The Drive Belt Pulley controls how effectively your Mercedes alternator and water pump operate. If the belt marker does not fall within the hash mark indicators on the tensioner you need a new belt or tensioner. However, if the pulley is noisy, you can most likely get away with just replacing the idler pulley.
2007-2008 GL320
from engine # 40 777760
from 11/10/07
2013 GL350
thru engine # 41 276768
thru 02/20/12
2007-2008 ML320
from engine # 40 777760
from 11/10/07
2012 ML350 Bluetec 4M
thru engine # 41 276768
thru 02/20/12
2007-2008 R320
from engine # 40 777760
from 11/10/07
2012-2013 S350
thru engine # 41 276768
thru 02/20/12