hmm, another intesting question. On my W126, i took it to three differant shops to have my alternator checked. MY battery checked out ok, and two of the shops passed my alternator as well. When i asked, the why does my battery not get recharged, they'd look at me with a dumbfounded look on their face. Finally, the third shop connected an amp meter(it may have been ohm also, i'm not sure), but it turns out that even though my voltage was ok, my regulator was causing my amps(or ohms i forget) to jump drastically. This wasn't allowing the battery to charge, thus my car wouldn't start. On my car the regulator was inside the alternator and my alternator bearings were going anyway, so i just replaced the entire alternator(actually the bearings locked up and then i didn't have a choice anymore
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good luck, im sure somone more knowledgeable will chime in shortly,
Ryan