I agree with regulator diodes, failed letting it backfeed itself to dead. Should be a replaceable part. Pending the mileage on it, you might be well off to just get an alternator rebuild kit (bearings, brushes, brush springs, regulator) and fix it up like new again, much cheaper option than a new or rebuilt.

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Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale