Drive Axles and Differentials
Axle and carrier fundamentals
The carrier houses the pinion, ring gear, differential case with side gears and spider gears, and bearings set with preload. Single-reduction hypoid axles are most common; two-speed and planetary double-reduction axles also exist. Tandems use a power divider with an interaxle differential; locking the interaxle or wheel differential is for slippery surfaces only and must never be done on dry pavement at speed.
Lubrication: check level at the fill plug on the carrier - level with the bottom of the hole. Use the OEM-specified GL-5 grade. Limited-slip units need friction modifier or the specified lubricant to prevent chatter in turns. A leaking pinion or wheel seal is often caused by a plugged housing breather or excessive bearing preload, not just a failed seal.
Noise diagnosis traps:
- Ring-and-pinion whine that changes between drive and coast indicates a gear pattern or backlash problem.
- Noise in turns but not straight ahead points to side gears, spider gears, or limited-slip clutch packs.
- A rumble that varies with load and persists while coasting in neutral suggests a pinion or carrier bearing.
- On a tandem, identify which axle is noisy with a chassis-ear test before removing anything.
Ratio and setup: divide ring gear tooth count by pinion tooth count. A 41-tooth ring gear with a 6-tooth pinion gives 6.83:1. Backlash and pinion depth are set with a dial indicator and marking compound; replacing a seal does not change either.
Rule of thumb: never reuse gear oil, and after any carrier work re-check oil level, breather condition, and fill-plug torque.