First check that the alt light (and other dash lights) is on when key is in accessory position II but engine not started. If it isn't then check to make sure that the exciter wire (thin red) is properly connected to the alt. As well as the short stiff blue grounding wire from alt case to alt bracket.

If light is on, then when engine is started it should go off. (If it remains on and bright then exciter wire may be shorting to ground.)

Now check voltage at alt. If 14V then may be thick blue grounding wire from engine block to battery. If much less then alt problem (eg. regulator brushes)
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1980 245 Canadian B21A with SU carb and M46 trans in Brampton, Ont.