If you follow the path of the serpentine belt on your engine, you will notice that it passes over several accessory pulleys. It also passes over a small pulley on the belt tensioner. The belt tensioner is a small component that consists of four main parts:

Base: The metal part that holds all the other components in place

Tensioner arm: Located at the bottom of the tensioner; applies pressure to the belt and allows for enough slack to help you adjust or remove the belt

Spring: Provides pressure to keep the tensioner tight

Pulley: The rotating part that makes the belt move

For the most part, all belt tensioners look the same. Some are larger, some are smaller. However, the design remains more or less the same.

The belt tensioner is always located in the path of the serpentine belt. It has a smaller pulley than the other pulleys in that path, making it easy to identify. If you look at a Mazda CX7 engine, you will notice that the belt tensioner is located on the right side.

Tensioners can be either static (fixed) or dynamic (moving) when they are working. A static tensioner is one where the tension is set and the tensioner is fixed in place to maintain a specific tension on the belt while the drive system is operating. Dynamic tensioners use some sort of spring arrangement to provide a known reaction force that pushes the tensioner into the belt, thereby tensioning the belt.