Ron Pope Motorsports                California Custom Roadsters               

Electric Fan Is a Generator


A mystery is 'afoot'. After a fresh, ground-up wiring job this week, my kill switch would not kill the running engine immediately. What the heck?

When the kill switch is operated it removes +12 volts from the ignition system which SHOULD stop the engine. A voltmeter on the ignition assembly showed that when the kill switch was operated the voltage on the ignition system was at 12 volts or so then declined over a period of several seconds to 0 volts during which time, the engine would continue to run, falter and finally shut down.

I determined that the source of the voltage was coming from the electric radiator fan by pulling the fuse. If it is running at shutdown, the inertia of the spinning fan blades generated plenty of DC voltage and current - enough to run the ignition for a few seconds.

Problem solved! The fix was to install a hefty diode in series with the +12volt fan motor lead so that the motor received running voltage when the ignition was on but when acting as a generator after the ignition was killed, the diode blocks voltage from the fan motor getting into the ignition circuit.
That is one of the reasons I use a Fan relay.
Mine car does the same thing. I use a manual switch for the fan when needed. If I forget to turn the fan off, the engine just keeps running. No big deal as I am as done with that car as I will ever be. Unless I sell it.

     Ron Pope Motorsports                Advertise with Us!