Manual Cooling Fan Control

Updated July 31, 2000.

Some people have expressed interest in being able to either manually control their engine cooling fans, or have the cooling fans run at all times. On this page, I'll try to explain how to do it. Feel free to email me at slin01@mail.orion.org if anything is unclear.

Theory and Operation
 The cooling fans in all W-body cars are driven by electric motors. Some cars have one fan, but most have two, a primary and a secondary fan. Normally, the ECM controls the fans and tells them when to turn on. The ECM uses inputs from an engine temperature sensor and fan request lines to determine whether one or both fans should come on. The ECM controls the fans by grounding the coil on one or both relays. When the coil on a relay is grounded, it turns on and switches on the appropriate fan. In order to manually control the fans, what must be done, is to ground the relay coil. When the car is switched on, one terminal of the relay coil is +12V (or hot at all times). The other terminal is open. Ground that terminal, and the relay will switch on. You can add a switch between that terminal and ground in order to manually control the fan.

How To Do It

Precautionary Testing. First, you will need to find the OPEN end of the relay coil (the end that is 0V, and NOT +12V). This is very important, because you'll blow a fuse if you accidentally ground the hot side of the coil. Here's how to do it:

  1. Decide whether you want to manually control the Primary Cooling Fan, the Secondary Cooling Fan, or both.
  2. Find the relays. They should be in an electrical center under the hood. There should be a label on the cover telling you which fan is which. On my '89 Olds Cutlass Supreme and '90 Pontiac Grand Prix, the relays are under the RH (passenger side) electrical center.
  3. Pull the relays and look underneath them. Notice how the terminals are labeled. #85 and #86 are for the coil. When the coil switches on, terminals #30 and #87 are shorted together.
  4. Now you need to turn on the ignition. You don't need to start the engine.
  5. Get a voltmeter or test light. Connect one lead of the test light or negative lead of the voltmeter to any ground (any unpainted metal part of the car's chassis or motor).
  6. Using the other lead of your test light or the positive lead of your voltmeter, touch the metal terminals on the relay SOCKET that correspond to the relay's #85 and #86 terminals. See Fig. 1 and Fig. 2 below
  7. The terminal that lights your test light or shows close to +12V on your voltmeter is the HOT terminal of the relay coil. The side that is 0V is the OPEN terminal. This is the one you want to remember.
Fig. 1 - Checking the terminals in the Primary Fan relay socket. Notice the voltmeter reads close to +12V. That means this is the HOT side.
Fig. 2 - Checking the terminals in the Secondary Fan relay socket.

If you want to, you can just hope your car is wired the same as my Cutlass and not bother doing all the above testing. GM is usually pretty consistent, but sometimes the wiring varies. On my Cutlass, the terminal that's HOT is #85, so we need to ground #86. It's the same for both fan relays. If you ground the wrong side, you'll blow a fuse. The above procedure is just to make extra sure you do it right the first time.

The Actual Modification. Okay, now we're ready to do the actual wiring.

  1. Pull the relay(s) if you haven't already. If you only want to manually control one fan, just pull the relay for the one you want. If you want to control both fans, pull both fan relays.
  2. Using a length small gauge wire (22AWG or so), wrap it around terminal #86 on the relay(s) - or on some cars it might be #85. You'll need to use a pretty long wire if you want to switch the fan off and on from inside the car. Not so long if you just want the fan to run with the ignition on. You can get by with a small gauge wire since the relay coil only consumes 160mA to 200mA (mA = milliAmps). See Fig 3. for an example.
    DO NOT ACCIDENTALLY CONNECT THE WIRE TO TERMINALS #30 or #87! THIS COULD CAUSE SOMETHING TO MELT or BURN. PLUS IT WILL BLOW A FUSIBLE LINK.
    Fig. 3 - Wrap a wire around the appropriate terminal (#86 on my car).
  3. Don't wrap the wire around too much of the terminal, because you'll need to be able to get the relay back in its socket. Push the wire as close to the relay's body as possible. At this point, it would be a good idea to solder the wire to the terminal to make a reliable connection. Make sure to keep solder off of the terminal where the wire isn't touching it, because solder blobs on the terminal might prevent the relay from pushing far enough into its socket. For the sake of this demonstration, I didn't bother soldering it.
  4. Now plug the relay back into the socket. If all went well, the relay should go nearly all the way down into its socket with only the wire coming out from under it.
  5. Now take the other end of the wire, and connect it to an SPST (single pole, single throw) switch. Ground the other end of the switch, and you will now be able to turn the fan on and off with the ignition ON. If you wish to have the fan run constantly with the ignition on, then simply ground the wire to any available ground. Best way is probably to crimp a ring terminal to the end of the wire, and ground it to an existing unpainted metal bolt on the frame, strut tower, engine, etc. See Fig. 4 for wiring examples.
  6. Repeat steps 1-5 for the other fan relay if you wish to have manual control over both fans.

 
Fig. 4 - Here are a couple wiring examples.

Now you should be all finished. Test it out by turning the ignition on, and turning on your manual fan switch. The fan should be running. If it isn't, double check your wiring and make sure you wrapped your wire on the correct relay terminal. If you wrapped the wire on the wrong relay terminal, you may have blown the ALT fuse. Make sure you have a good ground and a good connection. If your fan(s) still won't come on, make sure the fan motors are good.
Fig. 5 - A quick example

I have quickly wired this up as shown in Fig. 5 and verified that it works fine. I didn't use a switch, I just grounded the black wire that I attached to relay terminal #86 to the engine using the white alligator clip. This does indeed work fine, although I just rigged it up temporarily for demonstration purposes.

Other Solutions...
There are other ways to implement manually controlled fans. If you want a more permanent solution, you can actually tap into the wire(s) that feed the relay coils using wire taps or splice right into it. The wires you'd need to tap into is DK GRN-WHT for the primary cooling fan, and DK BLU-WHT for the secondary cooling fan. You can then ground them or connect them to ground via switches similar to the method I've explained. I chose to just attach the wire to the relay itself because it's, a) easier, and b) very easily reversible. If you solder the wire to the relay terminal itself, that would probably be more reliable than wire taps. I chose to ground the relay coil instead of shorting across the relay with a switch because, a) it won't hurt the ECM (as the ECM uses open-collector outputs to the fan relays), b) the fan will switch off with ignition off, and c) you aren't dealing with high current, so you can get by with small gauge wire which is easier to conceal.

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