A timing belt tensioner is a device that places tension on the timing belt in an engine to ensure contact is maintained between the belt and the pulleys on which it rides.
In order for your engine to function properly, all of the moving parts inside must be synchronized. They need to move at just the right time and in the right relationships to one another, just like the instruments in an orchestra. Like the conductor, your timing belt serves to coordinate movement of the crankshaft, camshaft(s), intake and exhaust valves, and more.
Timing belt tensioners vary in design from one engine to the next, typically relying on spring tension. Some modern engines use hydraulic tensioners that use engine oil to self adjust automatically.
A significant amount of tools and training are necessary to replace a timing belt tensioner, as it is quite an involved process and one that can lead to engine damage if performed improperly.
To replace a timing belt tensioner, a technician needs to gain access to the front of the engine. This often requires lifting and supporting the vehicle in the air. Shields, guards, covers, ducts, drive belt, and even the radiator might need to be removed as well.
With those items out of the way, a typical timing belt tensioner replacement (the procedure differs from one vehicle make and model to the next) involves the following general steps: