. Petrol engines typically run 8:1 to 12:1, diesel engines 16:1 to 22:1.

Wet compression test

A compression test repeated after adding a little oil to the cylinder. If pressure rises significantly, the rings or bores are worn; if it stays low, valves or head gasket leak.

Valve clearance too tight

Insufficient clearance holds the valve slightly off its seat, so it overheats, burns and leaks compression. Symptoms include hard starting, misfire and eventually a burnt valve.

Clutch pedal free play

The pedal travel before the release bearing contacts the pressure plate, typically 10 to 25 mm. Too little causes slip; too much causes incomplete release and gear crunch.

Gear ratio calculation

Gear ratio equals driven gear teeth divided by driving gear teeth. Overall ratio multiplies the gearbox ratio by the final drive ratio to give total speed reduction.

Synchroniser function

Cone-shaped baulk rings friction-match the speed of the gear to the shaft before engagement. Grinding on downshift usually indicates worn rings or incorrect lubricant.

CV joint click diagnosis

Clicking or knocking during full-lock turns indicates a worn outer constant-velocity joint. Vibration under acceleration instead points to the inner joint or a bent driveshaft.

Spongy brake pedal

A pedal that feels soft and compresses indicates air or moisture in the hydraulic circuit. The system must be bled in the specified sequence, farthest wheel first.

Camber angle

The inward or outward tilt of the wheel top viewed from the front. Excessive negative camber wears the inner shoulder; excessive positive camber wears the outer shoulder.

Toe setting effect

Toe is the difference in track between wheel fronts and rears. Incorrect toe produces rapid feathered tread wear and is the most common cause of short tyre life.

Brake disc runout

Side-to-side wobble of the disc face measured with a dial indicator. Runout beyond about 0.05 to 0.10 mm causes pedal pulsation and requires disc replacement or machining.

Tyre size decoding

In 195/65 R15, 195 is section width in millimetres, 65 is aspect ratio in percent, R means radial construction, and 15 is rim diameter in inches.

Tread wear indicators

Raised bars moulded into the tread grooves at 1.6 mm height. When tread is level with them, the tyre is at the legal minimum and must be replaced.

Cupped tyre wear

Scalloped or cupped patches around the tread usually indicate worn shock absorbers, wheel imbalance or loose suspension parts rather than incorrect inflation pressure.

Static versus dynamic balance

Static imbalance is a heavy spot causing vertical bounce. Dynamic imbalance is a heavy spot off-centre causing side-to-side shimmy, corrected with clip-on or adhesive weights.

Ohm's law and power

Voltage equals current times resistance. Power equals voltage times current, so a 12 V 55 W lamp draws about 4.6 A and has roughly 2.6 ohms resistance.

Battery state of charge

A rested 12 V lead-acid battery reads about 12.6 to 12.8 V when fully charged and about 12.2 V at half charge. Charging voltage should be 13.8 to 14.5 V.

Voltage drop testing

Measuring voltage lost across a cable or connection while current flows. More than about 0.5 V on a high-current circuit indicates excessive resistance such as corrosion.

CAN bus termination

A twisted-pair differential network terminated at both ends. With the battery disconnected, resistance across the bus pair is commonly about 60 ohms from two parallel 120-ohm resistors.

Stoichiometric ratio

The air-fuel mass ratio giving complete combustion, about 14.7:1 for petrol. Lambda equals actual ratio divided by stoichiometric ratio, with 1.00 being ideal.

EGR valve function

Exhaust gas recirculation returns inert exhaust to the intake, lowering peak combustion temperature and reducing nitrogen oxides. Stuck open it causes rough idle; stuck closed it causes knock.

Diesel smoke colours

Black smoke means over-fuelling or restricted intake air. Blue smoke is burnt lubricating oil. White smoke is unburnt fuel or coolant, often from late injection timing.

DTC pinpoint test

A diagnostic trouble code names a circuit or system, not a failed component. The pinpoint test steps through voltage, ground and signal checks to isolate the actual fault.

Service interval rule

Maintenance is due at whichever comes first, the distance or the time interval. Short-trip and dusty use require shorter intervals than the standard schedule.

Brake imbalance failure

A difference in braking force between the two wheels of an axle, commonly above about 30 percent, causes pull and fails a roller brake test.