Most vehicles have a rubber belt on the front of the engine that drives accessories such as the air-conditioning compressor, power steering pump and alternator. If this accessory drive belt (also called a V or serpentine belt) breaks, the battery won't get charged, the air conditioner won't blow cold air and the power steering will go out. In addition, if the belt drives the water pump, the engine could overheat.
Most manufacturers call for periodic inspection of the belt as part of scheduled maintenance, but few list a specific replacement interval, and inspection intervals vary widely.
Mercedes-Benz, for example, says to inspect the belt every two years or 20,000 miles, while Volkswagen says to check it every 40,000 miles. On most Ford vehicles, the manufacturer says to start inspecting it after 100,000 miles and then every 10,000 miles. On many GM vehicles, the first recommended inspection is at 150,000 miles or 10 years.
Though these belts often last many years, they can become cracked or frayed and need to be replaced. That's why they should be inspected at least annually on vehicles that are more than a few years old. In addition, if a belt needs to be replaced, the pulleys and tensioners that guide the belt should be inspected to determine if they caused damage other than normal wear.
A belt that isn't cracked or frayed may look like it's in good shape, but grooves on the hidden side may be worn enough that the belt slips on the pulleys that drive the accessories. That will cause problems in systems that rely on the belt to keep things humming. For example, a slipping drive belt may cause the alternator to work intermittently or at reduced power, and the battery won't get fully recharged as a result, perhaps triggering a warning light.
Another sign of a worn belt is a squealing noise under acceleration. That could indicate that the belt is slipping because of wear, a belt tensioner is loose or a pulley is out of alignment.
Most modern vehicles use belts made from ethylene propylene diene monomer, a synthetic rubber that lasts longer than older types of engine belts. Most belt manufacturers estimate the typical lifespan of an EPDM belt to be 50,000 to 60,000 miles, and some say it's more than 100,000 miles. However, it can be hard to tell how worn one is with just a visual check because EPDM belts are less likely to crack or lose chunks of rubber than other types. They should be inspected by a professional.