Dave,
I'm mistaken. My voltage regulator is fine the way it is wired. If you aren't sure, I can walk you through it as my friend Rich Ludwig did for me last night. Everything is wired where it should be. The cure? Put a diode inline on the fan line so that when the key is turned off, the ignition will no longer sence a power source on the line sending that signal to the voltage regulator/ignition.
What is happening is when you have the fan on, it is drawing alot of current through the amp meter and is keeping the MSD (or whatever you have) ignition turned on. You can either install a diode as suggested or turn the fan off before you turn the engine off. Diode is the cure. Turning the fan off first is a mask.
Good Luck.
