Drivetrain and Transmission Flashcards

A Class 8 truck exhibits a harsh 1-2 upshift in an Allison automatic transmission. What is the MOST likely cause?

Answer: Faulty turbine speed sensor

A faulty turbine speed sensor provides incorrect shaft speed data, causing the TCM to miscalculate shift timing and produce harsh upshifts.

When performing a stall test on a torque converter, what does a stall speed BELOW the manufacturer's specification indicate?

Answer: Low engine power or a seized stator

A stall speed below spec indicates the engine cannot develop enough power, or the stator is seized and is not allowing proper fluid coupling multiplication.

Which component in a standard Spicer-style driveshaft prevents vibration caused by the angularity of universal joints?

Answer: Phased yokes

Phased (timed) yokes ensure the front and rear U-joints operate in the same plane, canceling out speed fluctuations and preventing driveline vibration.

A truck technician is diagnosing a manual transmission that pops out of fifth gear under load. What is the MOST probable cause?

Answer: Worn detent balls and springs

Worn detent balls and springs cannot hold the shift rail in the selected gear position, allowing the transmission to pop out under load.

In a tandem drive axle bogie, what is the purpose of the interaxle differential lock?

Answer: It couples both axle input shafts to distribute torque equally regardless of traction

The interaxle differential lock mechanically couples the forward and rear drive axle inputs so torque is equally distributed, preventing one axle from spinning freely when traction is lost.

Which type of lubricant is typically specified for a Meritor-style hypoid rear axle?

Answer: SAE 75W-140 GL-5 gear oil

Hypoid axle gears require GL-5 rated gear oil with extreme-pressure additives, and modern Meritor axles commonly specify SAE 75W-140 synthetic GL-5 for temperature stability.

A technician notices the speedometer reads higher than actual vehicle speed on a truck that recently had larger tires installed. What drivetrain calibration is required?

Answer: Reprogram the vehicle speed sensor (VSS) pulse count in the ECM

Larger tires travel farther per revolution, so the ECM's VSS pulse-per-mile calibration must be updated to reflect the new tire circumference.