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Wiring Electric Radiator Fans


Diamond Member
Sep 1, 2010
I sourced a replacement radiator fan assy. for my beater, which fits fine, but I need to wire it up. Doesn't matter if I need to manually switch it on as the car is for track/strip use only. I was going to get a pair of 40A relays and fuses along with some nice rugged wire and toggle switches.

Only thing I have is the radiator fans, which look like two 12" units, and the wires have been snipped. I could buy a harness but I should have everything I need to at least get it working. Anyone have some experience with this? The car's got a mechanical fan too, but tbh I'll probably take that off once the electric ones are working.


Diamond Member
Aug 11, 2001
Edit: Do the "Dual Fan Wire Example, Manual Control Version" on the following link. That will let you use a single switch to toggle between one or both fans on.

If you want them both off part of the time then use the same "12V fused source" going to both switches, then their outputs going to the individual relays #86 pins. It will then need no diode and you'd only need SPST switches.

Relay coil ground does not need to go to the battery terminal, any good chassis ground point will work. The fans themselves ought to ground back to the battery or a power distribution box capable of handling the extra current (which you may or may not have).


Below I have an example of a typical 40A automoative fuse layout and pinout. Pin 87A wouldn't be used at all, a 4 pin relay would work fine for this app.

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Diamond Member
Sep 14, 2010
Jegs has a number of thermostat switches and thermostat+fan wiring kits. I'd start there. I'd hate to trash an engine because I forgot to flip a fan switch...

Otherwise, here is a very comprehesive wiring diagram:

This setup lets you switch manually or with a thermostat switch, see a light illuminate if the fan is getting power, and won't run the fans when the car is turned off.

Also, you're fine to ground the fans to the body or block as long as it is a good, clean ground. After all, your starter grounds to the block!