Signs that a fan belt needs to be replaced include noise and cracks or frays on the belt. An overabundance of pinholes or a belt that is physically loose are also reasons for replacement.
Fan belts that need replacement sometimes squeal as they start to slip from the pulley. This could happen while the car is in motion, or it could happen only when the car is started. As the damage to the belt increases, the volume of the noise becomes louder.
A visual inspection of the fan belt may reveal physical damage. Using a flashlight and a mirror, check both sides of the belt for cracks, frays and pinholes. More than two cracks per inch indicate the belt needs replacement. Fan belts with deep cracks should be changed right away. Also, change the belt if any abrasions or pinholes are present.
To replace the fan belt, first stretch it as far as it can go. Remove the old belt and install the new belt following the same pathway of pulleys. Stretch the belt until it has no more than 1/2 inch of slack. Ensure that it is not bent, and then start the vehicle to test the placement. Readjust it if any squealing or smoking occurs.
Fan belts keep engines from overheating by circulating air. They should be changed every 50,000 miles if the vehicle is older, regardless of damage.