When this happened to me, it turned out to be worn brushes in the voltage regulator. 2 screws on the botton side of the alternator are removed. The old one pops out, the new one pops back in. Takes longer to setup for it than to actually do the work. This site and others sell a $15 volt. reg. that some believe to be as good as the Bosch counterpart which is more like $35 or so.

To test - start car. Hook up voltage ohm meter(VOM) - DC20 scale. Red lead to battery + and black lead to battery -

Running you will set anything from 13.x to 14.x

When the brushes are gone, your voltage reading while idling will be considerably less than 13.x

Also, new brushes are approx. 1/2" long when new.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"