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.
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
Using the ratchet and 10 mm socket, disconnect the red 12V power lead from the
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.
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.
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.
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.
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
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.
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.
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.
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
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.
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.
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.
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.
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.
Crawl back inside the car and take away the pliars. You'll notice that the
grommet seals itself around the wire.
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
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.
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.
Reconnect the red 12V power lead to the battery. Do not turn on the car.
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
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
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.
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.
Replace the stock air intake sections if you removed them earlier.
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 -.
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.
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.
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
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.
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.
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.