ASE H3 Drive Train (Transit Bus) key terms

The complete system that carries engine power to the wheels, including transmission, driveshaft, differential, and axles. In a transit bus it also includes the angle drive or drop box.

A rotating tube with universal joints that transmits torque from the transmission output to the differential or angle drive. It must be phased and balanced to avoid vibration.

A cross-and-bearing joint that lets a shaft transmit torque through a changing angle. Worn needle bearings or a dry cross cause vibration, clunking, or snapping under load.

The correct alignment of the universal joint yokes on a two-piece driveshaft so their angles cancel vibration. Yokes on opposite ends of the tube must lie in the same plane.

The working angle between the transmission output, driveshaft, and pinion or angle drive. Excessive or unequal angles cause vibration and shortened universal joint life.

A splined yoke that allows the driveshaft to change length as the suspension moves. It needs proper lubrication and a free-sliding spline to prevent binding or thrust loads.

A rubber-cushioned support bearing that carries the middle of a two-piece driveshaft. A collapsed or torn cushion allows shaft whip and vibration under acceleration.

A gearbox, sometimes called a drop box, that turns power from a longitudinal driveshaft to the transverse axle in a rear-engine transit bus. It contains its own lubricant and gearing.

A gear assembly that splits torque to the drive wheels while allowing them to turn at different speeds in a turn. It contains a ring gear, pinion, and carrier.

The matched gear set that changes driveshaft speed and direction into axle speed and torque. Wear, misalignment, or wrong backlash causes whine or howl.

The small measured clearance between ring gear and pinion teeth. Too little causes overheating and scoring; too much causes impact noise and tooth breakage.

The distance the pinion gear sits from the ring gear centerline, set with shims. Incorrect depth moves the tooth contact pattern and causes noise or rapid wear.

The contact mark left by gear marking compound on the ring gear teeth after a test rotation. It shows whether pinion depth and backlash are correct.

The assembly that carries the drive wheels and transmits torque from the differential. It includes the housing, shafts, bearings, and wheel-end components.

A lip seal that keeps axle lubricant in the hub and dirt out. A failed seal leaks oil onto the brake shoes or drum and can cause a wheel fire or brake loss.

The rotating assembly that mounts the wheel to the axle or spindle. It contains bearings and seals and must be adjusted to the correct end play or preload.

A transmission that shifts gears automatically using hydraulic pressure and electronic control. In transit buses it typically includes a retarder and a torque converter.

A fluid coupling that multiplies engine torque at low speed and lets the engine idle in gear. It contains an impeller, turbine, and stator.

The stationary vane element inside a torque converter that redirects fluid to multiply torque. A failed stator one-way clutch causes poor acceleration and low stall speed.

A clutch inside the torque converter that mechanically locks the turbine to the impeller at road speed. It improves efficiency and reduces heat and fuel use.

The hydraulic control block that directs fluid to shift clutches and control pressure. Sticking valves or clogged passages cause harsh, delayed, or missing shifts.

An electrically operated valve that the transmission control module uses to direct fluid and command shifts. A failed solenoid sets a code and can cause a default gear.

The computer that monitors inputs and controls shift solenoids, line pressure, and converter lockup. It stores diagnostic codes and can default the transmission to a limp mode.

The hydraulic pressure the transmission pump supplies to operate clutches and valves. Low pressure causes slipping; high pressure causes harsh shifts.

A gear or vane pump driven by the converter that supplies fluid pressure and flow. A worn pump causes low pressure, slipping, and delayed engagement.

A hydraulic or electric auxiliary brake that slows the bus without using the service brakes. It reduces brake wear and heat on long downgrades.

A test that holds the throttle and brake in gear to check converter and engine performance. Abnormal stall speed points to a stator, engine, or clutch problem.

A diagnostic test that measures hydraulic pressure at specified ports and conditions. Results are compared to specifications to isolate pump, valve body, or clutch problems.

A driving evaluation of shift points, shift quality, noise, and vibration under load. It confirms complaints and helps separate engine, transmission, and driveline faults.

A shake or shudder felt through the floor or seats from rotating driveline parts. It is classified by speed and load to isolate joints, shafts, or gears.

A stored code set by the control module when a circuit or performance fault is detected. It guides testing but does not by itself identify the failed part.

The color, smell, and contamination level of transmission or axle lubricant. Burnt smell or metal debris signals overheating, clutch wear, or gear failure.

The process of finding the source of a fluid loss using cleaning, dye, and inspection. Common sources are seals, gaskets, cooler lines, and breathers.

Isolating a sound by its pitch, timing, and load dependence. Gear whine changes with speed; bearing growl changes with load; joint squeak changes with angle.

The manufacturer's required tightening value for a fastener. Correct torque prevents joint failure, leaks, and distortion of housings and covers.

A protective strategy in which the control module limits gears or power after a fault. It lets the bus move to a safe stop but must be repaired before service.