A serpentine belt tensioner is designed to automatically hold a predetermined amount of tension on a drive belt while the engine is in operation. If the drive belt is loose, the tensioner has failed or the wrong size belt has been installed. The tensioner should be about half way through its travel to hold proper tension, most have a gauge stamped on them to determine belt stretch for replacement.
On rare occasions an engine can buck or jerk at start up and can suddenly cause a belt to jump out of place. Other problems can occur if the drive belt snaps, this can be due to fatigue of the belt. The most common reason for the belt to snap is age coupled with the natural wear and tear of the belt. Before belt tensioners there was a simple adjustment bolt that was used to set the tension on the belt. The disadvantage to this set up is once the belt stretches slightly it would have to be re-adjusted. If re-adjustment is not performed the belt can create a loud screeching noise. A belt tensioner consists of a main body, swing arm, tension spring, pulley and pulley bearing.
The drive belt or serpentine belt is an essential component of your vehicle. The primary belt function is to supply power to the steering system, water pump, air pump (if equipped) air conditioner and alternator. This belt is connected to the drive pulley of the engine to supply power. If the belt fails it will almost always render your vehicle inoperable until the drive belt is replaced. Typically most people are able to replace a serpentine belt themselves with basic automotive tools. When replacing the belt draw a diagram of the belt routing before you start to avoid confusion. If you have removed the drive belt and need the belt routing you can look it up on a belt routing diagram.