Car differential: how it works, types, symptoms and rebuild

A car differential is the set of gears in the driveline that splits engine power between two wheels and lets them turn at different speeds in a corner. On front-wheel drive it sits inside the gearbox, on rear-wheel drive in the rear axle. A failing differential usually announces itself with whining, droning or knocking.

That last part is where it gets tricky. You hear a hum under the car that rises with road speed, but is it the differential, a wheel bearing or a driveshaft? Forums say all sorts of things, and a differential rarely sets a fault code that points you in the right direction.

This article explains what a differential is, how it works, which types exist and how to recognise a fault. After that you will read what a differential rebuild involves, what decides the cost and how to look after it. We base it on technical sources and on what we see in the workshop at ATR Transmissies in Montfoort, in the Netherlands, including two documented cases from our own work.

What is a car differential?

A car differential is a set of gears between two driven wheels that lets those wheels turn at different speeds while both keep receiving drive. The name says it: it handles the difference.

You need it in every corner. The outer wheel travels further than the inner one, so it has to turn faster. If both wheels were rigidly connected, the tyres would scrub and skip, wear quickly and put needless strain on the driveline. Wheels that are not driven, such as those on a trailer, can turn independently and do not need a differential.

The differential has a second job. In many cars it is also the final drive: the last reduction in speed between the gearbox or propshaft and the wheels, because the pinion has fewer teeth than the crown wheel it drives.

How does a differential work?

Drive enters the differential through a small gear, the pinion. The pinion turns a large crown wheel (in American English the ring gear, hence “ring and pinion”). The crown wheel is bolted to the differential carrier, which holds small planet gears, often called spider gears. These pass the drive to two side gears, and each side gear is connected to a driveshaft for the left or right wheel.

The parts of a differential

Straight ahead and in a corner

When you drive straight ahead, the whole carrier turns as one block and the planet gears do not spin on their own shaft. Both wheels turn at the same speed. As you steer into a corner, the inner wheel slows down. The planet gears then start to roll around their shaft, so the outer wheel speeds up by exactly as much as the inner wheel slows down.

The weakness of an open differential

An ordinary (open) differential can send most of the drive to the wheel with the least grip. If one wheel is on ice, mud or wet grass, that wheel spins and the wheel with grip gets almost nothing. That is why there are differentials that limit or block the difference. More on those under the types.

Where is the differential in your car?

Where the differential sits depends on which wheels are driven:

• Front-wheel drive: the differential is built into the gearbox, which is then called a transaxle. It often shares oil with the gearbox, although some gearboxes have a separate oil compartment for the final drive.
• Rear-wheel drive: the differential sits in the middle of the rear axle, between the rear wheels, and receives drive through the propshaft. On vans and older cars the whole assembly is often just called “the rear axle” or “the diff”.
• Four-wheel drive: there is a differential on the front axle and one on the rear axle, which is why people talk about front and rear differentials, and often a centre differential or clutch in the transfer case that splits drive between front and rear.

On four-wheel drive the differentials and the transfer case work closely together, but the transfer case is a separate part with its own complaints. If you drive a BMW with xDrive, read about BMW xDrive transfer cases. For Audi we cover the 0CX transfer case and for Mercedes the A251 transfer case.

Does an electric car have a differential?

Yes, in most cases. An electric car with one motor per axle has a fixed reduction gear with a differential, usually in the same housing. Only cars with a separate motor for each driven wheel can do without a mechanical differential, because software then controls each wheel’s speed. How an EV drivetrain is built up is explained under gearboxes in electric cars.

What types of differential are there?

There are roughly five types. They differ mainly in how they deal with a wheel that loses grip.

What is a limited slip differential?

A limited slip differential (LSD) limits the speed difference between the two wheels. If one wheel spins, the other still gets drive. A full locking differential goes a step further and ties both wheels rigidly together for a while. That helps off road, but on the public road and in corners on tarmac the lock should be released.

