How to Wire a Relay for a Radiator Fan

by Don Bowman

When installing an auxiliary fan or simply wiring the factory fan so that the driver can manually turn the fan on and off, make sure the fan is turning in the right direction. If the fan is not turning in the right direction, air will be blown away from the radiator, and will not help to cool the radiator. To check which way the air goes, just put your hand in front of the fan to make sure the air is blowing through the radiator. The fan will turn in either direction depending on which of the two wires the power is supplied to. There are two methods to wire the relay, one with manual control and one with automated control.

Manual control relay wiring:

Step 1

Purchase a 4-post relay with a 30-amp rating. Locate the relay in immediate proximity to the fan being used. Find a spot to secure the relay with a screw or a bolt. Install a 10-gauge wire to the battery positive terminal with the use of a round electrical connector. Remove the nut on the battery terminal and put the wire connector over the bolt and tighten the nut. Remove the negative terminal of the battery first so that there is no chance of sparks. Make sure the wire is long enough to route it out of sight and away from any moving parts.

Step 2

Cut the wire about 6 inches from the connection to the relay and attach a 30-amp inline fuse with two wire connectors (do this on the longer side of the wire, but do not attach it to the shorter, relay side of the wire yet). Before the inline fuse is attached to the relay side of the 10-gauge wire, insert a 16-gauge wire that will be long enough to go to the switch side of the relay. Just cut the insulation off of the 16-gauge and twist the wire end and insert the 10-gauge and the 16-gauge together into the wire connector and crimp it. Use a female terminal on the other end and attach it to the switch terminal of the relay. Cut another piece of the 10-gauge wire long enough to go to the fan. Use a blade terminal for the relay and a barrel connector for the fan.

Step 3

Attach the wire to the relay terminal opposite the battery terminal. Route the 16-guage wire up the fenderwell and through the firewall to just below the dash. You will see rubber grommets usually around or just under the brake booster where the throttle cable comes through the firewall. Use these for access into the cabin.

Step 4

Attach the wire to the relay with a female blade to the negative side, which is the last free terminal. Attach the dash side of the 16-gauge wire to the switch with a female blade terminal and mount the switch to the bottom of the dash. Run a ground wire on the other terminal of the switch. Use a good ground such as the steering column or the dash mount bolt at the kick panel for the ground. Any good ground to the frame will work. Now when the switch is turned on it will supply a ground for the relay causing the relay to actuate and turn the fan on. In the event the fan fails there is a fuse inline to protect the wiring.

Automatic control of fan:

Step 1

Purchase an automatic heat-sensing switch at any auto parts store. It consists of a small square and adjustable relay and thermostat box, a gas filled tube with a sensor on the end and the correct wiring connectors. This works great in that when it is turned on the fan will only operate when the temperature gets to a predetermined figure. It is a stand-alone unit that is not effected by the computer or anything else.

Step 2

Locate the heat-sensing switch as close as possible to the radiator. Attach the box with the screws provided. Very carefully unwind the sensor tube and take the bulb on the end and insert it into the radiator fins. Just line it up between the cores and push it all the way in. This senses the heat.

Step 3

Wire it up the same way that the manual unit was wired. The difference is that the manual turns the fans on continuously where this one will allow the fans to operate as temperature requires. If you leave the automatic fan switch on it will not normally effect anything because the fans will not operate when the engine is cool, and it will not draw from the battery.

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