Motor Vehicle Mechanics flashcards

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

Stoichiometric air-fuel ratio

The mass ratio of air to fuel that gives complete combustion; approximately 14.7:1 for gasoline. Deviations change emissions, power, and fuel economy.

Compression ratio

Ratio of cylinder volume with the piston at bottom dead center to volume at top dead center. Higher ratios raise efficiency and require higher-octane fuel.

Four-stroke cycle

Intake, compression, power, and exhaust strokes complete one cycle in two crankshaft revolutions. The camshaft therefore rotates at half crankshaft speed.

Ohm's law

Voltage equals current multiplied by resistance (V = I × R). Used to calculate expected current draw or voltage drop in a vehicle circuit.

Hydraulic brake pressure

Pressure equals force divided by piston area. A smaller master-cylinder bore produces higher line pressure for the same pedal force.

Diesel compression ignition

Diesel engines ignite fuel by heat of compression rather than a spark. Ratios of 16:1 to 23:1 raise intake air above fuel auto-ignition temperature.

Torque converter stall

The maximum engine speed reached with the turbine held stationary and full throttle applied. It indicates converter and engine matching.

Final drive ratio

Ring gear teeth divided by pinion gear teeth. Multiplying it by the gearbox ratio gives the overall ratio and determines engine speed per road speed.

Cold cranking amps

Current a battery delivers for 30 seconds at minus 18 degrees Celsius while maintaining at least 7.2 volts. Indicates starting capability in cold weather.

Tire aspect ratio

Sidewall height expressed as a percentage of section width. A 205/55 tire has a sidewall height of about 113 millimeters.

Clutch slip

Occurs when transmitted torque exceeds clutch capacity, allowing engine speed to rise without matching vehicle speed. Caused by wear, weak pressure plate, or oil contamination.

Cylinder leakage test

Pressurized air is applied to a cylinder at top dead center. Escaping air located at intake, exhaust, crankcase, or coolant identifies the leaking component.

Compression test procedure

Warm engine, throttle held open, all spark plugs removed, and a gauge fitted. Compare cylinders; low readings rising with added oil indicate ring wear.

Brake fluid hygroscopy

Brake fluid absorbs moisture from the air over time, lowering its boiling point. Moisture causes pedal fade under heavy braking and internal corrosion.

Tread wear indicator

Raised bars molded in tire grooves at 1.6 millimeters of remaining tread. Tires at or below this depth no longer meet minimum legal tread.

Torque-to-yield bolt

A fastener tightened past its elastic limit, usually by torque plus angle. It must be replaced rather than reused because it has stretched permanently.

Parasitic drain test

Measures current with the ignition off and modules asleep. A reading above roughly 50 milliamperes indicates a component not shutting down.

Fuel trim

Short-term and long-term corrections to injected fuel based on oxygen sensor feedback. Large positive trim suggests a lean condition such as a vacuum leak.

Voltage drop test

Measures voltage lost across a circuit while current flows. More than about 0.1 volts per connection indicates excessive resistance.

Freeze-frame data

Snapshot of sensor values and conditions stored when a diagnostic trouble code sets. It helps reproduce the fault and guides testing.

Readiness monitors

Self-tests the engine computer runs to verify emission systems. Continuous monitors run constantly; others need specific drive cycles to complete.

CAN bus termination

Two 120-ohm resistors at bus ends give about 60 ohms measured across the data lines. An incorrect value indicates an open or shorted network.

Ignition secondary waveform

Oscilloscope trace of firing voltage and burn time. High firing voltage suggests a wide plug gap or lean mixture; short burn time suggests rich mixture.

High-voltage safety

Hybrid and electric systems use orange cables. Remove the service disconnect and verify zero voltage before any work near high-voltage components.

Airbag deployment wait time

Period after battery disconnection during which the module retains backup power. Servicing before it expires risks unintended airbag deployment.

Refrigerant recovery

Refrigerant must be recovered and recycled with equipment matched to its type. R-134a and R-1234yf systems use different fittings and oils.

Jack stand rating

The load a stand safely supports, which must exceed the lifted corner weight. Stands are placed on level, firm ground at manufacturer lift points.

Three-C repair order

Complaint, cause, and correction documented on the repair order. This format supports warranty claims, customer communication, and technician accountability.

Technician efficiency

Flat-rate hours produced divided by clock hours worked, expressed as a percentage. It measures productivity against published labor time guides.

Symptom-to-system strategy

Diagnostic approach that maps the complaint to the affected system, checks service bulletins, then tests the most probable cause before replacing parts.

Supplemental estimate

Additional authorization required when diagnosis reveals faults beyond the original estimate. Work proceeds only after customer approval.

Caster angle

Tilt of the steering axis from vertical in side view. It produces self-centering and straight-line stability; too little causes wander at speed.

Brake rotor runout

Side-to-side variation measured with a dial indicator while the rotor turns on the vehicle. Excessive runout causes pedal pulsation during braking.