A Torsen only works when there is torque on it. Audi describes how its Torsen centre differential locks only under load and unlocks when you lift off. If a wheel is completely off the ground, a Torsen behaves much like an open differential.

Electronic limited slip

Many modern cars have no mechanical LSD and let the stability control (ESP) brake the spinning wheel instead. That sends drive back to the wheel with grip. There are also electronically controlled multi-plate clutches that decide how much drive goes where in each situation.

How do you know your differential is failing?

You almost always recognise a failing differential by sound first: a whine, howl or drone that rises with road speed, or a click or clunk in corners. Vibration, oil leaks and play in the driveline often follow.

Watch out for these symptoms:

• Whining or howling that gets louder with speed, sometimes only when accelerating or only when lifting off.
• Droning or humming that rises with speed and can change tone in a corner.
• Clicking, knocking or rattling in corners, especially when turning slowly and tightly.
• A clunk or jolt when pulling away, when getting on and off the throttle or when switching between forward and reverse.
• Vibration through the floor or steering wheel that increases with speed.
• Oil leaks at the rear axle, the pinion seal or the driveshafts. Differential oil often has a sharp, sulphurous smell.

Which noise points to which cause?

The moment you hear the noise says a lot about the cause. Use this table as a first estimate. Certainty only comes after a test drive and inspection.

Why a differential rarely sets a fault code

A conventional differential is purely mechanical. There is no sensor to report a worn bearing, so the warning light usually stays off. In one of our own cases, a Mercedes Sprinter 906 with a whistling noise under light load that rose and fell with revs, there were no fault codes at all. The cause was in the rear axle. You do not find a complaint like that with a code reader. You find it with a test drive and a targeted inspection.

That is also why we at ATR Transmissies always drive the car ourselves first. A differential complaint can sound very much like a failed wheel bearing, a worn driveshaft or a bearing in the gearbox. Replace parts without measuring and you can easily pay for the wrong one. Not sure whether the noise comes from the gearbox? See also symptoms of a failing gearbox.

What causes a differential to fail?

Most differential problems come from wear combined with oil that falls short. The gears run under high pressure, and once lubrication fails, bearings and gears wear out fast.

• Too little or old oil. Oil that was never changed, or that drops below the right level through a leak.
• Leaking seals or gaskets, especially the pinion seal and the seals at the driveshafts.
• Worn bearings. The pinion and carrier bearings are the most common source of droning and whining.
• Heavy loads. Towing a caravan or trailer, lots of power or lots of hard driving. On a Mercedes C63 AMG (C205) that we rebuilt, the bearings in the rear differential had failed early under that kind of load, and the other bearings already showed clear wear on their running surfaces.
• Incorrect set-up after an earlier repair, where the contact pattern or bearing preload was not set properly. That can bring the noise and wear back within a short time.
• Wheelspin. A wheel spinning for a long time on ice or mud makes the planet gears turn at high speed.
• Dirt and water. Through worn seals, water and sand can get in, especially on cars that are driven off road or through water.

Can you keep driving with a bad differential?

We advise against driving on with a bad differential. A bearing that drones slightly may last a while, but the damage grows, and what starts as a bearing problem can end with damaged gears. In the worst case the drive locks up while you are driving, and that is dangerous.

If you hear a new noise or see oil under the car, drive gently and have it checked as soon as possible. The earlier the cause is found, the better the chance that a targeted repair is enough.

Differential rebuild: what actually happens?

A differential rebuild means a specialist strips the differential, cleans and measures it, replaces the worn parts and then sets the gear contact and the bearing preload again. A good rebuild stands or falls with that set-up.

