VW TDI serpentine belt tension pulley is a cast aluminum assembly. It has the spring loaded tension arm and the idler pulley integrated in one part. Typically if you begin to hear squealing or grinding noises from your engine, the pulley has gone bad. Poor belt tension can also cause your serpentine belt to slip, which is clearly marked by extremely glossy spots on your belt, this can lead to premature belt failure. If you find that one of these parts is failing the entire assembly must be replaced.
Along with your VW TDI serpentine belt tensioner make sure to replace the serpentine belt; it may have been damaged by a faulty tensioner. The Robert Bentley series of manuals is the industry standard, reference material for working on your Volkswagen. Make sure to have one whenever you start working on your Jetta or Golf TDI.