You need to see which field wire on the alt goes back to the voltage reg. I think its the green one but sometimes I have seen different colors. But you want to unhook the field wire going to the reg and see if it still charges. If it does then the alt has a grounded rotor and if stops charging the the field wire back to the reg is shorted to ground. You want the field wire that does not have 12 volts with the key on. Leave the 12 volt field wire hooked and unhook the other field wire then let us know what you get. Ron