I've been going through the same exact same low voltages as you have with yours. I have a 1982 300 Turbo. I found that the Glo-plug relay timer is not kicking off. I unplugged the relay after some deep head scratching and three used altenators last known to be good. Now my voltages are up to 13.4 at 1000 RPM just like yours. Wiring goes to a junction on the right fender near the battery. The wiring just felt warm while running and that was my clue. Tomorrow I'll see if I can find out what up with the glo-plug timer.

Great post by the way. I checked my ground wire on the body near the battery and there was some ugly stuff in the bolt threads. Cleaned it up and put some anti-corrosion goop on.

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1984 280SEL, 62,000 miles
Euro model in the USA
1983 300D (Totaled out 10/2004)
New Factory Mint Green paint
Palomino int
1982 240D (Steak dinner for two)