The engine accessory drive is called the Front End Accessory Drive (FEAD) or the Accessory Belt Drive System (ABDS). The alternator, power steering, air conditioning and water pump are typically driven by a serpentine belt via the crankshaft. If any of these components fail, with the exception of the air conditioning, an engine will not function and the driver will be left stranded. The serpentine belt that drives these components must be maintained at a proper tension. If the belt tension is too low, the belt will slip causing noise and premature belt wear, which can also cause the accessories to not function properly; if the tension is too high, this will cause excessive belt wear and the accessories can fail as well. In the past, there were several belts on the engine and they needed to be tensioned periodically as the belts would stretch and wear prematurely. In today's modem engines, there are one or two belts and the tension is maintained by an "Automatic Belt Tensioner". This ABDS tensioner sets the belt to the proper tension and maintains the belt tension maintenance free.
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