Hello,

I am having trouble diagnosing a charging system issue. The car is a 1984 Mercedes 300D.

My battery kept dying on me a month ago. I replaced the battery. The new one died too. Charged it back up and began to look at the alternator. The voltage at the battery was not rising when the car was on.

Took the alternator out of the car, got a new voltage regulator for it, put it back in car. Still no rise in voltage. Took the alternator to the autozone to bench test (it was fine), put it back in the car, no rise in voltage.

Checked continuity between the alternator plug (both B+ terminals) and the battery + (checked out fine), and between battery ground and chassis and engine (all checked out fine). Even took the ground terminal off the chassis and battery, cleaned it, put it back on. Took the alternator off the car, and back to the store to have retested. This time it failed.

Bought a rebuilt bosch 55amp alternator, put it in the car. Battery reading 11.8V before turning the car on. Did not rise with new alternator when car started. Took it back out and back to the store. Bench tested it again (it was fine).

Read about the battery light on the instrument cluster. Took the cluster apart. Checked the bulb with a bench power supply (it lit up, works). Put it back in the cluster, and tested continuity from the plug (hole 6) to the fuse (#12), and from the plug (hole 7) to the D+ third terminal on the alternator. This is essentially both ends of the dash bulb. Both tested okay.

Put the dash back together, and plugged the alternator back into the car (just wires, not belts). I turned the key and no battery light comes on.

What am I doing wrong here? What is left to check? I've been on this on and off for a month now, and now have a new alternator and battery and a car that will not charge the battery. I can now install or remove an alternator from this car in five minutes flat. Any help here would be awesome.

Thanks,
Blake