What is a diff? Here’s the run-down

You’ve hit the open road, but after a while you notice a strange noise coming from underneath your car. When it comes to cars making odd clunking, whining, whirring or other sounds, it’s always best to play it safe and get it checked. It could be your diff.

At its core, a differential, or ‘diff’, is a group of gears underneath your car that let the wheels spin independently of each other – but most importantly convert the energy output from your transmission to forward momentum! Without it, you wouldn’t be able to turn corners properly or drive in any direction – suffice to say it’s an integral part of your driveline function!

Where is the diff in a car?

The diff can be in a few different locations depending on what kind of car you drive. To find the diff, you’re basically looking for a housing at the centre of the axles that connects them to the driveshaft.

In 2WD cars, there are only two tyres that are getting power from the engine - either the front or the back set, which means these cars have just a single diff. But in AWD cars, all the wheels get power from the engine, so they’re fitted with a front, rear, and centre diff (also known as a transfer case). 4WD cars are different still, and usually have a heavy duty transfer case that allows the user to switch between 2WD and 4WD most commonly with 2 options for 4WD, 4, ‘high’, and ‘low’.

What does a failing diff feel like?

As the diff controls the power to your wheels, it’s pretty important to make sure it’s in good working order. Noticing any changes will help uncover problems early. The signs of a failing diff are usually heard and felt, so watch out for these common symptoms:

Does changing differential fluid make a difference?

As the car is driven and the gears are moving, the diff fluid heats up and can degrade over time. That’s why you need to make a diff oil change part of your regular maintenance.

If the oil is old, it can’t lubricate and the gears can become worn, leading to the issues above. An oil change also lets you see if there’s metal shavings in the oil. These will need to be removed by taking off the cover and cleaning inside the diff, as metal shavings can cause problems even after the oil has been changed. This is also an indication your differential may need mechanical work to rectify the issue.

When should you change your diff oil?

The frequency depends on your car. Your owner’s manual will show the recommended timing for your specific vehicle.

But there are a few things that can affect how often you’ll need to do it. Lots of driving, off-roading, and hauling heavy loads can all mean more wear, so keep this in mind. If you’re doing a lot of towing, or touring – best to stick to the ‘severe’ maintenance schedule in your owner’s service and warranty handbook and prevent a major failure.

What is a diff lock?

Although the diff’s primary purpose is to let the wheels spin independently of each other, sometimes it’s good to be able to turn this feature off. This is where the diff lock comes in, letting you power the wheels equally when you need it.

This feature is found on full, 4WD systems with selectable transfer cases.

When to use the diff lock

A good example of when you would want to use diff locks is during 4WDing. When you’re off-roading, your car can often have one wheel in the air, or one wheel on a slippery surface like mud, sand, or gravel. If your diff isn’t locked, this means the car will recognise this wheel as the one with the least resistance, and put all the power into it. But you don’t really want this, as the wheel with traction is the one that needs the power. If your diff is locked however, both wheels will have equal power, giving you greater control.

If you have an automatic diff lock, there will be a button on your dashboard with a diff lock symbol on it. This can be a picture of a lock, a line drawing with an X, or similar.

What is a diff drop?

If you’re into 4WDing, you might have had your vehicle raised for better clearance underneath. But if you’ve raised it more than two inches, it may be problematic by putting pressure on your CV joints. That’s where a diff drop comes in, which is basically a spacer that’s added so that the diff can sit further down and lessen the risk of breaking these joints.

This is usually an aftermarket modification whereby the diff is physically lowered.

Diff ratios explained

The diff ratio is basically the relative size of the crown wheel to the pinion, which is one of the gear sets housed within the diff. You can change this ratio by switching out the set.

Why would you do this? If you’ve put in a bigger engine, or changed the tyre diameter, this can affect your vehicle’s performance, which will require a different ratio.

A higher ratio will mean a lower top speed, but faster acceleration. It will lower your fuel economy with the higher RPMs needed, and increase your vehicle’s noise on the highway. All of this is pretty subjective, so you’ll need to work out what’s most important to you.

Need help with your diff? Give us a call and one of our qualified automotive mechanics can book you in for a service.