What does a differential do and how do you know it has a fault?

Every time you take a corner, the outer wheel travels further than the inner one, so the two driven wheels must turn at different speeds. The differential is the set of gears that allows this while still sending power to both.

On most front-wheel-drive manual cars it sits inside the gearbox casing and shares the same oil. On rear-wheel-drive cars and many four-wheel-drive vehicles it is a separate unit in the axle, with its own oil. That difference matters when something goes wrong.

How do I know if my differential is faulty?

Differentials give warnings through noise. A whine that changes with speed, a hum that varies when you accelerate or lift off, or a clunk when you take up drive are all possible signs.

Because the noise travels through the car, it can be hard to place. A wheel bearing, a tyre or a driveshaft joint can all sound alike, and only a proper inspection will separate them.

• Whine rising and falling with speed
• Rumble or growl in corners
• Clunk on pulling away or changing direction
• Oil seen leaking near the axle or gearbox

What causes differential wear?

Low or old oil is a major cause. Gears and bearings depend on a film of oil, and leaking seals or missed oil changes allow metal to wear faster. Hard use, heavy loads and sudden shock loads add to the strain.

Bearings can also fail on their own, creating a roaring noise. In a front-drive car the differential sits in the gearbox, so a fault there often shows up as a gearbox noise.

What can I check myself?

Look under the car for fresh oil marks on the ground and note which area they are from. Listen for the noise in different situations: straight driving, gentle bends, accelerating and coasting. Make notes, because that information is useful to a technician.

Check your handbook for the correct oil type and service advice for your axle or gearbox. Do not top up with the wrong oil, and do not open any filler plugs unless you know what you are doing, as the right level and grade vary between vehicles.

What does a garage do about differential problems?

We would inspect the unit for leaks, play and noise, and check the oil level and condition, as metal particles in the oil are a clue to internal wear. Sometimes a leaking seal or a bearing can be dealt with, while heavy gear damage may mean replacing or reconditioning the unit.

We cannot promise a repair without inspecting the car. The cost drivers are the type of unit, whether it is part of the gearbox, the parts needed and the labour to remove and refit it.

When should I stop driving?

Stop if the noise turns into grinding, if there is a serious oil leak, or if the car lurches or loses drive. A failed differential can lock up or break, and a locked axle is dangerous at speed.

If you notice only a faint whine, book it in soon rather than waiting, as running with damaged gears usually makes the repair larger.

When you ask for a quote, tell us the make, model, whether it is front or rear drive and exactly when the noise appears. That helps us judge whether the unit, a bearing or something else is the likely cause.