What is an auxiliary drivebelt?
The aux drivebelt is also known as the serpentine belt, and shouldn't be confused with the timing belt which serves a completely different purpose.
The auxiliary belt's role is to 'power' a variety of important peripheral components via a system of pulleys driven by the main crankshaft pulley.
On old cars the belt simply connected the crank with the fan, so as the engine rotated so did the fan. But as engines became more complex – and fans became electrical rather than mechanical – the fan belt was superseded by the V-belt, which as the name suggests is a V-shaped belt that powers one or more additional components.
It's not uncommon to find more than one V-belt, depending on the application.
In more recent years the aux belt has become the preferred choice. Its wide, flat, often toothed design means that it can power several different components at once and has become the norm in engine bays where space is restricted.
On a regular front-wheel-drive car you'll see the aux belt down the side of the engine (access may be very tight). The aux belt is virtually always visible, unlike the timing belt which is much more likely to be hidden behind a cover.
It's not uncommon for the aux belt to power the alternator, power steering pump, air-con pump and water pump.
Your Haynes manual will show you which belt powers which ancilliary.
As the belt needs to be long you're also likely to find an idler pulley and/or a tensioner pulley in the mix. These keep the belt correctly tensioned so it doesn't deviate from its course, and so that it applies sufficient friction to the pulleys.
What is an aux drivebelt made of?
The belts are made of reinforced rubber and are usually 'toothed' on one side. It's interesting to note that as the belt snakes its way around the various pulleys, both the toothed side of the belt and the smooth side apply friction to the different pulleys depending on the level of 'load' they place on the belt.
When should an auxiliary drive belt be changed?
Unlike your car's timing belt, there are no hard and fast rules on when the auxiliary belt(s) should be changed. This is because if it snaps it's not going to cause catastrophic engine damage. However, other important ancilliaries will stop working, so it's important to inspect the belt(s) at regular intervals – we recommend once a year.
Take a good look at the belt, checking it for cracks and making sure there are no chunks missing from it. Also check its tension - if it's too slack it's likely to squeal, but if it's too taut it'll put a strain on the ancilliaries' bearings.
Head here for instructions on how to change your drive belt.