When you take a corner in your car, the outside wheels have a slightly longer distance to go than the inside wheels. That means that the outside wheels have to turn a bit faster than the inside. The piece of mechanical wizardry that makes this possible is called the differential.
The differential allows the drive wheels to rotate at different speeds in turns without the wheel binding or hopping. If you have a rear-wheel drive vehicle, the differential is on the rear axle. You've seen that bulge in the middle of the axle when you're behind a truck – that's the differential.
If you have a front-wheel drive vehicle, the differential function is handled by your transaxle. Of course, all-wheel drive vehicles have differentials on both axles. They also have a center differential or a transfer case between the front and rear axles to compensate for speed differences between the front and rear.
Because all the power of the engine is transferred through the various differentials, you can imagine that they are very strong and are built last a long time. That's why it's important to keep your differential properly lubricated. Differential fluid cools and protects the gears.
Your service technician will check differential fluid level and top it off if necessary. With low fluid, the differential will run too hot and wear prematurely. Ask your service advisor for when it's recommended to change your differential fluid. Fresh fluid will extend the life of your differential. Your technician will also inspect the u-joints which connect your drive shaft to the differential and may recommend service. Some u-joints can be lubricated as part of a routine lube, oil and filter change as well.
Now, of course differentials eventually wear out and need to be replaced. You might notice a strange noise from you axle area as one of the first warning signs. When the differential shows signs of failing, it's important to repair it. If you leave it too long and it freezes up when you're driving you could lose control of your vehicle and other parts like the axle, driveshaft and transmission could be damaged.