Can't stop the engine from running

[lalessi1]

Well, this is first for me... I tightened the very loose fan belt on my FE and started the engine. The "GEN" light came on. Drove the car a bit pulled into the garage, the car would not shut off, even with the key removed. Disconnected the battery, still running. Finally pulled the coil wire. I have a Pertronix II ignition system, a new alternator with an older "solid state" voltage regulator. I am thinking voltage regulator? Any thoughts.. My car has been sitting a year...new engine, front end rebuild, transmission rebuild, various body work projects, new exhaust as of today. I was really looking forward to driving it. 

[BWhitmore]

It could be that you have a bad diode in your alternator which is allowing 12 volts to be fed back to the ignition switch and the coil when the switch is turned off.  Remove the wires from your alternator and see if the engine turns off.  If it does you have found the problem.

[lalessi1]

Well I pulled the alternator and the car started and shut off as designed. I had the alternator tested and it bench tested "OK".
I put it back on and pulled the regulator to find the stator spade connector was off. Not sure if it was loose and came off in the process of removal. Re-did the stator and field wire connectors at the regulator and started the car...all was good. Summit Racing is sending me another alternator after agreeing a bad diode could have caused the problem. I think the diode may fail at elevated temperatures, the engine was running at 220 degrees (another issue) when I couldn't shut it off.

It could be that you have a bad diode in your alternator which is allowing 12 volts to be fed back to the ignition switch and the coil when the switch is turned off.  Remove the wires from your alternator and see if the engine turns off.  If it does you have found the problem.

Thanks for the help.

[BWhitmore]

If the problem re-occurs with the new alternator you may need to install a diode in the wire going to the alternator from the ign switch that energizes the alternator.  As you know a diode is a one way check valve and allows current to flow in one direction but not the other.  I had the same problem on my Ranchero (312 with a XX alternator with internal regulator).   I was able to source from the maker of my new wiring loom a loom that included the diode I am referring to however it would be easy to add this diode to your existing wiring loom.