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How to check and change your car’s auxiliary drive belt

when drive belt replacement

Rob Keenan is the interim digital editor of
He runs a Mk2 Ford Focus ST and an aging Mercedes SLK55
Find him on Twitter @zorba_t_greek

An auxiliary drive belt powers many of your car's vital functions. You'll find at least one, often two and sometimes three auxiliary belts under the bonnet, typically driving the alternator, power steering pump, water pump and air-con compressor.

Sometimes known as the serpentine belt because of the way it snakes around a series of pulleys, the auxiliary belt is powered by the crankshaft pulley; it’s important that the belt is at the correct tension if it’s to operate properly.

My drive belt is cracked!

On average, a drive belt should last between 50,000 and 100,000 miles, but extreme temperatures and contaminants, such as oil and coolant, can shorten that lifespan. When a belt fails it tends to do so without much warning - the power steering pump will stop providing assistance, the alternator won't generate electricity or the water pump will stop. All are important so you'll have to pull over to the side of the road.

The belt(s) should be inspected at least every time the vehicle is serviced. First, check that the tension is correct. Using your thumb, press on the belt mid-way between a couple of pulleys. It will deflect slightly but there shouldn't be a noticeable deformation and it should immediately take up the slack when you let go.

Now take a look at the front and back of the belt for the symptoms shown in the picture below. If you see small cracks running across the 'V' portions of the belt you have nothing to worry about. But if you spot cracks running along the length of the belt or chunks are missing from the belt, you need to change it immediately.

Common problems with auxiliary belts

drive belt is glazed

Where’s my drive belt’s tensioner?

It's a jungle down there. You probably won't be able to see the extent of the belt's run, even with your head buried in the engine bay (with the engine turned off, of course). So take a look at your car's handbook or your Haynes manual - the latter will have at least one diagram (such as the one below) showing how the aux drive belt is routed.

The key item here is the belt tensioner, because you need to manoeuvre it to release the tension on the belt to be able to remove it. You can't simply cut the belt off. Well, you could, but you won't be able to get the new belt on.

Understand how your car's aux belt is routed

Can drive belt be repaired

How do I fit a new drive belt?

Replacing an auxiliary drive belt isn't a difficult or lengthy job but it's important that you buy the right belt (make sure it's designed for your precise model - and takes account of whether you have air-con or not) and check that it's routed correctly around the pulleys. It should also be seated properly on each pulley's grooves.

Once that's done you can allow the tensioner to do its thing, start up the engine and make a quick visual check. Job done!

Learn more about fitting the belt

Watch this video to see how to change your drive belt