When to replace the serpentine belt tensioner

When should you replace the serpentine belt tensioner

Let’s get this straight from the get-go, a serpentine belt tensioner doesn’t last the life of the vehicle. In fact, most start to show serious wear right around the 100K mile mark. You may think a worn tensioner just means your belt loses tension; you’d be wrong. It’s just the opposite. A worn tensioner actually increases tension on the belt and that increased tension can damage the nose bearings on your AC compressor, power steering pump and alternator. Here’s why.

A serpentine belt tensioner is also a dampening mechanism

Every time a cylinder fires it produces a power pulse that pulls the belt tighter. Inbetween power pulses the belt tension relaxes. To avoid the constant pull/release, every tensioner is built with a dampening mechanism; it’s kinda like a shock absorber for the belt system. When the dampening mechanism wears, the belt experiences more pull/release motions, and that jerking/release motion is transmitted to the belt and then to the bearings on your compressor, pump, and alternator. This belt movement is called belt flutter.

Belt flutter causes tensioner idler movement

The easiest way to assess the belt tensioner condition is to

belt tensioner movement while running

observe it while the engine is running. A good belt tensioner will dampen the power pulses so much that the tensioner pulley arm barely moves. As the dampener wears the pulses cause the belt to flutter and that can be seen on the pulley arm. A belt tensioner pulley arm should not move more than 1/32”. Once worn, that same pulley arm can move up to ¼”; it almost “dances” as the engine runs. That dancing movement creates a vibration, almost like plucking a guitar. So in addition to increasing tension on the compressor, pump and alternator bearings, the plucking vibrations add to the bearing wear.

Repeat alternator, compressor and pump failures may be due to a worn tensioner

If your alternator

To learn more about serpentine belt tensioners, see this post

©, 2020 Rick Muscoplat