Differential: Operation, Diagnosis, and Setup
Differential fundamentals
The differential lets drive wheels turn at different speeds while delivering torque. The ring-and-pinion set changes direction and ratio; the carrier holds the side gears and spider gears. In a turn, the spider gears rotate on the cross pin so one side gear can speed up while the other slows. Limited-slip designs add clutch packs, cone clutches, a viscous coupling, or a Torsen gear set to limit speed difference. Clutch-type units often require friction modifier.
Diagnosis traps
Differential whine must be separated from tire noise. Gear whine usually changes with throttle and load: acceleration whine relates to the drive side of the ring-and-pinion; deceleration whine relates to the coast side. A howl that changes when turning can involve side gears or bearings. Chatter on turns in a limited-slip unit often means worn clutches or missing friction modifier. A single spinning wheel is normal for an open differential; both spinning under power suggests a limited slip or locker.
Setup concepts
Ring-and-pinion setup depends on pinion depth, pinion bearing preload, carrier bearing preload, backlash, and contact pattern. Backlash is measured with a dial indicator while holding the pinion; the pattern is checked with marking compound. Do not rely on backlash alone-pattern shows whether pinion depth is correct.
Rule of thumb
Acceleration whine: inspect the drive-side pattern and backlash. Deceleration whine: inspect the coast-side pattern. Always follow the manufacturer's specification; typical passenger-car backlash is small, often a few thousandths of an inch.