All modern automobiles have several accessories on the front of the engine including an air-conditioning compressor, alternator, power steering pump, water pump and cooling fan. A drive belt -- commonly referred to as a fan belt -- drives one or more of these accessories. When a component in the fan belt system fails, you may hear a squealing sound from the fan belt. Four main components can cause a fan belt to squeal.
Failed Fan Belt
A failed belt is the most likely cause for it squealing. As the fan belt ages, it becomes glazed and stretched. Glazing and stretching causes the belt to slip on the pulleys that it drives -- air-conditioning, power steering and alternator, for example. This slipping noise causes a squealing sound from the fan belt, and the only way to rectify this is to replace the belt.
The fan belt typically drives several components besides the fan. If one of these components fails and locks up, it can cause a loud squealing sound from the belt. The only way to rectify this problem is to replace the failed component. The belt also runs over several idler pulleys only there to guide the belt. If the bearing in an idler pulley fails, you may hear a squealing sound as the belt drags along the pulley.
Just as the fan belt glazes, sometimes a pulley can glaze. Plastic pulleys are more susceptible to glazing than a metal pulley. This can cause the belt to lose its grip on the pulley, causing a squealing sound. The only way to rectify this problem is to replace the glazed pulley.
The fan belt requires a certain amount of tension to operate correctly -- the tension varies, depending on the vehicle. Some vehicles have a manual tensioner pulley and that allows an adjustment of the tension on the belt as it stretches with age. If your vehicle has an automatic belt tensioner, it does not require any adjusting by you. In the case of a loose automatic tensioner, replace the tensioner to rectify the problem.
Temporary Squeal Elimination
Several companies sell a belt dressing that reduces or eliminates belt squeal. This dressing is only for temporary use and using it as a repair will eventually lead to belt breakage. If the belt breaks on the road, it can leave you stranded, due to overheating or a drained battery.