MTTC Automotive Trade Test cheat sheet

Gasoline stoichiometric air-fuel ratio is about 14.7:1 by mass; rich mixtures (lower AFR) raise power and CO, lean mixtures raise NOx and misfire risk.

Compression ratio = (swept volume + clearance volume) / clearance volume; a leak-down result above 20% loss indicates a sealing fault (rings, valves, or head gasket).

Four-stroke order: intake, compression, power, exhaust; a cylinder needs spark, fuel, compression, and correct timing to fire.

Engine vacuum at idle is normally 17-22 in-Hg at sea level; a steady low reading suggests intake leakage or late valve timing, a fluctuating needle suggests a sticking valve.

MAP sensor reports manifold absolute pressure; a high reading at idle (near barometric) indicates a vacuum leak or restricted intake.

Fuel trim: positive long-term trim (adding fuel) means a lean condition; negative trim (removing fuel) means rich; trims beyond about +/-10% warrant diagnosis.

Timing belt/chain alignment uses crankshaft and camshaft marks; interference engines bend valves if the belt fails, so replacement intervals are mileage-based.

Torque-to-yield head bolts are tightened in stages and often require angle torque; they are single-use and must be replaced, not reused.

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

A clutch that slips under load or engages at the top of pedal travel indicates worn friction disc or insufficient release bearing clearance.

Automatic transmission fluid level is checked with the engine running and fluid at operating temperature in most designs; low fluid causes slipping and delayed engagement.

Torque converter multiplies engine torque at low speed through stator action; the lockup clutch eliminates slip at cruise for efficiency.

Shift solenoid and pressure control solenoid faults set codes and cause harsh or missing shifts; line pressure is modulated by the PCM on electronic units.

A CVT uses a steel belt or chain between variable-diameter pulleys; ratio changes continuously and requires specified CVT fluid, not ATF.

Manual transaxle final drive and differential share lubricant; gear oil level is checked at the fill plug with the vehicle level.

A slipping automatic with burnt-smelling fluid often needs overhaul; a delayed cold engagement may be low fluid, filter restriction, or a sticking valve body.

Camber is wheel tilt viewed from the front (negative = top inward); caster is steering-axis tilt viewed from the side and provides straight-line stability.

Toe is the difference in distance between front and rear of the tires; toe-in improves stability, toe-out improves turn-in response.

Included angle = camber + SAI; a bent steering knuckle changes included angle, while a bent strut changes camber only.

Thrust angle is the average rear toe; a nonzero thrust angle causes dog-tracking and off-center steering wheel.

Shock absorbers control spring oscillation; a worn unit causes bounce, cupping tire wear, and longer stopping distances.

MacPherson strut combines spring, damper, and upper pivot; lower ball joint and control arm bushings carry lateral loads.

Tire wear patterns: center wear = overinflation; both edges = underinflation; one edge = camber; feathered = toe.

Power steering whine at full lock with foamy fluid indicates air in the system or a restricted return line; bleed by cycling lock-to-lock.

Hydraulic pressure = force / area (P = F/A); a larger master cylinder bore reduces pressure for a given pedal force.

Dot 3 and DOT 4 brake fluid are glycol-based and hygroscopic; DOT 5 is silicone and must never be mixed with glycol fluid.

Brake fade from heat reduces friction; DOT 4 has a higher dry boiling point than DOT 3, which matters for heavy use.

A low or spongy pedal indicates air in the hydraulics or a leaking component; bleed order is typically farthest wheel first.

Disc brake runout and thickness are measured with a micrometer and dial indicator; below minimum thickness the rotor must be replaced.

ABS prevents wheel lockup by modulating pressure; a wheel speed sensor fault disables ABS and usually illuminates the warning lamp.

Parking brake adjustment and equal cable tension are checked at the rear wheels; a dragging brake shows heat and uneven pad wear.

Proportioning valve or electronic brake distribution limits rear pressure to prevent rear-wheel lockup during hard stops.

