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EFV-8 Club Forum / General Ford Discussion / Engine Run-On problem with electric fan

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Discussion Topic: Engine Run-On problem with electric fan

47coe

10-11-2009 @ 9:55 PM

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I posted this on Fordbarn as well...maybe I can find some more expertise here.

I have a 47 Ford COE with an aftermarket electric fan (which I intend to remove once I obtain all the parts to install the engine driven fan). The truck is mostly stock and I recently rewired the ignition circuit to work through the original steering column switch. Previously, the truck had a modern key start with an accessory position. Since I don't have an accessory position on the original switch, I wired the fan and coil together so that the fan only runs when the engine is running. Now when I shut off the engine, it will run for about 3 seconds with slowly dwindling rpm. If I disconnect the fan, the engine dies instantly when the ignition switch is opened...this is the desireable condition.

My questions are:
1) can an electric fan cause run-on problems because the spinning fan generates electricity (the motor becomes a generator briefly as no power is applied but the blades are spinning) which feeds back into the coil?
2) Obviously, I've identified the fan as the problem...how are other folks wiring electric fans when using only the original ignition switch?
3) Do I need a reverse-current relay, diode, or a separate fan relay (to isolate the coil)?

Any response would be appreciated...

Dan

TomO

10-12-2009 @ 6:56 AM

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You should be using a relay to feed the fan. The ignition switch was not designed to handle the high current of the fan.

Tom

supereal

10-12-2009 @ 8:29 AM

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Yes, the fan can generate enough power to feed the coil if it is turning after the ignition is turned off. There are a couple of ways to prevent this. First, an electric fan should always be operated with a relay to prevent overloading the ignition switch, as Tom says. That will physically disconnect the fan when the relay is de-energized. You can install a diode in the line to the fan, but due to the high current requirements of low voltage electric fans, the relay is a better choice. Run on is also aided by failure to completely close the throttle, and a too high idle speed.

47coe

10-12-2009 @ 11:09 AM

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Thanks guys, appreciate the advice. I didn't mention it for simplicity but I am using a relay for the ignition switch. The original switch is only carrying enough current to close the relay. The relay I'm currently using for that purpose is a 60A relay and my total current draw through the relay is about 30A tops.

I've got the stock flathead and idle is set around 400 rpm. The engine is running a bit rich after I rebuilt the carb so maybe the extra gas is adding to the run-on.

I'm inclined to add another relay for the fan. Although it will increase the number of wires required, it is more reliable than a diode. I was planning on using the diode from an alternator application so I can guarantee the current carrying capacity but I think I'll go with the relay.

Thanks again...

supereal

10-12-2009 @ 12:17 PM

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As long as you have the ignition and fan tied together on the output side of your relay, you will have the problem. You need a separate reply for the fan only. Just parallel the relay coil with the one for your ignition, but don't interconnect the relay points. If you have a double throw relay now, use a separate set of points on that relay for the ignition and fan, using those contacts to separate the fan and ignition. We prefer to use a thermostatically controlled relay for fans so it comes on and off as needed.

47coe

10-12-2009 @ 12:40 PM

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Thanks again. I plan to go with a relay in parallel to the ignition relay. I don't need a thermostatically controlled relay since I have a thermocouple and temperature rheostat downcurrent from the ignition relay now. I'll just isolate the +12V from the coil and work the problem that way. Then I can be certain of eliminating any run-on issues.

TomO

10-13-2009 @ 7:29 AM

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If you intend to change back to the standard fan, why don't you just use a toggle switch capable of handling 30 amps to control the fan? It will require that you remember to turn it on and off, but it is less expensive and less complicated than a relay.

Tom

47coe

10-13-2009 @ 5:44 PM

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Tom...good idea. But I don't know how long that swap will take. I need an original 42-47 COE radiator (same as big trucks) and a COE-unique fan shroud. The truck has a 53 radiator which doesn't fit well (sits 3 inches forward of the original radiator) and there is too much distance between the fan and radiator.

So, I'm going to go with the relay and avoid any forgetfulness on turning on the fan.

Dan

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