Regarding the VAC test across the battery terminals with the engine running, pulsed DC may read as VAC depending on your meter as another board member pointed out.

I'll have to verify my original assertion before I can again stand behind it 100%. Wish I had a good oscilloscope.

On my car, I was getting 12.75VDC across the battery engine running, so I switched to AC and got the 24.9VAC. For me, that was enough to diagnose the bad rectifiers.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)