Is the belt properly tensioned. Mercedes and other German cars tend to run with a high belt tension. An old test (not sure it really applies here) is no more than a 1/2" deflection in a long belt span (between two pullies).

If the belt is tight Sixto is probably right: the tensioner has failed. The tensioner is made up of an outer housing of metal and an inner shaft that is attached to the outer by rubber. If the inner twists too far is breaks and you loose the tension.

It also simply be that the tech who changed your water pump didn't understand how to adjust the belt in the first place. BTW, the tensioner is locked in place by a bolt, 17mm I believe, but is tensioned by the long nut in your picture.

If you are not sure, have someone who has "been there and done that" look at it for you.

Haasman

__________________
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)