Inspect the belts for signs of glazing or cracking. A glazed belt will be perfectly smooth from slippage, while a good belt will have a slight texture of fabric visible. Cracks will usually start at the inner edge of the belt and run outward. All worn or damaged drive belts should be replaced immediately. It is best to replace all drive belts at one time, as a preventive maintenance measure, during this service operation.
The maintaining of proper tension on belts is perhaps the most important thing in ensuring long life and the proper functioning of the components they drive.
If belts are installed too tightly, they put excess pressure on pulleys and bearings causing failure of such components as the water pump, alternator, power steering pump, air conditioning compressor, etc. Installing belts too loosely can cause slippage, which permits improper operation of the components that they drive, plus glazing and cracking of the belt.
A general rule for proper belt tensioning is simply this: find the longest straight run of the belt and depress it at its mid-point with your finger. You should be able to depress it about 1 / 2 inch; no more and not much less. Belts are tightened by loosening the components which they drive and moving it to increase tension. Some components are made of soft or brittle metals such as aluminum or magnesium, so don't pry on them with a metal prybar.