How do I stop engine run on caused by the cooling fan? I am using a Mallory Unilite distributor and a Spal radiator fan controlled by a standard relay and temp sender. What kind of diode should I use? How and where should it be connected? Also, how do you determine the polarity of the diode?

I also read in the forums about EMF spike caused by the cooling fan damaging electronics. Would the above diode solution also solve EMF spike problem?

Thanks in advance for help with this.

Not sure how you have your fan wired, but you should have a relay in between the battery and the fan. The relay controlled by the ignition switch. This way, your fan will only get powered when the switch is turned to the run position.

wiring this way should prevent what your experiencing, unless your timing and or idle settings are way off .

How do I stop engine run on caused by the cooling fan? I am using a Mallory Unilite distributor and a Spal radiator fan controlled by a standard relay and temp sender. What kind of diode should I use? How and where should it be connected? Also, how do you determine the polarity of the diode?

I also read in the forums about EMF spike caused by the cooling fan damaging electronics. Would the above diode solution also solve EMF spike problem?

Thanks in advance for help with this. Your right you need a diode and if this is 1st gen its best installed up at the voltage regulator on the radiator support. We tried to install it at the alternator and i didn't help, so we installed it at the voltage regualtor...I cant remember but if you look into some threads you may find the diode.

Sorry, I did not give vehicle info. 1969 Camaro 454. Single wire alternator.

A diode should do it (only allows electric to flow one way)

A diode is not needed if wired correctly. You probably have the ign hot wire for the fan hooked into the coil positive wire (key hot which is usually the easiest key hot wire to find) to allow the relay to operate. Napa sells a diode protected relay to stop the spike when the relay opens, but will have nothing to do with run on. Key hot needs to be from a source other than the coil key on wire, and the supply wire for the relay comes from your horn relay junction or battery. Terminal 85 on the relay is key hot, 86 is the sending unit wire, 87 is constant 12 volts and 30 is power to the fan. Here is a link to good info on relays...http://www.autoshop101.com/forms/hweb2.pdf