The hydraulic belt tensioner keeps the main drive belt tight, ensuring there is enough traction for the water pump and alternator to work. Something as simple as a loose belt can cause a failure of both the cooling and electrical systems. This is a BMW Genuine assembly, but has had the BMW logos removed so it can be reboxed as an IPD Germany product. 

This kit come complete with the pulley, assembly, and all fitting components required, and can be used to change your tensioner system from a mechanical to a hydraulic version.