Wild Weasel's Automotive HowTo Repository
Skip Navigation LinksInstall a Cooling Fan Bypass Switch

HOWTO: Install a Cooling Fan Bypass Switch

Written by Wild Weasel


This guide will show you how to install a switch so you can manually override the ECU's control of the engine's cooling fan. This can be useful in several cases. Firstly, the ECU will not turn on the fan itself until the engine is quite a bit over the normal operating temperature. If you're sitting in traffic and just don't want the engine running so hot, you can use this switch to bring the temperature back down. You can do this by turning on the A/C if you have it, which automatically turns on the fan, but if you're driving with the windows open then this is a big waste of power.

The other, perhaps more important use, is to cool down your engine on track days. Whether you're drag racing or autocrossing, a hot engine is a less powerful engine and if your engine is hot enough to turn the fan on by itself, then it's already way too hot for you to be competitive with it. This will allow you to run the fan with the engine turned off for maximum cooling. Note though that you may want to install a volt gauge so you can keep an eye on the status of your battery.

Wiring Diagram

This is the diagram for what we're doing here. The parts in black are the stock wiring for the cooling fan. The parts in red are what you're adding to make this work. For those of you that know how to read a circuit diagram, this should be all you need to know. Make sure you get the right green wire and go make it happen.

It's been a long time since I've had the upper portion of the stock air intake system under my hood. I would rather not post instructions to remove it than attempt to do so and get it wrong. If you can't get that piece out of the way, then maybe working under the hood isn't your thing.

The main wiring bundle goes through a large grey grommet behind the clutch pedal up into the fuse box under the hood just in front of the driver's side strut tower. The top of the fuse panel is labeled so you can see exactly where the fan relay is. Since there's more than one green wire going into the box, you'll want to pop it open to ensure you're splicing into the correct one.

  1. Using the ratchet and 10 mm socket, disconnect the red 12V power lead from the battery.

  2. Release the fuse box by pulling out the plug holding it down toward the front of the car. Use pliars to pull up the tab in the middle of the plug and it will pop right out without breaking. Now you can lift up that end of the fuse box.

  3. To disconnect the bottom, use a flat screwdriver to release the tabs all around it. With the bottom off, look for the green wire going to the fan relay.

  4. Being very careful not to cut into any wires, use a utility knife to cut through the tape holding the large wire loom closed leading into the fuse box. Cut and peel away the electrical tape holding the wires together.

    Separate the green wire leading to the fan relay from the other wires.

  5. Using the wire cutters, cut that green wire in half. This is where you're going to attach the diode. You can now clip the bottom back onto the fuse box and use the plastic plug you removed in step 2 to secure it back in place.

  6. Take the two red butt connectors and the diode. Trim down the terminals on the diode so you've got just enough to fully go into the butt connector while still being able to see the diode outside of the plastic. Using the crimping tool, attach a red butt connector to each side of the diode. It's important that you still be able to see the diode so you can tell which end of it has the stripe indicating the Cathode.

    Now is a good time to explain what a diode does and why we're putting one in this system. A diode acts as a one-way street for electric current. Power can flow one way through the diode but not the other way. By putting the diode between the new switch and the ECU, the ECU is still able to complete the circuit and turn on the fan but cannot detect that we've manually completed the circuit on our own. Without the diode, the ECU may be able to detect that the circuit has been completed and may throw a code indicating a failure of the cooling system. It is not fully known at this time whether the diode is necessary in this application. If it is unnecessary then no harm will come of it. If it is necessary, however, then it may prevent your ECU from throwing a code. If you understand electronics and are questioning the application here, this diagram explains how the ECU could potentially look for an alternate ground in the system and how the diode is preventing it from finding it.

  7. Using the wire stripper, strip 1/4" from the end of the green wire going down into the engine bay away from the fuse box. Look closely at the diode and note that one end of the diode has a white stripe on it. That end is the cathode. The other end is the anode. Power can flow from the anode to the cathode but not the other way. Since the ECU provides a ground in this system, you need to allow power to flow toward the ECU. Crimp the wire you just stripped into the butt connector on the side of the diode that has the white stripe.

  8. Now strip 1/4" from the end of the green wire that goes into the fuse box and crimp a blue butt connector onto it.

  9. Cut a length of wire that you will use to bridge the two connectors. You may want to use a 6" or longer piece to give you enough wire to loop back and around so you can fit it all nicely back into the wire loom. It's always better to cut longer than shorter since you can always trim it down later to fit how you want. Strip 1/4" off one end of that wire and crimp it to the red butt connector that is attached to the anode side of the diode.

  10. If you don't have heat shrink tubing, wrap up the section of wire with the two red butt connectors and the diode in it with electrical tape.

    If you do have the shrink tubing, slide a piece over that whole section of wire and use the heat gun to seal it up. Be careful not to overheat it and melt anything.

  11. Get a length of wire long enough to go from the fuse box to the firewall, through the firewall into the interior, and to wherever you intend to put your switch. Basically, get a great big piece of wire.

