I went through this on the 84SD. Buy a Bosch regulator. At least 2 others didn't help and I was considering buying an alternator. Also, be 100% certain that the glow plugs aren't drawing current. Autozoo condemned the alternator then I thought to disconnect the glow wiring and the alternator miraculously started working. Oops!!

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.