(Updated on September 29, 2020)
A differential is the part of a vehicle’s powertrain that allows two wheels to turn at different speeds. Differentials have one input and two outputs, with each output going to an axle or drive shaft which then connects to a wheel.
What is the Purpose of a Differential?
When you take a turn in your vehicle, the left and right wheels will take a slightly different path. The tires on the inside of the turn will make a circle with a smaller radius, which means it requires less revolutions of the tire to complete the turn.
If vehicles were not equipped with front and rear differentials, the left and right tires would be forced to spin at the same speed despite these different paths. This would cause one or both tires to slip, causing loss of traction, excessive tire wear, and additional stress on other powertrain components.
The difference in speed between the two tires is due in part to the vehicle’s track width (that is, the distance between the left and right wheels). The greater the track width and the tighter the turn you take, the more difference in speed there is between the inside and outside wheels.
How Many Differentials Does My Vehicle Have?
Vehicles have a different number and placement of differentials depending on the number of driven wheels it has. “Driven” wheels are the wheels which receive torque from the engine.
Front and rear wheel drive vehicles are two-wheel drive vehicles. They have a single differential.
Front Wheel Drive
Front wheel drive vehicles often have a transaxle, which is a single integrated unit that serves the purpose of the transmission, the differential, and the axle. On front wheel drive vehicles, the term “transaxle” may be used interchangeably with “transmission” or “differential” depending on who you talk to.
Rear Wheel Drive
Most rear wheel drive vehicles will have a standalone differential between the rear wheels that is linked to the transmission via a driveshaft. A rear differential is colloquially called a “rear end” or rear diff by some people.
All Wheel Drive
All wheel drive vehicles have three differentials: a front, rear, and center differential. The center differential splits the torque between the front and rear differential inputs, which each then split the remaining torque from left to right.
Four Wheel Drive
Four-wheel drive systems found on trucks and SUVs typically have a transfer case instead of a center differential.
Types of Differentials
An open differential is the simplest and most common differential. Equal torque is applied to both outputs. This means if one tire loses traction, the other tire will be limited in the amount of force it can apply to the ground to propel the vehicle forward.
Locking differentials are typically found on purpose-built off-road vehicles such as trucks and Jeeps that come with off-road packages.
A locking differential spins the left and right wheels at the exact same speed, regardless of wheel slip. This differential is great for tough terrain when one tire could be in the mud, snow, or even the air.
You should take care not to lock the differential in a high-grip situation, such as on dry asphalt. This could put excess stress on the powertrain and damage expensive components.
Limited Slip Differential
A limited slip differential is a middle ground between an open differential and a locking differential. These are typically found on sports cars and higher end luxury cars.
There are many different types and implementations of limited slip differentials. The basic idea is that when a tire slips, the maximum wheel speed of a slipping tire is limited. Torque is then transferred or multiplied to the opposite tire that still has grip. Ideally, this allows a vehicle to keep moving forward even if one tire has little or no grip.
Bad Front or Rear Differential Symptoms
There are a few noticeable signs that will present themselves if your front and/or rear differentials start to wear out or get damaged. Here are the most common symptoms that you can expect to happen if your differential goes bad.
1) Whining Noises
Whining noises are one of the most common and noticeable symptoms of a bad differential. This could be due to the components of the differentials not being lubricated properly.
Perhaps the differentials are leaking fluid and that is causing them to wear prematurely. Whenever their components are not being lubricated properly, you are likely to hear whining noises coming from the end of your car where the differential sits.
2) Tire Damage
If your left and right tires are traveling at the same speed as you turn a corner, the inner tires are going to have more wear put on them because they’re being forced to rotate faster than they should be. This will cause your tires to wear prematurely.
If your differential is locked you may also experience tire scrubbing, which is the sound of your tires gripping and slipping in quick succession. Depending on the rubber compound, this may sound like a rapid squeal or a chirp. There will likely be some vibration to the rhythm of the sound.
3) Difficult Handling
Differentials are what allow your vehicle to corner comfortably when you make a turn. A failing differential is likely to upset the cornering ability of your vehicle and make it feel unstable.
Anytime you notice handling difficulties when turning or going around a corner, get your vehicle to a mechanic as soon as you can to have it checked out.
If the universal joints of your differentials get too worn out, the drive shaft will start to vibrate. You will feel the vibrations even more as you step on the gas pedal to accelerate your vehicle. The vibrations will get even worse if there is a differential fluid leak.
Although this might not seem serious, you should let this be an early warning sign that you need to have your differentials checked out.
5) Gear Grinding
If you have a worn differential, its gears may start to grind more. This may even cause humming noises to emerge from the grinding of the gears. You will hear the humming sound get louder as you accelerate the vehicle.
If you hear this, it’s a good idea to have your differential inspected, and possibly the transmission too. If you catch it early, the fix could be as simple as a gear oil change.
Front or Rear Differential Repair Cost
The average cost of repairing bad front differentials and rear differentials will depend on the severity of their condition. Generally speaking, it should not cost more than $400 to repair most rear or front differentials. Most people pay around $200 to $300 though.
This would involve repair work related to adjusting the backlash and replacing the oil seal of a differential. But if you need to completely rebuild a differential, then your repair costs will be between $400 and $800.
This is still cheaper than having to replace an entire differential, which would be between $1,000 and $2,000.