A few weeks ago, I had an alternator belt fail on my 616 diesel 240D. I replaced the belt, but now the alternator light stays on at idle and, albeit faintly at cruise. At first it would go out at normal highway cruise RPM. I have checked the belt for tension, and it seems to be OK, but I have not precisely measured the tension.
Any ideas would be greatly appreciated.
Probably a slipping belt, but also could be something dragging which caused the old belt to die and will kill the new one soon too!
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.
You might need new brushes on the alternator. Two screws take out the regulator/brushes assembly (small black box on the lower rear of the alternator). It's not expensive. Your battery is likely discharging as a result, so hook up a charger overnight if you have one. Don't overtighten the belt, or it will wear the bearings.
Measure the voltage to see if the alternator is putting out at least 13.5V at idle... and 14.4 when revving at higher RPM. If not, you could try changing the voltage regulator (aka brushes)... it is cheap to do.
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