Belt Tensioner

Tensioners use a spring, a hydraulic damper or both, and a pulley that presses on the belt. Idler pulleys guide the belt around the engine. Bearings inside the pulleys wear over time, so tensioners and idlers are usually replaced together with the belt. Automatic tensioners have a mark or pointer that shows whether belt tension is within range. Manual tensioners are set with a tool during fitting.

The tensioner moves to take up belt stretch and absorbs vibration from the engine, keeping the belt pressure within a set range.

A worn tensioner can cause squealing, rattling or belt flutter. A failed timing belt tensioner can let the belt jump teeth, with possible engine damage. Noise from a tensioner is often worse at idle or when the engine is cold. A bearing that feels rough when turned by hand should be replaced.