What is a rear differential, and what does a rear differential do? Differentials are found in all types of vehicles, but not all have rear ones, depending on the type of drive system.
Continue reading for a complete guide on differentials and learn exactly which type your vehicle has.
What Is a Rear Differential?
A rear differential, or “rear diff” for simpler reference, can be found in a rear wheel drive, four wheel drive, or all wheel drive automobile. It is the gearbox situated between the rear wheels. The rear differential’s core responsibility is to shift torque to the wheels while permitting them to rotate at different speeds.
This relatively simple but very important function enables turning and traction on roads while maintaining control and stability. The vehicle will not have the issue of rear wheel skipping and scuffing when turning even slightly.
Its typical design consists of:
- Housing: this part encases internal components and serves as a support structure.
- Ring and pinion gears: These components transfer power from the driveshaft into the rear axle shafts.
- Spider gears: These allow movement of the rear wheels at varying speeds during turns.
- Axle shafts: This enables the rear wheels to receive power from the differential.
- Bearings and seals: These serve to support system parts and prevent leaking, excess friction, and contamination.
- Differential housing cover: Enables repairs and fluid changes to be performed.
- Limited-slip or locking differential parts: Increase traction and stability of the vehicle.
- Fluid: Minimizes the friction and abrasion of the moving parts.
How Does the Rear Differential Work?
To learn how a rear differential works, we must first know the inner workings of its gears and the forces acting upon them. Simply put, a differential has the fundamental function of distributing power to the two wheels, which, under normal driving conditions, will rotate at different speeds.
For example, during straight-line driving, both wheels receive a certain amount of torque simultaneously, and the differential gears do not turn. However, while turning, the outer wheel traverses a greater distance than the inner wheel.
To achieve the required torque distribution, the differential gears must turn to different speeds relative to each other.
Earlier, icy roads and off-road conditions might have required more control over wheel speed. Better traction and performance can be gained using different types ofials such as limited slip differentials and locking rear differentials.
With these kinds, a clutch mechanism within the differential senses wheel slip and engages, locking the rear wheels so they rotate together. This is known as either mechanical or electronically locking, usually termed an e-locker.
Repairing rear differentials
As with any part of your car, the rear differential needs attention and servicing. Like all rear differentials, they require lubrication for the rear differential to function correctly.
The parts of a differential must be submerged in gear oil as it not only lubricates the gears, but also the bearings. It is also useful to cool the components so they don’t overheat.
This fluid will be subject to degradation over time. Also, metal shavings are produced from the bearings and gears and settle within the fluid. Due to the metal shavings as well as the fluid’s degradation, changing the rear differential fluid is required at some intervals.
You will also service this component by changing the gear oil periodically to prevent damage to the interior pinion and ring gears.
Your mechanic will remove the cover from the component during the rear differential service. They will go on to wash all of the old fluid within your differential cases. After this, they will seal the cover once more and fill it with the correct quantity of fresh fluid.
For the majority of cars, just changing the rear differential fluid within a mileage interval of 20,000 to 40,000 miles will suffice. I recommend checking the owner’s manual for your vehicle to determine the optimal service interval, as there is some variation.
At this stage, you can consult your mechanic to enable you to estimate the expense that will be incurred in servicing the rear differential.
How Do You Know the Rear Differential Is Bad?
Differentials are robust constructs that do not simply break apart at once. Nonetheless, the internal parts and bearings do erode with usage and may suffer some form of damage because of rust, off-road driving, or other reasons. The following are some symptoms that are classically associated with a failing rear differential:
- Overheating. Differentials can overheat while running at high speeds or when dealing with extremely heavy payloads. Overworking the lubricants can have a catastrophic effect on the internal components of the differential. Regular overheating may indicate that the diff and/or gear ratios are not appropriate for the application.
- Grinding, howling, or clicking noises. Grinding, howling, or clicking noises and all sorts of sounds are associated with the slower wearing out of components. Pay attention to the respective clicking or grinding sounds during acceleration or cornering. Both high speeds and acceleration are susceptible to howling-like noises.
- Vibrations and shaking. Anyone who drives with a faulty diff is bound to experience noticeable shaking, especially while accelerating or steering. However, excess shaking signifies a critical issue, and to stop further damage to the internal component, it should be repaired as soon as possible.
- Difficulty steering. Difficulty in steering is a clear giveaway of a worn-out diff, although it may lead to the vehicle drifting off one side.
