Sorry to dig up an old thread, but my problem follows along close to this. About 1.5 years ago my battery light started coming in and out while driving. After about 4 hours of driving over the course of a week it went to on while the engine was running.

W140 S350D 603 engine.

I bought a Bosch voltage regulator/brush assembly and had a local auto electric shop install it. We never could get the battery to charge so I ended up parking it.

Two weeks ago I started playing again. New battery (old one was shot). I bought a plug in volt meter that goes in the cigar lighter.
Test today went like this:
Charged overnight to 100% by a battery charger.
Plugged in volt meter and turned key to first position -> 12.8 Volts
Turned key to turn on the glow plugs -> voltage drops to 11.2 Volts
All idiot lights light up and stay on, including the battery symbol.
Light goes out at 12 seconds or so -> voltage stays at 11.2 Volts (Normal, the glow plug light only indicates it is ok to try starting. The glow plug relay stays energized for 30ish seconds.)
At 20 seconds I started the car, fired right up -> voltage went back up to around 12.4, battery light stays on and does not go off until I turn the car off.
Went to the battery and hooked up my Fluke multimeter -> 12.4 Volts DC.
Switched multimeter to AC ->started at .008 Volts AC, went down to .002 Volts AC. That is 2 mVac.

Why the heck is it not charging!!! What do I check next? Voltage on the blue wire right at the alternator?

(Other notes, yesterday I used the cigar lighter volt meter to verify that voltage did not change with engine speed. I ran a two mile straight away at speeds up to 65 MPH and it stayed between 12.1 and 12.3 Vdc.)

Jim

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.