The differential is a relatively simple mechanical device that most car owners take for granted and in which they don't invest in proper maintenance and auto care. On this post, we demystify the rear differential and teach you how to spot and troubleshoot differential problems, as well as how to go about fixing them. First, we ask:

What Does the Rear Differential Do?

To put it simply, the differential is used to deliver engine power to the driving wheels. The engine power is delivered to the differential housing by the driveshaft and is transmitted to the wheels through a system of gears. The differential is a crucial part of a car's drivetrain system and without some kind of differential, the car wouldn't be able to run.

There are several different types of car differentials, but they all do the same thing, which is powering the driving wheels, helping the traction and even allowing each driving wheel to turn at a different speed to help smooth running of the vehicle. Rear wheel drive vehicles have the differential mounted on the rear axle, while front wheel drive cars have a transaxle acting as both the transmission and the differential. All-wheel or four-wheel drive cars, on the other hand, have differentials on the front and at the rear.

How to Care for Your Car's Differential

The first and the most important way of caring for your car's differential is regularly scheduled differential fluid change. The differential or gear oil is much denser than motor oil and depending on the type of differential the changing intervals can vary from 30 to 50,000 miles. Also, since the differential is located beneath the car, be sure to avoid hitting it on the rocks and debris since it can damage the differential housing, cause oil leaks, or even break the whole assembly.

Causes of Rear Differential Failure

Since the differential is a relatively simple mechanical device, there are a few problems which can occur. The first and most common is loss of differential oil which can cause gears inside the differential housing to break, grind, or even block the rear wheels. The loss of oil can happen for several reasons and it often happens when differential seals are worn and start leaking or when the differential housing has been hit and oil starts dripping to the ground.

However, differentials can fail if they are pushed too hard which means that you have performed numerous burnouts, drag racing, and so on. Also, it is known that differentials can break if the inside gears are made of softer-type steel which is not able to withstand engine power and torque.

Symptoms of a Bad Rear Differential

The faulty rear differential can easily be spotted if it is leaking oil or if the differential housing is bent or punctured. If you notice something like this, you definitely have a problem on your hands. Other signs of a bad differential are grinding noises coming from the differential while the wheels are turning and even blocked rear wheels. The worst-case scenario is that the differential internals are so broken that the wheels cannot turn at all.

Why is my differential making noise?

If you notice a grinding sound, this means that the gears are not lubricated or that they're even broken and that the differential grinds itself inside the housing. The first sign is humming noise coming from the differential, similar to the failing wheel bearing noise. However, it will get louder and louder until it develops the grinding sound in which case you will have to consider differential rebuilding.

What happens if my rear differential is leaking?

If your rear differential develops an oil or fluid leak it will prevent the inside gears to be properly lubricated which will heat the differential internals and cause extensive wear. The oil inside the differential is not only a lubricant for gears but also a coolant which will take the heat away from metal parts and keep the working temperature under control. In some performance cars, there is a separate differential oil cooler which is designed to cool down hot oil in the differential.

Types of Differential Repairs

There are several types of differential repairs and here they are:

1.  Differential replacement

In case of a fully worn out differential or a broken differential housing, it is recommended to replace the whole unit with a new part or with a used one in good working condition.

2. Differential Rebuild

If the differential gears and internals are worn out, the best thing you can do is to replace them with a new set of gears and have basically a new unit in an old housing. This way your rear end will perform as new.

3.  Swapping gears

Swapping gears is similar to a differential rebuild since you will need to physically remove the old gears and install the new ones. However, in this case, you are installing gears with a different rear end ratio than before. Changing the differential ratio will have a big impact on your performance. If you install higher gears, you will have better acceleration and lower top speed while if you install lower gears, you will experience slower acceleration and higher top speed. Swapping gears can improve fuel economy or help you tow or carry load easily and more efficiently.

Costs of Rear Differential Repair

To be able to properly determine the cost of rear differential repairs, we first need to classify them into three categories:

1.  Repair Costs

If you need only to replace differential oil or differential oil seal, those are small repairs which are fairly inexpensive. For this, you will only need new oil, a set of common tools, and an hour or two in your driveway. Regardless of the type of differential and spare parts, differential repair services of this type can go well below $100.

However, if you need to replace bearings, drive shafts or gears, it will take longer and cost more.

2. Replacement Costs

In the case of terminally ill differentials, replacement is the only solution. If you decide to replace the whole unit, with or without drive shafts, you are looking at a serious job which will require lifting the car and using advanced tools and help to install the differential.

As you might expect, the replacement cost is significantly higher and the amount of time required to replace the whole differential is longer. You can save some money if you buy a used differential in good working condition instead of a brand new one.

3. Rebuild Costs

If you decide that rebuilding your differential is the way to go, you will need a set of gears, bearings, fresh oil, and sealing set, all of which are reasonably priced and can be obtained from most car part shops. Rebuilding doesn't require removing the differential from the car and it can be done in your driveway. Same goes if you want to swap gears and change your rear end ratio to achieve better performance from your engine.


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