DTC DTC Drivetrain and Transmission Systems 2 — Questions and Answers
Question 1: What is the primary function of a differential in a diesel truck's drive axle?
- To increase ground clearance
- To allow drive wheels to rotate at different speeds during turns (Correct answer)
- To engage four-wheel drive
- To regulate axle lubrication pressure
Correct answer: To allow drive wheels to rotate at different speeds during turns
The differential allows the inner and outer drive wheels to rotate at different speeds while still receiving engine torque, preventing tire scrub during cornering.
Question 2: Which fluid is typically used in heavy-duty diesel truck manual transmissions?
- Engine oil (15W-40)
- Automatic transmission fluid (ATF)
- Gear oil (GL-4 or GL-5) (Correct answer)
- Power steering fluid
Correct answer: Gear oil (GL-4 or GL-5)
Heavy-duty manual transmissions use gear oil rated GL-4 or GL-5 to lubricate gears, bearings, and synchronizers.
Question 3: A diesel technician notices a differential whine that increases with vehicle speed but disappears when coasting. What is the most likely cause?
- Low differential fluid level
- Pinion gear bearing wear (Correct answer)
- Ring gear damage
- Axle shaft spline wear
Correct answer: Pinion gear bearing wear
Pinion bearing wear typically produces a load-dependent whine that is most pronounced under acceleration and quiets during coast when load is reduced.
Question 4: What is the purpose of a limited-slip differential (LSD) in a diesel work truck?
- To allow free-spinning of one wheel for turns
- To transfer torque to the wheel with more traction when one wheel slips (Correct answer)
- To disengage the rear axle at highway speeds
- To reduce differential gear noise
Correct answer: To transfer torque to the wheel with more traction when one wheel slips
A limited-slip differential uses clutch packs or a locking mechanism to bias torque toward the wheel with the most traction, improving off-road and low-traction performance.
Question 5: During a transmission service on a heavy-duty diesel, what is the correct procedure for checking fluid level in a manual transmission?
- Check the dipstick with the engine at operating temperature
- Remove the fill plug; fluid should be at the bottom of the fill hole thread (Correct answer)
- Check the sight glass on the side of the transmission
- Remove the drain plug and catch fluid to measure volume
Correct answer: Remove the fill plug; fluid should be at the bottom of the fill hole thread
Manual transmissions use a fill plug on the side of the case; fluid should be level with the bottom of the fill hole opening to indicate a proper fill.
Question 6: What is the most likely cause of a diesel truck automatic transmission that shifts harshly or bangs into gear?
- Low engine coolant level
- Worn throttle position sensor or miscalibrated shift points (Correct answer)
- Cracked bell housing
- Faulty air filter
Correct answer: Worn throttle position sensor or miscalibrated shift points
A worn or miscalibrated throttle position sensor sends incorrect load data to the TCM, causing harsh or abrupt shift events.
DTC DTC Drivetrain and Transmission Systems 2
What is the primary function of a differential in a diesel truck's drive axle?