Differentials and transfer cases: the units people remember after the whine

Differentials and transfer cases have neither a warning lamp nor a line on a standard service schedule. So they get remembered once the whine starts, when the repair costs several times the oil change that was skipped.

What they do

A differential lets the wheels on one axle turn at different speeds in a corner. Without it the inner wheel would scrub at every turn.

A transfer case splits torque between axles on four-wheel drive.

A rear axle coupling on crossovers is a separate unit with its own oil and its own electronics.

The oil schedule people skip

Axle and transfer case oil has a replacement interval, and it is in the handbook. The problem is that it is not on the standard service menu and the owner does not know about it.

What happens to it: the oil works in harsh conditions — high tooth loads, heat, wear products. It loses its properties, and wear then accelerates.

Worth checking: level and condition. A metallic film on the magnetic plug is normal fine debris; large particles or swarf are a reason to stop and investigate.

Especially important after water: driving through deep water draws it in through the breathers. Emulsion in an axle destroys bearings within months; covered in the flood water article.

Wear signs

A whine that changes with load — rising under acceleration, changing on lift-off. That is the key difference from tyres and bearings, whose noise tracks speed; covered in the humming article.

Howling at a specific speed.

A clunk when pulling away or changing direction — backlash in the teeth or splines.

Leaks — pinion and flange seals.

Vibration at speed — the propshaft and its centre bearing.

Why mismatched tyres destroy four-wheel drive

The most important practical part.

Four-wheel drive assumes the wheels turn at roughly the same rate. A diameter difference creates a permanent speed difference the unit has to absorb — and it absorbs it through friction and heat.

What creates the difference:

• Different tread wear across axles.
• Different tyre models or sizes.
• Low pressure in one wheel.
• Fitting one new tyre instead of a set.

The consequence: an overheated coupling, differential wear, and on some cars a limp mode and a code.

The rule: on four-wheel drive, tyres are replaced as a set and pressures checked regularly; wear is covered in the tyre wear article.

What diagnostics shows

Coupling codes — overheating, mismatch, actuator error.

Coupling temperature in live data where available — under hard driving it rises and the system limits torque.

The four wheel speeds — a persistent discrepancy indicates different diameters.

ABS codes — the transmission uses the same signals for its calculations.

In short

• Axle and transfer case oil has a schedule absent from standard servicing.
• A whine that changes with load is a final drive, not tyres.
• Water from deep puddles enters through the breathers and emulsifies the oil.
• On four-wheel drive, tyres go as a set or the coupling suffers.
• Swarf on the magnetic plug is a reason to stop, not to top up.

Download the app — transmission codes and wheel speeds are read with an ELM327 adapter over Bluetooth SPP on Android.