What did the slip ring on the alternator look like.

There is an AC voltage test (not DC voltage) that you can do to see if there is an issue with diodes inside of the alternator. I can't remember the details but perhaps someone will chime in.

If you are in good with a mechanic they could hook the alternator up to the engine analyzer and it would show what used to be an oscilloscope pic of the alternator output and it would show up a bad diode.

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84 300D, 82 Volvo 244Gl Diesel