Fan Switch Modification

This article documents something that I did to MY car. This document may contain errors. If you choose to attempt to duplicate this modification, you do so at your own risk

The electric cooling fans on the 3rd generation Rx-7 don't tend to activate until the coolant temperature has become very hot. Those of you with aftermarket coolant temperature gauges know exactly what I'm talking about.

One trick is to turn the A/C on low. This will cause the fans to come on (low to medium speed usually).

Another trick is to turn on the parking lights. This will cause the ECU to turn the fans on at a lower temperature than it usually would.

My problem is that 1) my driveway is 1/4 mile long and very steep 2) I park on the top (6th) floor of the parking garage at work, you enter from the bottom and creep up to the top in 1st gear... avoiding all the idiots who think the parking garage is their own personal autocross course. Both of these situations tend to get the engine temps up just when you need to shut the engine off.

So, I rigged up a switch to activate the cooling fans, instantly. This way I can activate the fans early and avoid letting the engine temps get up.

First, a little information on how the fans work.

The fans are controlled by four relays that live on the passenger's side of the engine compartment. I will name them 1, 2, 3, and 4.

4 1
3 2

(as viewed from the passenger's side fender)

Relay 1 is connected to the heater control unit and activates when you turn the A/C on.
Relay 2 and Relay 4 are connected in parallel.
Relay 3 is connected to the water thermoswitch and increases the fan speed.

Various combinations of relay activation yield different fan speeds. There are many combinations, but these are the ones I am concerned with:

2 + 4 = low fan speed
2 + 3 + 4 = medium fan speed
1 + 2 + 3 + 4 = high fan speed

The ECU just grounds a pin to activate a given relay. I don't think it can check the state of the relays, so I should be able to activate the relays manually without upsetting the fuel computer.

So here is what I did:

Locate the relays (click the image below for a larger picture).

The set of relays can be seen in the picture above. The two relays with the colored dots on them are the relays that I tapped into.

Removing the two 10mm bolts that hold the relay bracket in place gives easy access to the wiring.

The red-dotted relay should have a GREEN/BLACK (green wire with a black stripe) wire going into it. Splice a piece of insulated 16 or 18 gauge braided wire into the GREEN/BLACK wire. A vampire type connector can be used or just strip the wires and solder them together. Be SURE that you don't cut the wires going to any of the relays. Do the same for the BLUE/GREEN wire going to the green-dotted relay. Check the wires with a voltmeter before splicing into them. If they have voltage on them under any circumstances (ignition on, off, etc.) then you have the wrong wires. There are large, high current wires in that same harness that you don't want to mess with.

You should now have two wires coming from the two relays. Now would be a good time to test them. With the ignition switch on, grounding the wire from the red-dotted relay should give you low fan speed. Grounding both wires should give you medium fan speed.

It is time to make a decision.

  1. Splice the two wires together with a single, longer wire. Run the single wire into the passenger compartment and connect it to a SPST switch (RS# 275-324b). Splicing the two wires together has the side effect that any time the ECU tries to turn the fans on, it will get one speed higher than it asked for. It will get medium speed instead of low, and high speed instead of medium. This is fine with me and that is how I did mine.

  2. You can run both wires into the passenger compartment and connect each one to its own switch, or use an single DPST switch, and the ECU will get the fan speeds it asks for when the switch is open.

In either case, the other side of the switch (the side not connected to the relays) needs to be connected to a ground.

I used a piece of convoluted tubing (also available at Radio Shack) to protect the wires running from the relays. The tubing is indicated by blue dots in the picture above. I routed the wiring through the ECU harness grommet at the firewall. The ECU harness grommet is a major pain to get to, so, in the future I will probably just drill a hole in the firewall somewhere and add a grommet.

Here is a picture of my switch, mounted next to my water temp gauge, in a home-made gauge panel.


Thanks go to Tom Johnson for this great idea!