I would say it could be the externally mounted voltage regulator. Its on the back of the alternator. Two screws to take off and look to see if the bars are worn unevenly or worn down.

Should be less than $55 to replace. I use dental floss or fishing line to tie the bars down while installing.

Just a thought....

I have had this happen many a times, and its usually the culprate.

Ryan

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Ryan
1985 300D Turbo Diesel
1984 200 Euro