ASE T-Series cheat sheet

Stoichiometric air-fuel ratio for gasoline is about 14.7:1 by mass; mixtures near 12.5:1 make best power and near 15.5:1 give best economy.

Cylinder compression: all cylinders should be within 10% of the highest reading; a wet test that raises pressure indicates worn rings, while no change indicates valve or head gasket leakage.

Cylinder leakage test: air escaping at the oil filler = rings, at the throttle body = intake valve, at the tailpipe = exhaust valve, bubbles in the radiator = head gasket or crack.

Steady engine vacuum at idle near sea level is roughly 17-22 in-Hg; a low steady reading suggests a vacuum leak or retarded timing, and a fluctuating needle suggests valve or ignition faults.

Oil pressure rule of thumb is about 10 psi per 1000 rpm, with a minimum of roughly 10-15 psi at hot idle.

Base ignition timing is set with the timing connector (SPOUT/EST) disconnected at the specified idle speed; the PCM controls timing once the connector is restored.

Valve clearance is measured with the lifter on the base circle of the cam; adjustable rockers are set to zero lash plus the specified number of turns.

A clogged PCV system raises crankcase pressure and causes oil leaks and rough idle; a stuck-open PCV valve leans the mixture and also causes rough idle.

Cooling system pressure testing at the cap's rated pressure (typically 14-16 psi) with the engine off finds external leaks; a gauge that falls with no visible leak suggests an internal leak.

Diesel engines are compression-ignition; typical compression ratios are 16:1 to 24:1, and the heat of compression ignites the fuel as it is injected.

In a diesel cylinder balance test, the lowest cylinder should be within about 20-25% of the highest; a larger spread indicates a mechanical or injector fault.

Cetane number rates diesel ignition quality; 40-50 is typical for highway fuel, and higher cetane shortens ignition delay for easier cold starts.

Common-rail injection pressure ranges from roughly 5,000 psi at idle to more than 25,000 psi under load; high-pressure fuel can penetrate skin.

Exhaust smoke diagnosis: black = overfueling or air restriction, white = cold engine or retarded timing with unburned fuel, blue = lube oil being burned.

An engine compression-release (Jake) brake opens the exhaust valve near TDC of the compression stroke, releasing compressed air and adding retarding force.

A diesel that cranks but will not start with air in the fuel system must be primed; suction-side air leaks cause low power and stalling.

Coolant nitrite/SCA concentration and pH are checked at every service on wet-sleeve diesels to prevent liner pitting and cavitation.

Turbo boost is limited by the wastegate or VGT; low boost with black smoke points to a leaking charge-air cooler or boost pipe.

Axle ratio equals the ring gear tooth count divided by the pinion tooth count; a 41-tooth ring gear with an 11-tooth pinion gives 3.73:1.

Driveshaft U-joint operating angles should be equal and opposite within about 1 degree, and typically fall between 1 and 3 degrees, to prevent vibration.

Differential backlash is measured with a dial indicator at the ring gear and is commonly 0.005-0.010 in; pinion preload is set with shims or a crush sleeve.

Limited-slip differentials require friction modifier in the lubricant; chatter in tight turns indicates the modifier is missing or depleted.

Clutch free play is measured at the pedal or release bearing; too little free play wears the release bearing and causes slipping, too much causes incomplete release and gear clash.

An automatic transmission stall test compares engine stall speed to specification; low stall in all ranges suggests a restricted exhaust or weak engine, high stall suggests a slipping clutch pack.

Manual transmission lubricant level is correct when oil is at the bottom of the fill plug hole with the vehicle level.

A missing driveshaft balance weight or a seized U-joint produces a vibration that rises with road speed and often changes with load.

On a driveshaft without a double-cardan joint, the yokes at each end must be in phase (aligned in the same plane) to cancel speed variation.

Air brake governor cut-out is typically about 120-135 psi, with cut-in roughly 20-25 psi lower; a compressor that runs constantly indicates a governor fault or a leak.

A low-pressure warning device must activate before system pressure drops below 60 psi, and spring (parking) brakes begin to apply automatically at about 40-45 psi.

With the engine off and a full brake application, system leakage should not exceed about 3 psi per minute on a single vehicle and 4 psi per minute on a combination.

Slack adjuster pushrod stroke must not exceed the adjustment limit; the slack adjuster should sit near 90 degrees to the pushrod with 60-75 psi applied.

DOT 3 and DOT 4 brake fluids are glycol-based and compatible; DOT 5 is silicone-based and must never be mixed with them.

Brake drum diameter is measured and compared to the maximum turning diameter stamped on the drum; exceeding it requires drum replacement.

A pull to one side during braking indicates a restricted hydraulic hose, contaminated or wet lining, or a seized caliper or wheel cylinder.

