Drivetrain and Transmission Flashcards

A fleet technician notices that a coach's Allison transmission defaults to manual mode and will not auto-shift. Which system failure would MOST likely cause this?

Answer: Loss of communication between the TCM and engine ECM (J1939 datalink fault)

Allison transmissions rely on J1939 CAN bus communication with the engine ECM for throttle position and load data; a datalink fault causes the TCM to enter a limp-home mode.

When measuring pinion bearing preload on a rear axle after a ring and pinion replacement, what tool is used?

Answer: Inch-pound torque wrench and rotating torque method

Pinion bearing preload is measured using an inch-pound torque wrench to check the rotational drag (rotating torque) required to turn the pinion, compared to the manufacturer's specification.

What is the function of the synchronizer blocking ring in a manual transmission?

Answer: It prevents gear engagement until the gear and shaft speeds are matched

The synchronizer blocking ring uses friction to equalize the speed of the sliding clutch and the gear being engaged, blocking engagement until synchronization is complete to prevent gear clash.

During a transmission fluid analysis on a heavy-duty automatic, the technician finds the fluid is milky white. What does this indicate?

Answer: Water or coolant contamination

Milky or foamy white transmission fluid is a classic indicator of water or engine coolant contamination, often from a leaking transmission cooler inside the radiator.

On a Spicer angle-spring style clutch, what is the purpose of the torsional springs in the clutch disc hub?

Answer: They absorb driveline shock and dampen torsional vibration

Torsional damper springs in the clutch disc hub flex to absorb sudden torque spikes and smooth out driveline vibration between the engine and transmission.

A bus technician finds that the Allison transmission will not upshift past 3rd gear and the 'Check Trans' light is on. After pulling codes, a P0721 (Output Speed Sensor rationality) code is present. What is the BEST next step?

Answer: Check the OSS wiring harness for damage, shorts, or poor connections before replacing the sensor

A rationality code often indicates a wiring issue rather than a failed sensor; inspecting the harness for chafing, connector corrosion, or opens should precede component replacement.

What is the correct method for checking backlash in a heavy-duty rear axle ring and pinion gear set?

Answer: Mount a dial indicator on the axle housing with the plunger against a ring gear tooth and rock the ring gear

A dial indicator mounted with its plunger contacting the face of a ring gear tooth measures the rotational play (backlash) while the pinion is held stationary.