Richard that is what I thought, somthing had to be done with that extra wire and I could not remember what. It could also be used to excite an idiot lamp if one chose to do so.
I would suggest running the excite wire to the switch at a differant (on hot) lead than the ignition. I realy think he has blown a diode and is now back feeding the coil from the alternator, and that is why it would not turn off.
The diode trio assy. is inside the alt. Tony one of the diodes is probably blown.
