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I have a 92 hb dx with a z6 swap. The problem is alternator stops charging intermitantly, but if i turn on the lights or blower it comes on and stays on. It will cut out again if none of the accesories are on.

Things i have done:
1 load tested the battery which is fairly new. (bought last summer) and showed it was good
2 changed alternator twice and bench tested newest one, which tested good
3 disconnected sound system-no change
4 take off battery connections and cleaned terminals
5 checked ground points in engine bay

I know that the voltage regulator is built in the alternator but what actually turns the alt on to charge mode? Is there a control unit in the cabin?
Has anybody had the same problem? Any help would be great cuz this is driving me nuts!