I had exactly the same problem about a year ago. The alternator light/battery light on the instrument cluster is not coming on when I turn the ignition key to ON position, and at the same time, my battery is draining which means the alternator is not charging the battery. The bulb is good, the fuse is good, the voltage regulator is good, and the overvoltage protection relay fuse is also good. I finally replaced the alternator and fixed the problem. I had that old alternator bench-tested at AutoZone and their tester determined that the alternator diodes are bad.

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'85 300D Turbo - CA Version