What is a rear differential and what does a rear differential do? While every vehicle has a differential, whether you have a rear one or not depends on the drive system. Read on for the ins and outs of differentials and how to know which your vehicle has.
What Is a Rear Differential?
A rear differential, or simply “rear diff”, is the gearbox located between the rear wheels of a rear-wheel-drive, four-wheel-drive, or all-wheel-drive vehicle. Its primary function is to transfer torque to the wheels while allowing them to rotate at different speeds.
This simple yet crucial task is what makes turning and road traction possible, ensuring control and stability, without having the rear wheels skip and scuff when you turn even slightly.
Its typical design consists of:
- Housing – Encases internal components and provides structural support.
- Ring and pinion gears – Transfer power from the driveshaft to the rear axle shafts.
- Spider gears – Allow rear wheels to rotate at different speeds during turns.
- Axle shafts – Transmit power from the differential to the rear wheels.
- Bearings and seals – Support parts and prevent leaks, excess friction, and contamination.
- Differential housing cover – Provides access for repairs and fluid changes.
- Limited-slip or locking differential parts – Enhance traction and stability.
- Fluid – Reduces friction and wear between moving components.
How Does the Rear Differential Work?
Understanding how a rear differential operates requires a grasp of its inner gears and the forces that act upon them. The basis of a diff boils down to its ability to distribute power to the two wheels, which vary in speed during typical road conditions.
In straight-line driving, both wheels receive equal torque, and the diff’s gears remain relatively still. However, when you turn, the outer wheel travels a greater distance than the inner one.
To accommodate this, the differential gears rotate at different speeds, achieving the necessary torque distribution.
Experiencing icy roads or off-road conditions may require more control over wheel speed. This is where various types of differentials, such as limited-slip differentials and locking rear differentials, come into play, providing better traction and performance.
With these types, a clutch system inside the diff detects wheel slip and engages or locks the rear wheels so they spin at the same speed. It can be mechanical or electronic locking, typically called an e-locker.
Servicing Rear Differentials
You need to take care of your rearend differential just like you would any other component of your auto. One important aspect of servicing rear differentials is their lubrication.
The gears within the differential need to be covered in gear oil, a substance that lubricates not only the gears but also the bearings. The oil also helps keep the components cool, so they do not overheat.
This fluid will break down as time passes. Additionally, metal filings come off the bearings and gears and collect in the fluid. Between the metal filings and the breakdown of the fluid, you must change the rear differential fluid every once in a while.
Your maintenance for this component will also involve changing the gear oil as a preventative measure against damaging the interior gears, which are the pinion gear and ring gear.
During rear differential service, your mechanic will take off the cover from the component. From there, they will clean any of the old fluid found in your differential cases. They will then reseal the cover and add in the appropriate amount of clean, new fluid.
For most cars, you just need to change the rear differential fluid every 20,000 to 40,000 miles. Because there is some variation, you should look at the owner’s manual for your vehicle to confirm your ideal service interval.
From there, you can ask your mechanic about the cost to service your rear differential.
How Do You Know the Rear Differential Is Bad?
Differentials are tough assemblies that rarely fail all at once. However, the internal components and bearings wear out over time, and they may become damaged (especially due to rust or off-road driving). These are some of the most common symptoms of a failing rear diff:
- Overheating. Diffs may overheat when operating at high speeds or when hauling heavy loads. This reduces the performance of the lubricants and can damage internal components. Regular overheating may be a sign that your diff and/or gear ratios are unsuitable for your application.
- Grinding, howling or clicking noises. Differentials make all sorts of noises as they wear out. Listen for clicking or grinding noises when you accelerate or steer through a corner. Listen for howling noises when driving at high speeds.
- Vibrations and shaking. Faulty diffs often cause noticeable vibrations, especially while accelerating. Excessive vibrations are a sign of a major fault and should be repaired as soon as possible to avoid further damage.
- Difficulty steering. A worn out diff can cause your vehicle to pull to one side or become difficult to steer.
- Oil leaks. Diffs contain oil that provides lubrication and keeps the internal components cool. Oil leaks should be addressed as soon as possible.
- Excessive tyre wear. Worn out diff centres can often cause tyres to wear down quickly. Look for signs like excessive wear, uneven wear marks, or damage towards the outside of the tyre (e.g. cracks or chunks of rubber missing).
How to Fix a Noisy Differential
One of the most common signs of an issue with your rear diff is an unusual differential sound. This may sound similar to howling. This pinion bearing noise may have variations in pitch, and it may come and go. These variations will depend on the load on your differential, as well as the severity of the problem.
