Well, there is no evidence of any slippage or dragging of the belt, but
then again, the W115 alternator is in a place where it is difficult to
see much of anything. Another part of the mystery is that
occaisionally, the alternator light will not come on when the
electrical (key)switch is turned on, but then will come on as soon as
the start switch is pulled. This could be related, or not, I do not
know, and the only manual I have, the Haynes, is mum on the subject.