You may need to replace the pulley if there is play in the bearing, the pulley is out of alignment, or the pulley is squealing(because the bearings are bad).
Additional Fitment Specifications:
2006 C350 Engine Range: From 30 058377 Smooth
2006 CLK350 Engine Range: From 30 058377 Smooth
2006 E350 Engine Range: From 30 058377 Smooth
2006 ML350 Engine Range: From 30 058377 Smooth
2006 R350 Engine Range: From 30 058377 Smooth
2010-2011 R350 4Matic 3.5L 3498CC 213Cu. In. V6
2005-2006 SLK350 Engine Range: From 30 058377 Smooth