How to Connect a Radiator Fan to a Hot Wire

by Don Bowman

A radiator fan requires an average of 15 amps to operate. The fan uses a relay in the fuse relay box under the hood. The relay has four terminals--one that is hot all the time, even with the key off. A second terminal becomes hot only when the key is on. This is the terminal that actuates the relay. This terminal cannot be used for the fan because it is designed to carry low amperage from the key, and if the fan is attached to it, a fire could result because the wire is too small for the amperage required. The fan must have a 10-gauge wire to carry this amount of amperage. You need to hard wire the radiator fan to a toggle switch on the dash in such a manner that the fan will shut off when the ignition key is off. The fan can also be turned off manually when cruising to extend the life of the fan.

Install the inside toggle switch where convenient. Remove the radiator fan relay from the fuse and relay box on the fender well. Test the relay terminals for power with the ignition key off (use the voltmeter). There will be one terminal that has power with the key off. Mark this terminal on a piece of paper so it can be used later.

Turn the key on and check for a second terminal to have power. This is the one from the key that will be used to actuate the relay. Mark it down. Put the voltmeter on ohms and check the remaining two terminals for a ground. Mark the terminal showing ohms down as a ground. The last terminal is the one to the fan. Now all four terminals have been marked.

Place a jumper wire across the terminal that had power with the key off and the terminal that was dead and did not go to ground. This is the fan hot wire. Set the voltmeter to volts and check the fan wires to see which of the two is the power wire. Mark this wire or remember which one it is.

Disconnect the negative cable to the battery. Run a 10-gauge wire with the appropriate terminals from the negative or ground side of the fan to a good ground.

Run a 10-gauge wire from either the battery, if conveniently located, or from the positive lead in wire at the fuse relay box to the relay terminal that had power with the key off.

Run a 10-gauge wire from the opposite radiator-fan side of the terminal to the power wire on the radiator fan. This is the wire that operates the fan when jumped. Run a 16-gauge wire from the ground terminal of the relay to a good ground.

Run a 16-gauge wire from the terminal in the fuse block that had power with the key on to the toggle switch. Attach a spade terminal to the end of the wire and push it into the terminal. Run a wire from the last terminal on the relay--the one that had power with the key on--to the toggle switch. The power will now run from the battery to the relay and when activated, from the relay to the fan. To activate the relay, the power comes from the terminal in the fuse relay box that has power with the key on to the toggle switch. When the switch is turned on, the power is sent back to the relay, through the relay and to ground, which activates the switch. The fan switch should be left on except when cruising. When the vehicle comes to a stop and no air is moving through the radiator, the fan should be on. The fan should be on continuously when the air conditioning is on.

Items you will need

About the Author

Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).