- Oil leaks. An oil leak is of great concern because differentials are self-contained units that require oil for cooling and lubrication purposes. Oil leaks should be dealt with right away.
- Excessive tyre wear. Destruction of diff units will frequently lead to tyre wear. Look for excessive wear, marks of wear that are not uniform, or signs of wear on the outer side of the tyre in the form of cracks or even chunks of the rubber cut off.
How to Fix a Noisy Differential
Hearing unusual differential sounds is one of the tell-tale signs the rear diff has a problem. It can be a howl of some sort. This noise is likely coming from the pinion bearing and varies in how loud it is and how often it is heard. It will depend largely on how much load is on the differential and the degree of the problem of the differential.
If you suspect hearing differential noise from the rear, mention it to your trusted mechanic. This noise is very commonly confused with that generated by tires, which people tend to disregard. Subsequently, the owner might run a greater risk of encountering more severe issues and far more complex repairs.
What About a Rear Differential Leak?
Aside from noise, other possible symptoms that you may notice include fluid leaks or low oil levels in the rear diff. Most rear differentials have fluid leaks at the axle seals, the rear differential cover, or the pinion seal (the area where the driveshaft joins the differential).
Most differential fluid symptoms are low in value, but it is easy to recognize if you have a leaking rear diff and park your auto on the ground, which has light brown or grayish fluid stains.
How much it costs to repair leaks at the rear end depends on how serious the damage is. Usually, the oil seal replacement and backlash readjustment take a short time and are relatively inexpensive.
Contrary to that, fixing the leak will at times need a rear differential rebuild or other extensive repairs.
Most common repairs for the rear differentials
Each case is unique, but most of the common rear differential repairs will likely fit one of several categories.
Replacing the Differential Gasket
The rubber/silicone cover protecting the rear differential is susceptible to wear and will most likely leak in the long run.
Repairing this issue does not require advanced skills because replacing the cover involves removing the existing differential cover, cleaning the surface of the differential seal, and resealing the cover. The entire process will take less than one hour.
Sealing the Rear Differential Piston
A driveshaft yoke is attached to the front of the rear end differential. The driveshaft yoke is usually sealed, but aging or simple, everyday damage will most probably leave this seal prone to wear and tear and add a leakage problem.
A simple fix is to take the yoke off, remove the seal, and replace it with a new one, without breaking the old one first. But this last step, which is the most difficult, is going to be the hardest as it is very easy to shatter the new seal while trying to use it.
Sealing the Differential’s Side
Leaking differentials can also be fixed by sealing the side of the differential. The diff side seals stop the fluid from leaking to the rear brakes.
This method of repair is the most complicated and time-consuming of the three because the axle shafts, which have to be removed, are part of the assembly. The task on hand involves removing the old seals and replacing them with new ones before completing the assembly.
Replacing the Rear Differential Bearing
The other rear diff bearings include the pinion bearing and side bearing. All of them suffer from some form of decay or pitting, which causes rear diff noise.
A fully equipped mechanic can complete the task in three to five hours with all the required materials at hand. This makes it less affordable than the previous repairs.
Replacing Rear Differential Gears
A common repair is the replacement of the gears in the rear differential. As the gears are used, their teeth may get mangled or chipped over time. Without replacing them, you will incur noise complaints and a greater chance of failure.
The costliest parts to replace in a differential, not including labor, the the differential gears.
Replacing the Entire Rear Differential
In the absolute worst-case scenario, you will need a rear differential replacement. That may become necessary in some situations, including when the gears “grenaded”. In that case, the differential case will likely not be fixable.
Your mechanic will then have to replace the entire rear differential, the housing, bearings, seals, and gears included.
Rear Differential Repair Cost
Like most repairs, the extent of the damage will determine the cost of getting a rear differential fixed. Thank your lucky stars if the only issue is degraded oil. Generally, the seals and bearings aren’t too expensive, so with the labor costs, the price for oil replacement will typically be around $300.
If the damage is more extensive, I will likely have to bring the vehicle to a more specialized workshop, which increases the price significantly.
In the worst case, if new gears are required, I will likely be looking at a price of around $1500. If the component is completely beyond repair and needs a complete replacement, you can anticipate rear replacement prices reaching up to $4000.
With this information on a rear differential repair that you have now, you can determine if a replacement is worth the cost, or if a new vehicle would better suit your needs.