ASE T-Series key terms

The ideal mass ratio of air to gasoline, about 14.7 to 1, at which the fuel burns completely with the oxygen available. The control module uses oxygen sensor feedback to hold the mixture near this ratio during light-load, closed-loop operation.

A piezo-electric sensor bolted to the engine block that generates a voltage when it detects the vibration of abnormal combustion. The control module responds by retarding spark timing to protect pistons and bearings.

A sensor mounted in the intake ahead of the throttle that measures the mass of air entering the engine, usually with a heated element. The control module uses that signal to calculate base fuel delivery and engine load.

A transformer that steps battery voltage up to the thousands of volts needed to jump the spark plug gap. Coil-on-plug designs mount one coil directly above each plug and eliminate spark plug wires.

A system that routes blow-by gases from the crankcase back into the intake so they are burned instead of escaping to the atmosphere. A stuck valve can cause oil consumption, rough idle, or seal leakage.

A diesel layout in which a high-pressure pump fills a single accumulator rail and the control module opens each injector electrically. Rail pressure and injection timing are controlled independently of engine speed.

A valve that diverts exhaust gas around the turbine once boost reaches its target, limiting maximum boost pressure. It may be opened by a spring-and-diaphragm actuator or controlled electronically by the engine module.

An exhaust aftertreatment device that traps soot from diesel exhaust. The engine module periodically raises exhaust temperature, a process called regeneration, to burn off the collected soot and restore flow.

An aftertreatment system that injects diesel exhaust fluid into the exhaust ahead of a catalyst, where it reacts with nitrogen oxides to form nitrogen and water. The module monitors dosing and derates power if fluid runs out.

A small electric heating element threaded into the combustion chamber of a diesel engine. It warms the air during a cold start so the injected fuel ignites reliably and white smoke is reduced.

A small, early injection of fuel delivered just before the main charge in a diesel engine. It raises cylinder pressure and temperature gradually, which reduces combustion noise and nitrogen oxide formation.

A flexible coupling between two shafts that transmits power while the shafts operate at an angle. A worn joint typically shows up as vibration, clunking, or looseness felt when the shaft is rocked by hand.

The alignment of the yokes at each end of a two-piece driveshaft so both joints work in the correct relationship. Incorrect phasing produces a vibration that changes with vehicle speed.

The small amount of rotational free play between the ring gear and pinion before their teeth contact. It is checked with a dial indicator and must fall within specification to prevent whine or tooth damage.

A fluid coupling that connects an automatic transmission to the engine, multiplying torque at low speed and allowing the engine to idle while the vehicle is stopped. Most units contain a lockup clutch for cruising efficiency.

The clearance between the release bearing and the clutch fingers, or between the pedal and its pushrod, that lets the clutch fully engage. Too little free play causes slippage and release bearing wear.

The lever on an air brake that turns pushrod stroke into rotation of the S-cam. Automatic versions self-adjust, but pushrod stroke must still be measured to confirm the brake is properly in adjustment.

A drum brake in which an S-shaped cam spreads the shoes outward against the drum when the pushrod moves the slack adjuster. It is the standard foundation brake on heavy truck steer and drive axles.

A combination air chamber that holds both a service diaphragm and a large spring which applies the parking brake when air pressure is released. The spring also applies the brake automatically during a loss of air pressure.

A valve that speeds application and release of the rear brakes by sending reservoir air straight to the chambers when it receives a control signal from the treadle valve. It shortens lag on long wheelbase vehicles.

Loss of braking force caused by heat, either from friction material outgassing or from drum expansion pushing the shoes away. It usually appears after repeated hard stops or a long loaded descent.

The forward or rearward tilt of the steering axis as viewed from the side of the vehicle. Positive caster improves straight-line stability and steering return, while too much raises steering effort.

The difference in distance between the front and rear of the tires as viewed from above. Incorrect toe scrubs the tires sideways and is one of the most common causes of rapid tread wear.

The direction the rear axle points relative to the vehicle centerline, calculated from rear toe readings. A thrust angle that is not zero leaves the steering wheel off center and makes the truck dog-track.

The distance between a specified suspension point and the frame or ground, used to set air suspension leveling valves. Incorrect ride height changes driveline angles, headlamp aim, and tire wear.

A steering box in which balls circulate between the worm shaft and nut to reduce friction, converting steering wheel rotation into Pitman arm movement. It is common on heavy trucks with solid front axles.

A measurement of the voltage lost across a cable or connection while current is flowing, taken with the meter set to DC volts. Excessive drop on a loaded circuit points to corrosion or a loose terminal.

Current that keeps flowing from the battery after the ignition and all accessories are switched off. Measured in milliamps, an excessive draw will drain the battery while the vehicle sits overnight.

The controller area network standard used on heavy trucks, in which modules broadcast data over a twisted pair at a defined bit rate. A shorted or open data line can stop communication across many systems at once.

A test that applies a small signal to estimate the battery's internal resistance and available cranking power without a heavy load. It is quick and does not require the battery to be fully charged first.

A set of six diodes inside an alternator that converts the alternating current made by the stator into direct current for the vehicle. A failed diode causes low output or an AC ripple that makes lights flicker.

A canister on the high side of an air conditioning system that stores liquid refrigerant and removes moisture with a desiccant. It often carries the sight glass and is replaced whenever the system is opened.

A valve that meters refrigerant into the evaporator while dropping its pressure so it boils and absorbs heat. It varies flow according to the temperature and pressure at the evaporator outlet.

The number of degrees a refrigerant vapor sits above its saturation temperature at the pressure being measured. Checking it shows whether the expansion valve is feeding too much or too little refrigerant.

Pulling a deep vacuum on an air conditioning system with a pump to remove air and moisture before charging. Holding the vacuum and watching the gauge also serves as a leak check.

A written report the driver completes at the end of the day listing defects found during pre-trip and en-route inspections. The carrier must review it and certify that repairs were made or were not needed.

A laboratory test of a used oil sample that reports wear metals, contaminants, fuel dilution, and additive levels. Read as a trend over several samples, it can reveal internal wear before a failure occurs.

Measuring how far the brake chamber pushrod travels with the brakes fully applied and comparing it to the manufacturer's limit. An over-stroke reading means the foundation brake or slack adjuster needs service.

An optical tool that measures coolant freeze point or concentration from the way light bends through a sample. It is more accurate than a hydrometer and needs only a few drops of fluid.