Proper Tension

• A properly tensioned belt should have about 1/2 inch of play when pushed side to side, with the engine not running. Belt tension should be checked in the middle of its span between pulleys. When the engine is running, there should not be any apparent flutter of the belt. Belts should be checked periodically to assure the proper tension is maintained.
  
  

Automatic Tensioners

• Most modern cars use a serpentine belt that drives many or all of the car's engine components (alternator, air conditioner, etc.). These belts are tensioned by a device called a tensioner, which is a spring-loaded pulley that holds pressure on the belt. If a tensioner is not working properly, it should be replaced.
  
  

Tension Problems

• An improperly tensioned belt can cause numerous problems. A belt too loose can slip, which causes noise, improperly powers components, and can fall off. A belt too tight can break or damage the components it drives.