Engineer Motor Vehicles Maintenance flashcards

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

Volumetric efficiency

The ratio of actual air mass inducted per cycle to the theoretical mass that would fill the swept volume at ambient density. It falls with intake restriction and high engine speed.

Compression ratio formula

Compression ratio equals swept volume plus clearance volume, divided by clearance volume. Petrol engines typically run 9:1 to 12:1 and diesels 15:1 to 22:1.

Diesel ignition delay

The time between start of injection and start of combustion. Long delay causes a rapid pressure rise and diesel knock; higher cetane number shortens the delay.

Cylinder leakage test

Pressurising a cylinder at top dead centre on compression and measuring leakage. Bubbles in the radiator indicate a head gasket fault; air from the dipstick indicates ring or bore wear.

Blue versus black smoke

Blue smoke indicates burning oil from worn rings, valve guides, or turbo seals. Black smoke indicates over-fuelling or insufficient air, typical of a restricted air filter.

Torque-to-yield bolts

Fasteners tightened past their elastic limit and therefore single-use. Reusing them risks loss of clamp load and joint failure; replacement is required after each strip-down.

Preventive maintenance trigger

Service is due at the first of the distance, time, or engine-hour limit specified by the manufacturer. Whichever limit is reached first sets the service date.

Job card purpose

A record of vehicle identity, odometer, complaint, work performed, parts used, technician, and time. It supports costing, warranty claims, and traceability of repairs.

Overall gear ratio

Gearbox ratio multiplied by final drive ratio. Road speed equals engine rpm times rolling circumference, divided by overall ratio with unit conversion applied.

Clutch free play

The pedal travel before the release bearing contacts the pressure plate. Correct free play keeps the bearing unloaded when the pedal is released and prevents slip.

CV joint click diagnosis

A rhythmic click from the front wheel on full lock indicates a worn outer constant-velocity joint. Vibration under acceleration points to the inner joint or shaft imbalance.

Wheel bearing play check

Rock the wheel at twelve and six o'clock to feel for play. Rumble that changes with load and cornering confirms bearing wear rather than tyre noise.

Ackermann steering geometry

The inner wheel turns more than the outer wheel so all wheels roll about a common centre. Incorrect geometry causes tyre scrub and uneven wear.

Spongy brake pedal

A pedal that feels soft and compressible indicates air in the hydraulic system. Bleed the circuit and check for leaks before returning the vehicle to service.

Brake fluid hygroscopy

Brake fluid absorbs moisture from the air, lowering its boiling point and risking vapour lock. Replace at the manufacturer interval or when moisture exceeds about three percent.

Brake force imbalance

A large difference in braking force between left and right wheels causes pull and instability. Roller brake tester comparison above about thirty percent usually fails inspection.

Ohm's law and power

Voltage equals current times resistance, and power equals voltage times current. A twelve-volt circuit drawing five amps has 2.4 ohms resistance and dissipates sixty watts.

Voltage drop test

Measuring voltage across a cable or connection while current flows. More than about 0.5 volts drop on a main circuit indicates excessive resistance and needs repair.

Alternator output check

With the engine running and loads off, charging voltage should be about 13.8 to 14.8 volts. Lower output suggests worn brushes, a failed diode, or a faulty regulator.

CAN bus idle voltage

With the network at rest, CAN-High and CAN-Low each sit near 2.5 volts. A short to ground or between the lines disables communication across the network.

OBD-II code prefixes

P denotes powertrain, B body, C chassis, and U network codes. A code identifies a circuit or system, not necessarily the component that must be replaced.

Four-gas exhaust analysis

Measuring carbon monoxide, carbon dioxide, hydrocarbons, and oxygen. High CO and HC with low oxygen indicate a rich mixture; high oxygen with low CO2 indicates a lean misfire or leak.

Freeze frame data

Engine data captured at the moment a fault code is stored. It helps reproduce the conditions and confirms whether the fault is intermittent or continuous.

Side slip test

A rolling test that measures lateral wheel movement caused by alignment error. Excessive side slip indicates incorrect toe or camber and predicts rapid tyre wear.

Break-even hours

Fixed costs divided by the difference between hourly labour rate and variable cost per hour. It gives the billable hours needed before the workshop makes a profit.

Reorder level formula

Maximum usage multiplied by maximum lead time. Stock reaching this level triggers a new order, with safety stock covering demand variability.

Technician efficiency

Standard hours produced divided by hours actually worked. It measures how much flat-rate work a technician completes relative to the time available.

Straight-line depreciation

Annual depreciation equals original cost minus residual value, divided by useful life in years. It spreads equipment cost evenly across its service life.

Carbon monoxide exposure limit

The eight-hour time-weighted average limit for carbon monoxide is typically thirty parts per million. Exhaust must be extracted at the tailpipe to stay below it.

Battery charging safety

Charge in a ventilated area with eye protection. Hydrogen is explosive above about four percent in air, and acid spills are neutralised with sodium bicarbonate.

Jack versus axle stands

A jack is for lifting only; never work beneath a vehicle supported solely by a jack. Use axle stands or a lift rated above the vehicle weight.

Galvanic corrosion

Corrosion caused when dissimilar metals touch in the presence of an electrolyte. The less noble metal corrodes; isolate the joint with coatings or insulating washers.

Micrometer reading method

Read the main scale, then add the thimble division multiplied by the least count. A 0-25 mm micrometer with a 0.01 mm thimble resolves to 0.01 mm.

Thermal expansion formula

Change in length equals expansion coefficient times original length times temperature change. A half-metre aluminium part heated 100 K expands about 1.15 mm.