Rewiring the Fan Relay

Rewiring the Fan Relay
Paradise Garage

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If you run your car really hard in the desert or something, you might need this switch. Or you might not. In any case, it's a cool party trick. On a Mustang, it's not too hard to splice into the fan relay and add a switch which allows you to manually turn the fan on. An actual practical application is to help cool the car faster at a dragstrip.

The Mustang receiving this switch is the oft-visiting 1995 Ford Mustang GT.

First and foremost, disconnect your battery. You run the risk of hitting a hot wire or frying the computer.

Find a place inside the car to put the switch. Our car doesn't have a rear window defroster, so the blank-off plate worked fine. It's located under the headlight switch.

This is how we wired up the dash switch.

Install the switch and LED into the plate of your choice. How you install the switch is quite up to you. We used a standard toggle switch and a 12-volt LED by drilling two holes into the blank-off plate. You'll have to snap off a couple of tabs to make it work.

Run a ground wire through the firewall into the engine bay. Since our switch is up on the dash, we ran it through a grommet near the steering linkage, by the top of the driver's side firewall.

Run the ground wire to the front of the engine bay. Keep it away from hot areas.

Remove the plastic shield over the radiator and route the ground wire away from the hot areas. You can tie it to the other batch of wires already there.

Find the Constant Control Relay Module (CCRM). It's on the passenger side of the radiator, just below the upper radiator hose, next to the coolant overflow bottle, in front of the alternator. It's a relay with about a dozen and a half wires all over the place.

Pull that sucker out!! Loosen its retainer with a 3/16" socket.

Here is the CCRM after we spliced into it.

Look for wire #17, a green/black wire. It's labeled on the CCRM.

Cut the wire, but keep enough wire on each end to work with -- it's all got to get wired together again.

Connect the ground wire to one of the ends of the green/black (#17) wire.

Connect the ground/#17 unit back to the other end of the green/black wire. It's best to use a connector wire -- reconnecting it directly will be pretty hard.

Find a power wire inside the car, preferably someplace near where you're installing the switch, and preferably an ACC source. We used the convertible power line -- which isn't exactly close to the defroster switch -- because we were already Rewiring the Fog Lights.

Run a power wire from the power source to one side of an LED.

Run the LED ground from the LED to the switch.

The final switch looks almost factory-fresh.

Run the fan relay ground (the wire from the firewall) to the same side of the switch as the LED ground.

Find a real ground. There's a metal panel just below the steering wheel; the bolts holding that panel on are grounded.

Run a ground from the unwired side of the switch to the real ground.

The switch should work -- the fan and LED should turn on, but only on RUN or ACC.

Sure, running the fan with the car off will drain the battery -- eventually. Use common sense in how long you keep it going, and you'll have a great trick to help cool the engine faster than the next guy.