Question: On an '85 300D Turbo:

Why is it that, at idle, if I disconnect the battery terminals off the battery and test with voltmeter, it's reading 14.7 something volts but if the terminals are reconnected to the battery and tested with voltmeter, it's only reading 11.8 something volts at idle and drops to 10.4 when all headlights, AC, etc. is turned ON, but voltmeter reading creeps up to 11.2 something volts when the engine is revved at higher than idle RPMs?

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'85 300D Turbo - CA Version

Looks like a bad battery to me, as long as the 14.7 is the reading on the alternator and not the battery itself. Shorted cell?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I had this same problem with my 240D. On the Bosch alternators, you can replace the voltage regulator and brushes. They are one unit, I replaced mine with one from autozone (or Oreilly) and now it reads about 13.7 volts at idle. About 25 bucks, I believe. It is an easy fix, the unit is held on the back of the alternator with two screws. I also tightened all the connections after cleaning them as well. Been fine for about a year now.

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'79 240D 4 speed manual 105k miles

Oh, as for the dead battery... I thought the same thing. I still don't have a logical explanation as my battery would read low too, and the voltage would go up when it was unhooked. Maybe just a big load lowered the available power, seems to make sense if the brushes are shot.

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'79 240D 4 speed manual 105k miles

So that being said, battery is still good? and the alt. bosch regulator is bad?

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'85 300D Turbo - CA Version