If you think you may hear a rear differential noise, let your mechanic know. Many people mistake it for tire noise and then just ignore it. If this happens, you may find yourself with much more serious problems and more expensive and extensive repairs.
What About a Rear Differential Leak?
Although noise is a common symptom, you are probably more likely to notice a fluid leak or just low oil for your rear differential. The leaks typically occur by the axle seals, the rear differential cover, or the pinion seal, the last of which is where your driveshaft connects to the differential.
You can tell that you have a leaking rear differential if there is grayish or light brown fluid on the ground where the rear portion of your auto was parked, as this is the most obvious of the low differential fluid symptoms.
The cost of a rearend repair for leaks will depend on the extent of the damage. Just adjusting the backlash or replacing your oil seal should be quick and fairly affordable.
By contrast, repairing the leak will sometimes require a rear differential rebuild or other massive repairs.
Common Rear Differential Repairs
Although every situation is different, most of the common rear differential repairs will fall into one of several categories.
Replacing the Differential Gasket
In most cases, the cover on the rear differential will be either rubber or silicone. This means that it can deteriorate over time, leading to leaks.
This is among the simplest repairs, as you just need to take off the current differential cover, clean the surface for the differential seal, and then reseal the cover. This should not take more than an hour.
Sealing the Rear Differential Piston
If you look at the front of the rear end differential on your car, you will see a yoke that connects with the driveshaft. Over time, the seal surrounding this yoke may develop a leak due to aging or nicks in its rubber.
To repair this problem, you will need to remove the yoke, pry out the seal, and drive a new seal into its proper place, all without causing any damage. The last step is particularly challenging, as it is easy to accidentally damage the new seal as you put it into place.
Sealing the Differential’s Side
Another common repair for leaky differentials is sealing the side of the differential. The side seals on the diff stop its fluid, so it cannot leak onto the rear brakes.
This type of repair is more time-consuming and challenging than the previous two since you have to also remove the axle shafts. This process involves prying out the old seals and carefully installing the new ones. Then, you can put the entire assembly together again.
Replacing the Rear Differential Bearing
The bearings in the rear diff include the pinion bearing and the side bearing. Both can deteriorate or pit over time, which will lead to rear diff noise.
If you need to replace one of the bearings, expect the process to take between three and five hours for a trained mechanic to complete. As such, it will not be as cheap as the previously mentioned repairs.
Replacing Rear Differential Gears
Another common repair is to have to replace the gears in the rear differential. Over time, the teeth of the gears may wear down badly or chip. When this happens, you will deal with noise and a higher risk of failure, unless you replace them.
You do not want to have to replace the differential gears, as they are the costliest components to replace, not even counting labor.
Replacing the Entire Rear Differential
In the absolute worst-case scenario, you will need a rear differential replacement. This becomes necessary in certain situations, including when the gears “grenaded.” In that situation, the differential case may not be fixable.
In this case, your mechanic will have to replace the entire rear differential, including the housing, bearings, seals, and gears.
Rear Differential Repair Cost
As with most repairs, the cost of getting a rear differential fixed depends on how extensive the damage is. If the only thing wrong is degraded oil, then you’re in luck. Seal and bearings generally don’t cost too much, so when you factor in labor costs the price for an oil replacement is usually in the range of $300.
If the damage is more extensive, the price is going to increase considerably. If new gears are needed the price can be at least $1,500. Going to the worst-case scenario, if the differential is beyond repair, you’ll need the component to be replaced altogether. You can expect a rear differential replacement to cost up to $4,000.
Now that you know what to expect for a rear differential repair you have the information to decide whether or not a replacement is worth the cost and if a new car might make the most sense for you.
FAQs.
What Causes a Rear Differential to Go Bad?
In most cases, a rear differential will go bad over time due to simple aging. Just like other components in your auto age and weaken, the same is true of the rear diff. This process may occur more quickly if you do not properly maintain the component. As such, you can avoid rear differential problems with regular maintenance, at least to some extent.
Can You Drive with a Bad Differential?
Technically, you can drive with a bad differential, but it is not wise. The problem may get worse, to the point where it leaves you stranded somewhere. It can also cause damage to other surrounding components. It is smartest and safest not to drive with a bad differential.
What Happens If Rear Differential Goes Out While Driving?
The severity of the complications if the rear differential goes out while driving with a bad pinion bearing will depend on the problem and the vehicle. In the case of chipped or broken gear teeth, there may a vibration or skip in power.
Multiple broken teeth will likely cause thunks and rattling sounds. If this happens, you should pull over, as it would be very easy to lose control of your auto.
In the worst-case scenario, you may experience a full loss of power when you turn. Eventually, your car may just stop and refuse to start, leaving you stranded.