Air dryer desiccant and purge valve service keeps moisture and oil out of the reservoirs; constant purging indicates a failed purge valve or excessive compressor oil carryover.

A damaged ABS tone ring or wrong wheel speed sensor air gap causes false ABS activation, often at low speed.

Positive caster improves straight-line stability and steering return; too little caster causes wander, and excessive caster increases steering effort.

Camber is the inward or outward tilt of the top of the wheel viewed from the front; the vehicle pulls toward the side with the more positive camber.

Total toe is the sum of both wheel toe angles, and incorrect toe is the most common cause of feathered-edge tire wear.

Included angle is camber plus steering axis inclination; an out-of-spec included angle with camber adjusted correctly indicates a bent spindle, strut, or knuckle.

Thrust angle is the rear axle centerline relative to the vehicle centerline; a non-zero thrust angle causes dog tracking and an off-center steering wheel.

Tapered roller wheel bearing end play is typically 0.001-0.005 in; excessive play causes wheel wobble and uneven brake pad wear.

Tire wear patterns: center wear = overinflation, both edges = underinflation, one edge = camber, scalloped or cupped = worn shocks or imbalance.

Air suspension ride height is set by the leveling valve; incorrect ride height changes driveline angles and headlamp aim.

A power steering flow and pressure test identifies a weak pump; relief pressure below specification with normal flow indicates internal pump wear.

Ohm's law is V = I x R and power is P = V x I; a 12-volt circuit drawing 4 amps has 3 ohms of resistance.

Voltage drop on a loaded circuit should be no more than about 0.5 volt total, with roughly 0.1 volt maximum across any single connection or ground.

Battery state of charge by open-circuit voltage at 70 degrees F: 12.6 V = 100%, 12.4 V = 75%, 12.2 V = 50%, 12.0 V = 25%.

A capacity (load) test applies half the CCA rating for 15 seconds; the battery should stay at or above 9.6 volts at 70 degrees F.

Charging voltage should be about 13.5-14.5 volts at 2000 rpm with accessories off; AC ripple above about 0.5 volt AC indicates failed alternator diodes.

Cranking voltage below about 9.6 volts while cranking indicates a weak battery, excessive circuit resistance, or a dragging starter.

Parasitic draw is measured with an ammeter in series at the battery after modules go to sleep; more than about 50 milliamps is excessive.

The J1939 CAN bus runs at 250 kbps (500 kbps on some newer networks) and measures about 60 ohms across CAN high and CAN low because two 120-ohm terminators are in parallel.

A relay is tested by measuring coil resistance across the coil terminals and by checking contact voltage drop while the circuit is loaded.

R-134a systems use PAG oil and R-1234yf systems use POE oil; the refrigerants and their lubricants are not interchangeable.

At 80-90 degrees F ambient, typical gauge readings are about 25-45 psi on the low side and 150-250 psi on the high side.

Low side low and high side low indicates an undercharge; low suction with high discharge indicates a restriction at the expansion valve or orifice tube.

Superheat is evaporator outlet temperature minus the saturation temperature for the low-side pressure; subcooling is measured at the condenser outlet.

The system must be evacuated and held at about 500 microns to remove moisture and confirm no leaks before charging.

Refrigerant leaks are located with an electronic detector, UV dye, or a nitrogen pressure test; refrigerant must never be vented to atmosphere.

A/C clutch coil resistance is typically a few ohms, and clutch air gap is usually 0.020-0.030 in; an open coil prevents compressor engagement.

No heat from the heater is caused by low coolant, a stuck-closed thermostat, a plugged heater core, or a mispositioned blend door.

Frost or condensation on the accumulator or receiver-drier downstream of the orifice tube indicates a restriction in the system.

FMCSA requires at least 4/32 inch tread depth on front steer tires and 2/32 inch on all other tires; tread wear indicator bars appear at 2/32 inch.

Cold tire pressure is set to the vehicle placard value; a hot tire must never be bled down because pressure rises normally with heat.

Air brake leak-down: with the system charged and the engine off, loss should not exceed about 2 psi per minute with brakes released and 3 psi per minute with brakes applied.

A periodic (annual) inspection is required every 12 months under 49 CFR 396.17, and drivers complete a DVIR when defects are found or as required by the carrier.

Coolant condition is checked with test strips or a refractometer for freeze point, pH, and SCA/nitrite concentration.

Serpentine belt condition is checked for glazing, cracks, and missing ribs, and deflection is compared to the manufacturer's specification, often about 1/2 inch per foot of span.

Battery terminals and cables are checked for corrosion and tightness; a voltage drop test under load is more reliable than an ohmmeter.

Oil analysis trends for iron, copper, silicon, fuel dilution, and coolant reveal internal wear or contamination before a failure occurs.

Wheel nut torque is verified with a calibrated torque wrench to fastener specification; rust streaks or loose fasteners indicate lost clamp load.