An automatic belt tensioner is a pulley assembly that may be hydraulic or spring loaded. It automatically keeps tension on the serpentine belt, keeping it tight against the pulleys that drive the alternator, power steering, air conditioning, or any other accessories the engine powers.
A belt tensioner is critical for maintaining proper belt tension, and for keeping the drive belt from slipping or coming off completely. Slippage can cause the engine’s accessories to run inefficiently, or lose power sporadically, and may also cause the belt to wear out more quickly.
Just like the serpentine belt, the automatic belt tensioner is a maintenance item that should be replaced at regular intervals. Over time, the tensioner may begin to lose tension due to rust, water damage, debris, or normal wear and tear. It is usually recommended to replace the tensioner whenever the serpentine belt is replaced, to ensure the new belt is operating efficiently, with the proper tension.