At ATR a differential rebuild consists of these steps:

1. Diagnosis beforehand. Test drive, check for leaks and play, and establish whether the noise really comes from the differential.
2. Strip-down and cleaning. The differential comes apart and every part is cleaned, so wear marks become visible.
3. Assessing and measuring. We assess bearings, tooth faces, planet gears and seals. Only then does it become clear what really needs replacing.
4. Replacing what is worn. Usually bearings, seals and gaskets. With tooth damage also the crown wheel and pinion, which belong together as a matched set.
5. Setting the contact pattern. We apply a marking compound (often blue) to the crown wheel and check where the pinion teeth make contact. If the contact area sits too high, too low or too far out, we adjust the position of the gears.
6. Setting the bearing preload. With shims we set the bearing preload to the manufacturer’s specification. Too much or too little preload makes bearings wear early.
7. Filling and testing. Fill with the specified oil and take a test drive to check the noise has gone.

On the C63 AMG mentioned above, after replacing the bearings we used marking compound to check that the gears meshed properly and adjusted their position where needed. That is exactly what stops a differential from starting to sing again after a rebuild. You can read the full report under differential rebuild of the Mercedes C63 AMG.

Not always a full rebuild

A full rebuild is not always necessary. The Sprinter 906 complaint can often be fixed under the van without removing the whole differential: the internals come apart, are cleaned and repaired, and the contact pattern is set again. See the photos of the contact pattern checks in our Sprinter rear axle repair. A leaking pinion seal needs something different again from a worn crown wheel. That is why we only decide what is needed after the diagnosis.

Rebuild or replace?

Rebuilding is usually the more logical choice than fitting a different differential. A used unit has an unknown history, may have worn bearings or seals of its own and must have exactly the same final drive ratio as the original. If that ratio does not match, it does not fit the rest of the driveline. Replacement does make sense if the housing itself is damaged or parts are no longer available. ATR makes some of its parts in house and carries out its own research into materials and tolerances. You can read more about how we work on our about ATR page (in Dutch).

What decides the cost of a differential rebuild or replacement?

What a differential rebuild costs depends on the cause and on what has to be replaced. A fixed price cannot be given without a diagnosis. These factors make the difference:

• The cause. Changing the oil or a seal is a much smaller job than new bearings, and bearings are less work than a new crown wheel and pinion.
• The type of differential. An open differential is simpler than a plate-type LSD or a Torsen.
• The car. On front-wheel drive the differential is inside the gearbox and the gearbox often has to come out. A rear axle is usually easier to reach.
• Removal and refitting. Can the work be done under the car, as with the Sprinter, or does the differential have to come out completely?
• Parts. Whether parts are available, whether they have to be replaced as a set, and whether specific brand parts are needed.
• Knock-on damage. After driving on for a long time, driveshafts, the propshaft or bearings in the gearbox may also be damaged.

How we budget driveline rebuilds in general is explained under gearbox rebuild costs. Want to know what it means for your car? Contact us for a price indication based on your registration number and complaint.

Differential oil and maintenance

Looking after a differential comes down to the right oil at the right level. Many rear-axle differentials use hypoid gears, which slide across each other under high pressure. They need gear oil with extreme-pressure additives, often to the API GL-5 standard. Engine oil is no substitute.

• Always use the specified oil. Specification and viscosity differ per car. On front-wheel drive it is often the gearbox oil itself. Which oil your car needs is explained under gearbox oil.
• Limited slip differentials often need a special oil with friction modifiers for the plates. The wrong oil can cause chatter or judder in corners.
• “Lifetime” does not mean never changing it. Oil meant to last the life of the car still ages and oxidises. We recommend having it changed after some years, certainly if you tow, drive hard or have a high mileage.
• Check for leaks. A wet rear axle or oil at the driveshafts is an early warning. Replacing a leaking seal is far less work than bearings that have run dry.
• Act on noise. A new hum or whine is the moment to have it looked at, not when it gets louder.

