I pulled the voltage regulator and found one clean brush and one carbon lined brush. Photos are available at photos.yahoo.com/tabijan in the MB Voltage Regulator album. Sorry about the quality, my camera doesn't take good close ups. I presume the problem stems from one brush not making contact with the armature (or whatever it contacts). I guess we'll find out when the replacement regulator gets here.

For the purpose of the archives, removing the regulator requires nothing more than a Philips screwdriver. I had to get to the alternator from below. Let's see how much fun it is to install the replacement regulator.

Thanks for all who offered help. It's all much appreciated.

Sixto
91 300SE
81 300SD