Hello - just wanted to relay the whole experience - I think there will be others who will encounter the same symptoms and end up with the same simple fix.
Friday the 13th - Was heading to my folk's house, an hour away by interstate, with the headlights on and A/C running plus my son listening to a Thomas the Tank Engine on the tape player...
Thrity minutes into the trip the speedometer needle started jumping, then few minutes later it was reading about 10 mph slower. Then the tape player quit playing. I turned off the headlights and the radio came back on and the speedo was back to reading normal. I turned on the headlights and this affected most of the gauge needles and speedo again... I started thinking to myself 'oh, s--t, I must have screwed up something while cleaning the engine'. When I used the turn signals this also affected the gauges.
When I finally got to the exit ramp, the A/C started putting out hot air. Crap, maybe they did something at the shop while servicing and recharging the a/c. It wasn't cheap and I got this sinking feeling having to visit the shop again. I arrived at the house and left the car running and looked in the engine compartment. Nothing unusual. All the wires and connections looked fine. The engine was turned off and I went inside, hoping the problem would mysteriously go away. When I came back out about 30 minutes later to start the car, the battery bogged down. It was discharged somehow and I start wondering if the alternator conked out and everything was running on the battery.
I called the MB dealer service department asking about an alternator replacement and was told they wouldn't be able to get to it until Thursday !
But he also told me, I think out of sympathy, that 9 out of 10 times it's not the alternator but the voltage regulator mounted on the back side of the alternator. Within an hour before the parts department closed at 5:30, I had the voltage regulator removed and replaced and everything was back to normal.
One thing that helped speed up the process was a very short screwdriver (about a 2" handle and 1" blade) that allowed me to reach in and remove the two screws without getting hung up on anything. (Be prepared for a messy arm and hand - roll up your sleeve up to your shoulder.)
The 1981 300SD, with 192,000 miles, still has the original alternator. The Bosche voltage regulator has two spring loaded friction contacts. They were both worn down, one of them more then half. There are two different types, one with a diode and the other without. ($73 vs. $50).
Mine had the diode in it.
While I was picking up the part, I made sure to thank Scott the Service Coordinator at West Motors Sales in Roanoke, Virginia. He saved my weekend and spared me from much anxiety.