ASE G1 Auto Maintenance and Light RepairManual Drive Train and AxlesHard
During a pre-purchase inspection of a four-wheel-drive (4WD) vehicle, a technician observes that the front and rear driveshafts are rotating at different speeds while driving in a straight line on dry pavement in 4WD HIGH. This condition is NORMAL for which type of 4WD system?
- AElectronic shift-on-the-fly 4WD system
- BPart-time 4WD system
- CFull-time 4WD system
- DLocked differential 4WD system
Show answer & explanationAnswer & explanation
Correct answer: C. Full-time 4WD system
Full-time 4WD systems use a center differential (or equivalent) to allow the front and rear driveshafts to rotate at different speeds, accommodating speed differences between axles during turns or on varied surfaces. Part-time systems lock the axles together, prohibiting this difference.
Why the other options are wrong
- A. Electronic shift-on-the-fly refers to the engagement method, not the fundamental operational principle regarding differential action between axles.
- B. Part-time 4WD systems lock the front and rear driveshafts together, requiring them to rotate at the same speed. Driving on dry pavement in 4WD HIGH in a part-time system would cause 'driveline bind' due to speed differences.
- D. A locked differential system (center or axle) would prevent speed differences, leading to driveline bind on dry pavement.
Full-Time 4WD
A four-wheel-drive system designed to be used on all surfaces, including dry pavement, because it incorporates a center differential that allows speed differences between the front and rear axles.
- Can be used on dry pavement without driveline bind.
- Features a center differential or equivalent power-splitting device.
- Provides continuous power to all four wheels.
Memory trick: Full-time is 'free-time' for axles to spin differently.