Drive belts keep everything turning off of the crankshaft pulley. It's easy to see when the belts are beginning to wear, the belt will begin to glaze on the underside, making it far less effective, or cracks can appear on the back of the belt from “dry rot,” often caused by age. Squealing is common on worn belts due to the belt is slipping on pulleys. The alternator, fan, power steering pump, and a/c compressor rely on this belt to keep everything turning and working properly. If your drive belt is getting old, cracking, glazed, or squeaking, replace it to keep your accessories functioning properly.