    Crawl under the steering column and locate the big gray grommet where the main wiring bundle goes through the firewall. The wiring bundle goes through the middle and is surrounded by about an inch of soft rubber.

  12. Use a permanent marker to mark a dot on that rubber, to the left of the main bundle of wires right in the middle of the bundle and edge of the grommet. This is where you're going to run your wire through the grommet. You need the dot because once you poke the hole and remove the awl, the hole closes and is very difficult to find again.

  13. Use the scratch awl to stab a hole exactly in the center of your dot. You can actually leave the awl in the hole for now.

  14. Using needle nose pliars, hold the end of your wire with the pliars and push them through the hole you just made. The grommet will stretch around the pliars. If you left the awl in the hole, remove it now. You can now leave the pliars in the hole, holding the wire through.

  15. Go back under the hood and look for where your pliars and wire are sticking out. Pull a couple feet of wire through the firewall. Pull enough to route over to where you need to connect it below the fuse box.

  16. Crawl back inside the car and take away the pliars. You'll notice that the grommet seals itself around the wire.

  17. Strip 1/4" of insulation off the wire you pulled into the engine bay in step 15 and do the same to the bare end of the wire you attached to the diode in step 9.

  18. Place both of those wire ends you just stripped beside each other and squeeze the threads together as though they were one wire. You don't need to twist them. Just squeeze them together with your fingers. Now feed the two of them together into the blue butt connector that you attached to the end of the wire coming from the fuse box in step 8. Ensure that the wires are going into the metal part of the butt connector together and crimp them. The insulation on the butt connector should be overlapping the insulation on both wires just like in all your other connections.

  19. You are now finished with the wiring under the hood. Before wrapping everything up, you'll want to test that it works. First, go into the car and find the other end of the wire you're going to be connecting to the switch. Strip off 1/4" from the end of it and set it on your seat, away from any metal objects.

  20. Reconnect the red 12V power lead to the battery. Do not turn on the car.

  21. Go inside the car and pick up the end of the wire from the seat. Take out your cigarette lighter if you have one and hold the end of the wire to the side wall of the lighter socket close to the outer lip. DO NOT TOUCH IT TO THE BACK WALL OF THE SOCKET. When you hold it to the side of the socket, you should hear the relay switch and your fan turn on. If you don't, go back and recheck all of your connections. When you're done testing, put the end of the wire back on the seat, away from any metal objects, and go disconnect the red 12V lead from the battery again.

  22. Wrap up the main bundle of wires with electrical tape as it was when you started. The wire with the diode attached will be wrapped up within this bundle. The wire leading inside the car for your switch can exit at the bottom of it.

  23. Put the large plastic wire loom back around the wire bundle and wrap it in a few places with electrical tape to keep it secure, just as it was when you started.

  24. Put plastic wire loom over your wire from where it joins the main cable bundle right back to the grommet. Put some tape around both ends of the loom to keep it secured to the wire.

  25. Replace the stock air intake sections if you removed them earlier.

  26. Back inside the car, determine where you want to mount your switch. Because there are countless different types of switches on the market, I will leave the mounting entirely up to you. Your switch will have 2 or 3 leads on it, depending on whether or not there is a lighted indicator on it. They will be labelled, either on the switch itself or in some instructions or packaging that came with it, as +, -, and LOAD or, perhaps, just + and -.

  27. Now you need to find a good ground for your switch. Mine is grounded to my steering column. There are also plenty of good grounds to be found along the top of the dash. Alternately, you can tap into an existing ground wire if you have a wiring diagram and can find one. If you can't find a ground, then maybe dealing with wires just isn't your thing and you should find someone else more knowledgable to help you. If you're not tapping into an existing ground wire, be sure to use a multimeter to check that you've got a good ground where you intend to connect to. Not all metal parts make good grounds.

  28. When you've found your grounding point, get a piece of wire long enough to run from there to your switch and attach your resistor in-line on the wire at the end near where the switch will be.

  29. Use your multimeter to test that the resistance is as you expect it to be from one end of the wire to the other. If you're using a 56 ohm resistor as recommended, the resistance should obviously be 56 ohms (within 1) on the wire now.

  30. If your switch has 3 leads, connect another wire to this wire BELOW the resistor. You can ground that wire separately if you'd like but I find it's easier to ground them together. Attach that wire to the grounding point you found in step 27.

  31. Now connect the wire coming in from the engine bay to the + connection on your switch and connect the wire with the resistor on it to the - and load connections on the switch.

  32. Go back under the hood and reconnect the red 12V cable to the battery.

Your installation is now complete. Your switch should now turn the fan on and off. Be careful not to just leave it running with the car off though as you'll just run your battery down.

If you found these pages helpful, please donate and sign the guest book.

In many cases, these pages aren't only helpful but can save you a little or a lot of money. If the tips you found here helped you save money by doing something yourself, or preventing you from making costly mistakes, then please show your appreciation by sending a few bucks my way to help pay for the site.

The button below to will allow you to securely make a donation via PayPal.

Sign my GuestBook! * View my GuestBook!

Return to the index page.

And this? That's not me.