Many modern accessory drivebelts need a special tool to set the tension properly, so they cannot be tensioned at home. This is especially true of the long serpentine belts that snake from one accessory to the next. These belts are used on some modern engines instead of multiple belts and they're not only very expensive, but can also cause damage to the accessories they drive if the tension is not correct. The modern flat belt, which runs over multiple-grooved pulleys, can be tensioned without a tool, but most people tend to run them at too low a tension, so that they squeal. They should have about six to eight mm of free play.

The old-fashioned triangular- section belt is very easy to tension but, before you start, the belt should be examined for damage. A really thorough examination is only possible once the belt has been removed, but you can often get a good idea of the condition by examining the belt in situ. Roll the longest free length between your fingers so that you can see the inside, and look for cracks and signs of old age such as fraying, splitting or a hard glazed surface.

If the belt has to be replaced then start by loosening the adjusting bolt as well as the alternator mounting bolts, so that you can move the alternator towards the cylinder block, making it possible for the old belt to slip over the pulleys and the new belt to slip on.