Motor Vehicle Mechanics cheat sheet

Four-stroke cycle order: intake, compression, power, exhaust - one power stroke per two crankshaft revolutions, so camshaft turns at half crank speed.

Stoichiometric air-fuel ratio is about 14.7:1 for gasoline by mass; rich mixtures (lower ratio) raise power and CO/HC, lean mixtures raise NOx and risk misfire.

Compression ratio = (swept volume + clearance volume) / clearance volume; gasoline engines typically 9:1–12:1, and raising it increases thermal efficiency but demands higher-octane fuel.

Engine displacement = bore² × π/4 × stroke × number of cylinders; torque peak depends on volumetric efficiency, not displacement alone.

Ohm's law V = I × R; series resistances add, parallel resistances follow 1/Rt = 1/R1 + 1/R2, and a parasitic drain above roughly 50 mA indicates a fault.

Hydraulic brake pressure P = F/A; a larger master-cylinder bore raises fluid volume but lowers line pressure for the same pedal force.

Wheel alignment angles: camber is vertical tilt, caster is steering-axis tilt affecting self-centering, toe is front-to-rear difference; total toe-in reduces straight-line instability.

Diagnostic trouble codes: P = powertrain, B = body, C = chassis, U = network; a P0300-series code indicates random or cylinder-specific misfire.

Diesel compression ignition needs 16:1–23:1 compression ratios to reach the 700–900 °C auto-ignition temperature at the top of the stroke.

Manual transmission gear ratio = driven gear teeth / drive gear teeth; overall ratio multiplies gearbox ratio by final drive ratio.

Torque converter multiplication occurs only when turbine speed is well below impeller speed; at coupling point slip is small and multiplication approaches 1:1.

A clutch slips when torque capacity is below engine torque: capacity ≈ friction coefficient × clamp force × mean friction radius × number of friction faces.

Tire markings: 205/55 R16 means 205 mm section width, 55% aspect ratio, radial construction, 16-inch rim; load index and speed rating follow separately.

Battery capacity is rated in ampere-hours at a 20-hour rate; cold cranking amps (CCA) measure 30-second discharge at −18 °C to 7.2 V.

Alternator output must exceed total electrical load plus charging current; a diode failure typically causes ripple and dim, pulsing lights.

Cooling system pressure raises boiling point by roughly 3 °C per 10 kPa of cap rating; a failed cap lowers boiling point and promotes overheating.

Preventive maintenance intervals follow the manufacturer's severe-service schedule when the vehicle does frequent short trips, towing, or dusty operation.

Engine oil viscosity grade (e.g., 5W-30) reflects low-temperature pumpability and high-temperature thickness; the second number is measured at 100 °C.

Brake fluid is hygroscopic; absorbed moisture lowers boiling point and causes pedal fade, so it is replaced on a time interval regardless of mileage.

Tire tread wear indicators appear at 1.6 mm; uneven wear patterns diagnose camber (one edge), toe (feather edges), or inflation (center or both shoulders).

A compression test on a warm engine with throttle open: low readings that rise after adding oil indicate worn rings; unchanged low readings indicate valve or head-gasket leakage.

Cylinder leakage test identifies the leak path by listening at the intake, exhaust, oil filler, or radiator; bubbles in coolant indicate head gasket or crack.

Battery parasitic drain test uses a clamp meter or series ammeter; measure after modules sleep, typically 20–60 minutes after key-off.

Torque specifications apply to clean, lightly lubricated threads unless stated dry; torque-to-yield bolts must be replaced, not reused.

A scan tool reads live data and freeze-frame; a digital multimeter verifies circuit resistance and voltage drop - a drop above 0.1 V per connection is excessive.

Voltage-drop testing under load is more reliable than resistance testing for high-current circuits such as starters and grounds.

OBD-II readiness monitors: continuous monitors are misfire, fuel system, and comprehensive components; the rest run only under specific enable conditions.

Fuel trim: short-term trim corrects immediately, long-term trim learns; positive trim above about +10% suggests a vacuum leak or low fuel pressure.

An oscilloscope pattern of the ignition secondary shows burn time and firing voltage; high firing voltage indicates a wide gap or lean mixture.

Actuator tests (injector balance, EVAP purge, throttle sweep) are performed with the key on and engine off only when the manufacturer's procedure permits.

CAN bus diagnosis: measure 60 Ω across the bus with the module disconnected; bus voltage near 2.5 V on both lines indicates an idle, healthy network.

Wiring repair: solder and heat-shrink for signal wires, crimped terminals for high-vibration areas; never pierce insulation for a permanent repair.

Personal protective equipment for vehicle work: safety glasses, gloves, steel-toe boots, and hearing protection when using air tools.

Lifting points are specified by the manufacturer; jack stands must be rated above the lifted load and placed on level, hard ground.

Hybrid and electric vehicles use orange cables and high-voltage systems; the service disconnect is removed and the system verified de-energized before work.

Airbag modules contain a stored gas or pyrotechnic charge; wait the manufacturer's specified time after disconnecting the battery before servicing.

Refrigerant handling: R-134a and R-1234yf require separate recovery equipment; venting refrigerant to atmosphere is prohibited in most jurisdictions.

Used oil, coolant, and brake fluid are hazardous waste and must go to licensed collection; never mix waste streams.

Fire classes: Class B for flammable liquids, Class C for energized electrical; a CO2 or dry chemical extinguisher suits engine fires.

Hand tool safety: use the correct size to avoid rounding fasteners, pull rather than push a wrench, and never use a cheater bar beyond tool rating.

Customer complaint to verification: confirm the symptom, gather conditions (hot/cold, speed, load), then test before replacing parts.

A repair order must record complaint, cause, and correction, plus parts and labor; the three-C format supports warranty and customer communication.

Labor time guides list operation times; the technician's efficiency = flat-rate hours produced / clock hours worked.

Diagnostic strategy: start with a symptom-to-system map, check for technical service bulletins, then test the most probable cause first.

Road test before and after repair to verify the fix; document baseline conditions such as ambient temperature and fuel level.

Parts return and core handling: tag warranty parts, retain them per policy, and record serial numbers on high-value components.

Estimates require authorization before additional work; a supplemental estimate is needed when diagnosis reveals further faults.

Suspension geometry: positive camber tilts the top of the wheel outward; excessive negative camber wears the inner tread edge.

Caster is not adjustable on many strut vehicles without plates; too little caster causes wander, too much raises steering effort.

Toe is the most tire-wearing angle; total toe should be set with the steering wheel centered and the vehicle at ride height.

Shock absorbers control spring oscillation; a worn unit causes cupped tire wear and continued bouncing after a bump.

Ball joint play is checked with the wheel unloaded; vertical play indicates wear, while radial play may be normal bearing clearance.

Wheel bearing noise typically changes with cornering load; a bad bearing on the loaded side gets louder when that side is loaded.

Brake rotor lateral runout above specification causes pedal pulsation; measure with a dial indicator on the vehicle.

Tire balance corrects vibration at a specific speed; runout and radial force variation cause vibration that balance cannot fix.