How ATR Transmissies handles a differential complaint

We understand that you want certainty about what is wrong with your car before you spend money on a repair. With a differential that matters even more, because the noise sounds so much like other faults. That is why we work in three steps:

1. Book your car in. Send us the registration number, the complaint and any fault codes. Also describe when you hear the noise: when accelerating, when coasting or in corners. We review the request and schedule the car.
2. Independent diagnosis. Our transmission specialists find out where the complaint comes from and take the car on a test drive to experience the problem themselves. If it turns out to be a wheel bearing or driveshaft and not the differential, we will tell you that too.
3. Drive without worry. Based on the diagnosis we discuss what is needed. Once you agree, our specialists get to work. Afterwards we test the car and carry out a thorough final check.

We only repair what is really needed, often with a targeted repair instead of replacing a complete differential. Book your car in if you do not trust a noise from the driveline.

For garages: stuck on a customer’s rear axle or differential complaint, or unsure whether the noise comes from the differential, the transfer case or the gearbox? Bring the car to us. If it involves an electronically controlled transfer case or limited slip system that does set fault codes, you can have the diagnosis done remotely with ATR Remote. That way you can help your customer even when the complaint goes beyond what your own workshop handles.

Car differential: frequently asked questions

What are front and rear differentials?

A front differential sits between the front wheels and a rear differential between the rear wheels. A front-wheel-drive car only has a front differential, built into the gearbox. A rear-wheel-drive car has one in the rear axle. A four-wheel-drive car has both, and often a centre differential or clutch in the transfer case as well.

Where is the differential located?

On front-wheel drive the differential is inside the gearbox (transaxle), between the front driveshafts. On rear-wheel drive it is the round housing in the middle of the rear axle, where the propshaft enters. On four-wheel drive you find one on each axle.

How do you know if a differential is bad?

You usually hear it: a whine or howl when accelerating or lifting off, a drone that rises with speed, or clicking in tight turns. Vibration, oil at the rear axle and a clunk when pulling away also point to a fault. Because a wheel bearing or driveshaft can sound similar, only a test drive with inspection gives certainty.

Can I drive my car if the differential is bad?

Only gently and only to the workshop. A droning bearing can last a while, but the damage spreads to the gears, and in the worst case the drive locks up while you are driving. If you hear a new noise or see oil under the car, have it checked as soon as possible.

How much does a differential cost to fix?

That depends on the car, the type of differential and the cause. Often replacement is not needed and new bearings and seals plus a correct contact pattern are enough. That is why we start with a diagnosis. Contact us for a price indication based on your registration number and complaint.

Is a differential rebuild cheaper than replacement?

Usually yes, especially if you catch it in time. A rebuild only replaces the worn parts and sets everything up again. Fitting another differential brings its own risks, such as unknown wear or a final drive ratio that does not match.

How do you check differential fluid?

On most rear axles there is a filler plug on the side of the housing. With the car level, the oil should reach the bottom edge of that opening. On front-wheel drive the differential usually shares the gearbox oil, and the level check depends on the gearbox type. Follow the manufacturer’s procedure, or have it checked during a service.

How often should you change differential oil?

That differs per make and type of drive, so follow the manufacturer’s service schedule. If it says “lifetime”, changing it after some years is still sensible, because that oil ages too. If you tow, drive hard or have a leak, change it sooner.

Conclusion: a noise under the car? Have it measured first

The car differential splits the drive between the wheels and lets them turn at different speeds in a corner. On front-wheel drive it sits in the gearbox, on rear-wheel drive in the rear axle. A failing differential almost always announces itself with whining, droning or knocking, and because it rarely sets a fault code, a test drive is the best diagnosis.

Act in time and a differential rebuild is often enough: new bearings and seals, and the contact pattern and preload set again. Unsure whether a noise comes from the differential, a wheel bearing or the gearbox? Book your car in at ATR Transmissies. We start with an independent diagnosis and tell you honestly what is and is not needed. Still unsure what your symptom means? Find out what your symptom could mean on our page about gearbox symptoms.