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What's a differential and how does it work?

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Updated 10 Oct 2019

Jason Unrau

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You drop the gearshift in ‘Drive’ and smash the accelerator.

Your car leaps into action, hauling you anywhere you point your car.

Sometimes it seems like magic, but there’s a mind-numbing amount of technology and mechanics behind your car’s motion.

One of those mystery items is the differential.

What is a Differential and What Does It Do?

A differential, known as a diff for short, is part of every drivetrain.

The root of a diff is a set of hardened steel gears that direct power from the engine and transmission to the wheels.

With a transaxle, commonly used on a front-wheel-drive car, the diff is integral.

For a rear-wheel-drive or 4WD vehicle, the differential is a separate component.

Its purpose is two-fold: to change the direction of torque heading to the drive wheels, and to allow for drive wheels to turn at different speeds.

All of an engine’s torque goes into a single transmission or transaxle, and it needs to be directed to two wheels.

Differential side gears mesh with the larger pinion gear to perform that function.

Spider gears allow for one wheel to turn faster than the other, preventing binding and wheel scuff when you’re taking a corner.

How Do I Know if I Have a Bad Diff?

For most vehicles, the differential is one of the most reliable components, even under as much stress as it experiences.

Still, problems can occur with the diff that can spell disaster.

Each gear has at least one bearing and race that must be kept well lubricated to prevent pitting, overheating, and noise.

As well, the gears themselves can fracture or chip under extreme use (or abuse).

Symptoms include:

  • Whining noises. If you hear a whine or howling noise from your car’s undercarriage that gets louder and higher-pitched as you drive faster, it could be the diff.
  • Burning smell. An obvious burnt, acrid odour under your vehicle can indicate the gear oil in the differential case is badly deteriorated, or the fluid is low.
  • Grinding or binding. In bad cases, damaged or disintegrated bearings can cause horrible grinding noises when you’re driving, or a jerky binding feeling when you’re in motion or turning.
  • No motion. The worst it gets is broken gears or axles. If that happens, your car isn’t going anywhere.

Can a Diff Be Repaired or Is Replacement Necessary?

Unless you’ve done a really good job and ignored early warning signs, a differential can be repaired.

New bearings and races will be installed and the gears properly seated or meshed.

However, if your differential has blown up on you, a new diff assembly will be required.

What's the Cost?

To simply replace bearings and seals, it may only cost between $400 and $620 for a rear differential repair.

That cost nearly doubles if it’s a transaxle.

And if a differential replacement is necessary, expect the parts and labour bill to be $1580 to $2660 – perhaps even higher!

If you’re experiencing any of the symptoms outlined in this article, you’ll want to quickly and easily book yourself in with a qualified mechanic on AutoGuru to get your diff inspected ASAP!

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Written By

Jason Unrau

Jason is a Canadian automotive content writer with a background in the auto service industry, but he’s been hooked on cars and mechanics since childhood.

One of his first cars was an ’80 Mazda RX-7 that’s sorely missed to this day. A ’68 Ford Torino GT, a ’66 Ford Country Squire Woodie station wagon, and a ’96 Suzuki GSX-R 750 have spent time in his fleet of cars, bikes, and trucks over the past two decades.

Jason’s pride and joy is under construction – a turbocharged ’88 Mazda RX-7 convertible. Also on his resume is CASCAR official certification.