Those old Chrysler external regulators were almost impossible to field test…They were tested by testing the alternator by “full fielding” it. If the alternator responded normally, then the regulator was judged to be at fault. Typically, owners of these vehicles replaced both the alternator and regulator at the same time as the regulators were not very expensive. Millions of Chrysler product vehicles used them, so you could get one at a salvage yard for a couple of bucks…They were pretty reliable, failure was unusual…