Ohm's law: V = I x R; power P = V x I = I squared x R. A 12 V circuit drawing 3 A has 4 ohms resistance and dissipates 36 W.

Series circuit: total resistance adds and current is the same everywhere. Parallel circuit: voltage is the same and total resistance is less than the smallest branch.

Battery nominal 12.6 V at rest, about 13.5-14.5 V charging; below 12.4 V indicates a discharged or failing battery.

Voltage drop testing under load locates high-resistance connections; more than 0.5 V drop on a main cable is excessive.

Alternator output is regulated by the PCM or internal regulator; a diode failure causes AC ripple and dim flickering lights.

Starter draw test measures current; excessive draw with slow cranking suggests a dragging starter or seized engine.

Relays use a small control current to switch a large load; the coil terminals are typically 85 and 86, contacts 30, 87, and 87a.

CAN bus is a twisted-pair differential network; a shorted or open bus disables communication and sets multiple module codes.

Refrigerant cycle order: compressor, condenser, expansion device, evaporator, then back to compressor.

Pressure-temperature relationship: R-134a at about 70 degrees F evaporator saturation is roughly 70 psi; R-1234yf pressures differ and require separate gauges.

Subcooling = condenser outlet temperature subtracted from condensing temperature; low subcooling indicates undercharge.

Superheat = evaporator outlet temperature minus evaporating temperature; high superheat indicates low charge or a restricted orifice.

A/C system must be evacuated before charging; moisture and air reduce performance and form acids.

Blend door actuators control temperature by mixing air through the heater core and evaporator; a stuck door causes one-temperature output.

Cabin air filter restriction reduces airflow; refrigerant leak detection uses electronic sniffers, UV dye, or pressure decay.

Heater performance depends on coolant flow through the heater core and proper thermostat operation; a stuck-open thermostat causes low heat.

A torque wrench is calibrated and stored at its lowest setting; torque values are applied to clean, lightly lubricated threads as specified.

Micrometer reads to 0.0001 in; vernier caliper to 0.001 in; dial indicator measures runout and end play.

Feeler gauges measure clearances such as valve lash and spark plug gap; plastigauge measures bearing oil clearance.

A multimeter in series measures current; in parallel measures voltage; the meter must be set to the correct function before connection.

Jack stands support the vehicle; a jack alone is never used for work under the vehicle.

Lift points are specified by the manufacturer; improper lifting can bend underbody panels and damage fuel or brake lines.

Personal protective equipment: safety glasses, gloves, and hearing protection; eye protection is mandatory when grinding or using compressed air.

Fuel and solvent storage follows local fire codes; rags soaked in solvent go in a covered metal container.

Lesson plans align to program standards and include objectives, materials, procedure, assessment, and safety review.

Formative assessment checks understanding during instruction; summative assessment measures mastery at the end of a unit.

Demonstration method (show, then guided practice, then independent practice) suits psychomotor tasks like brake service.

Competency-based instruction requires learners to demonstrate a skill to a stated standard before advancing.

Differentiation adjusts content, process, or product for varied learners, including English learners and students with IEPs.

Safety orientation and tool check-out procedures are documented before students enter the shop.

Assessment rubrics list criteria and performance levels so grading is consistent across instructors.

Student organizations and industry credentials connect coursework to employment and should be integrated into the curriculum.

Right-to-know and hazard communication require SDS access and labeling for every chemical in the shop.

Used oil, antifreeze, and refrigerant are regulated waste streams; refrigerant must be recovered, not vented, under the Clean Air Act.

Battery and scrap metal recycling is required; lead-acid batteries are never placed in general trash.

Shop ventilation and exhaust extraction are used when running engines indoors to prevent carbon monoxide exposure.

A spill kit and clearly posted emergency numbers are required; report injuries per OSHA recordkeeping rules.

Lockout/tagout and disconnecting the battery before airbag or electrical work prevent unexpected energization.

Professionalism includes accurate repair orders, customer communication, and honesty about what was and was not repaired.

Personal conduct and cleanliness of the work area support safety and reflect on the employer and the trade.