Cert III Heavy Commercial Vehicle flashcards

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

Compression ratio

The ratio of cylinder volume at bottom dead centre to volume at top dead centre. Diesel heavy vehicle engines commonly run between 14:1 and 23:1 for efficient ignition by compression.

Gross vehicle mass

The maximum total mass of a vehicle including chassis, body, fluids, payload, and occupants as specified by the manufacturer. Exceeding GVM is a compliance offence.

Hydraulic pressure formula

Pressure equals force divided by piston area. For a given load, increasing the piston area reduces the pressure required, which is why large bore brake pistons generate high clamping force.

Four-stroke cycle

The sequence of intake, compression, power, and exhaust strokes completed in two crankshaft revolutions. In a six-cylinder engine, power strokes occur every 120 degrees of rotation.

Voltage drop test

A live measurement of voltage lost across a circuit while current flows. Excessive drop on a heavy vehicle earth or cable indicates high resistance needing repair.

Battery isolation order

Disconnect the negative earth terminal first and reconnect it last. This sequence prevents a short circuit through the chassis when a tool contacts the positive terminal or bodywork.

Hierarchy of controls

The ranked order of hazard controls: eliminate, substitute, engineer, administrate, then personal protective equipment. Higher-order controls are more reliable because they do not depend on individual behaviour.

Gear ratio calculation

Driven gear teeth divided by driving gear teeth. A 12-tooth gear driving a 48-tooth gear produces a 4:1 ratio, reducing output speed to one quarter of input speed.

Constant mesh gearbox

A manual transmission where all gear pairs remain permanently in mesh. Sliding synchroniser sleeves lock the chosen gear to the output shaft rather than moving gears into mesh.

Synchromesh baulk ring

A cone-shaped ring that frictionally equalises shaft speeds before the dog teeth engage. Worn or glazed baulk rings cause gear clash, especially on downshifts.

Range change section

A gearbox section that doubles the number of available ratios by selecting high or low range. It is usually air-operated and must only be shifted when the vehicle is stationary or at low speed.

Splitter section

A gearbox section that divides each main ratio into two steps, commonly around 0.8. It provides closer ratio steps for gradient work and is operated by a pre-select switch.

Clutch free play

The clearance between the release bearing and clutch fingers when the pedal is released. Insufficient play causes the bearing to spin continuously, leading to slip and premature failure.

Twin countershaft design

A manual gearbox using two layshafts that share torque load. This halves tooth loading, increases torque capacity, and is common in heavy commercial vehicle transmissions.

Output shaft speed

Input shaft speed divided by the gear ratio. An input of 1800 rpm through a 4:1 reduction produces 450 rpm at the output shaft, assuming no slip.

Torque converter stall speed

The engine speed reached at full throttle with the output shaft held stationary. It is a key diagnostic reference for converter and engine matching.

Torque multiplication

The torque increase produced by a torque converter's stator redirecting fluid. Multiplication is greatest at stall and falls to approximately 1:1 at the coupling point.

Planetary gear set

A gear set with sun, planet carrier, and ring gears. Ratios are selected by holding one member, driving another, and taking output from the third.

Transmission fluid check

Automatic transmission fluid is checked at operating temperature with the engine running and the selector in the specified position. Incorrect procedure gives a false level reading.

Shift flare

A rise in engine speed during a shift without corresponding vehicle acceleration. It indicates slipping clutch packs, low fluid pressure, or a faulty control solenoid.

Transmission retarder

A hydraulic or electric device that slows the vehicle without using service brakes. It must be switched off on low-friction surfaces to prevent drive wheel lock-up.

Coolant contaminated ATF

Coolant entering the transmission cooler turns fluid milky and damages friction material. Repair requires flushing the system and fixing the cooler, not only replacing fluid.

Lock-out tag-out

The isolation procedure that identifies all energy sources, isolates them, applies locks and tags, and verifies de-energisation by test before maintenance begins.

Fire extinguisher classes

Class A for ordinary combustibles, B for flammable liquids, C for flammable gases, and E for electrical equipment. Selecting the wrong class can worsen the fire.

Manual handling control

Risk reduction by mechanical aids, team lifting, and keeping the load close to the body's centre of gravity. Personal protective equipment alone does not control the hazard.

Waste oil disposal

Used oil is contained, stored separately from solvents and coolant, and sent to a licensed recycler. Pouring it to drain or soil is an environmental offence.

Diagnostic process

Verify the complaint, gather information, test to isolate the fault, repair, then confirm the repair. Skipping verification risks repairing a symptom rather than the cause.

Diagnostic trouble code

A stored code identifying a circuit or system fault, not necessarily the failed component. It is a starting point for testing rather than a final diagnosis.

Compression test comparison

Cylinder pressures are compared against specification and each other. A low cylinder indicates ring, valve, or head gasket leakage and requires further isolation testing.

Torque-to-yield fastener

A fastener tightened to a specified torque then rotated through a set angle, stretching it into yield. It is single-use and must be replaced rather than reused.

Micrometer reading

The measurement is the barrel scale plus the thimble scale. A 0.01 mm resolution requires reading square to the scales to avoid parallax error.

Vernier zero error

A reading shown when the jaws are fully closed. A positive zero error is subtracted from each measurement to obtain the true dimension.