If you didnt already know, sorrry to point it out if you do, when you lose the belt and the alternator is not longer being driven, you typically have all the instrument cluster lights dimmly. You lose electricity and all the sensors voltage drops and the indicator lights all come on. It is typically the sure sign that the belt is off while driving. Also, the ABS light can be a reliable indicator sometimes since the voltage drop seems to happen there first and the ABS sensors will indicate trouble and the light will come on.
at least 3 times in various cars, I have seen basiclly the exact moment my car's battery has dropped in voltage after an alternator has died while driving. The car would then be on battery and the battery begins to drain down until eventually the voltage is low enough to drop first at the ABS sensors and turn on the light. it goes down hill from there and lots of other things start failling and losing power. It is time to turn everything off you can and search for a service station, Fast.

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red