Changing a radiator fan control switch to an ECU controlled fan with a manual
over ride switch.
This is a guide to allow your ECU (in this case an Emerald K3) to control the
switching of your radiator fan in lieu of the existing fan control switch (which is
plugged into your radiator). It also includes the installation of a manual over ride
switch as well.
The relay is included in the earth return lead to the relay to simplify wiring of the
override switch. It could just as easily be included in series with the fuse in the
positive feed. The override switch directly switches the fan rather than the relay so
that it will switch on the fan, even if the relay has failed.
Kudos to Paul Deslandes and JK who are the actual brains behind this; I am just the
monkey typing it up.
Tools you will need
Crimp connectors
Heat shrink & heat gun
Crimping pliers
Multimeter (always useful!)
Wire
A switch (for the manual override or use one of the existing ones on the dash)
A 12V 30A 4 pole ‘normally open’ relay
The existing fan circuit looks like this:
+12V green wire à Fuse 3 à green wire to rad fan control switch à black/green
wire from rad control switch to fan à black wire from fan to earth.
Emerald K3 ECU
You can set the temperature at which the ecu activates the fan in the software
menus (refer to manual). In common with most ECUs with a fan control facility, the
K3 has separate on/off temperatures so that the fan comes on at a preset
temperature but runs until a lower temperature has been reached. As well as better
controlling the temperature, it also stops the fan ‘hunting’ on and off around a single
temperature. The ‘ON’ temperature should be set approximately 5degC higher than
the ‘OFF’ temperature.
The ecu’s Pin 6 is an 'output swtiches to earth to activate relay and fan'
This means that you need to insert this ecu controlled ‘switched earth’ into the fan
circuit, after the fan (where the black wire currently is).
Don’t forget to remove the existing rad fan control switch from the circuit at the same
time!
The new circuit will look like this:
+12V green wire à Fuse 3 à green wire [OMIT to rad fan control switch à
black/green wire from rad control switch] to fan à black wire from fan à ECU
switched earth
So you will see that the circuit is left open (i.e. ‘off’) until the ecu decides to close the
circuit by earthing it at a predetermined temperature. This completes the circuit and
switches on the fan.
Manual Override Switch
The Manual OR switch works on the same principle as the ECU switch; it simply
completes the circuit by earthing the fan’s black wire. The difference being that you
can do this yourself on your dash or where ever you locate your OR switch.
For this to work however, you need to make sure that the Manual OR switch has a
chance to complete the circuit before the ECU does. This is achieved by connecting
a second black earth wire (leading to your OR switch) to the fan black earth wire,
before the ecu is connected to it. Your circuit will look like this:
+12v green wire à Fuse 3 à green wire to fan à
black wire from fan
à ECU switched earth
à Manual OR switch
So how do you wire the ECU in then?
Hopefully the above information provides an overview of the logic behind how the set
up works. Now all you have to do is get the ECU into the circuit somehow. To do
this you will need to insert a relay.
For those who don’t know or are terrified by the thought of relays (like me!) this can
seem a little daunting, however JK posted a link to an idiots guide that even I, as an
idiot, managed to understand: http://www.autoshop101.com/forms/hweb2.pdf
You need to buy a 12V 30A 4 pole ‘normally open’ relay. eBay/Halfords etc all have
these for a couple of quid. Here’s the one I bought:
http://www.ebay.co.uk/itm/111635180009?_trksid=p2057872.m2749.l2649&ssPageN
ame=STRK%3AMEBIDX%3AIT
Here is a diagram of how it is all wired up:
Mount the relay close enough to the fan so that you can take the exiting fan wires
directly to the relay without extending them. If you have a relay with a mounting tab
that allows it to be screwed to a solid surface, so much the better. Ohterwise cable
tie it to a solid point or the loom so that it doesn’t flap about and risk fatiguing the
wires over time.
So you connect the black wire from your fan to pin 30 on the relay, then take a new
wire from pin 87 to earth.
The circuit is completed when the ECU switches pin 85 to earth and the ignition is
turned on and 12V connected to pin 86.
Hang on a second, what’s this +12V ignition thing all about then?
Well you’re going to need to find your +12V ignition wire. This is a wire that is only
live with 12v once you’ve turned your ignition on. Thankfully there is a logic to the
way these little cars of ours are wired up.
On the k series (I can’t speak for newer models I’m afraid) this is the white wire
behind your dash or that runs to the grey engine plug in your engine bay.
It is not the red and white wire. This wire only triggers when you press the start
button/turn the key to start the car.
If you are unsure, use your mutlimeter and test that the white wire is only live when
the ignition is on.
Connect into this white wire and connect it to pin 86 on the relay.
Wiring OR switch
Simply run your new (second!) black wire from the fan –ve (black wire) all the way to
your switch. I used a spare fan switch that was already on my dash. Connect your
black wire to this and make sure you take a final earthing wire from the switch to a
common ground on the car (otherwise it won’t work!). I used the ground located
behind the dash on the drivers side.
And there you have it. Just hold your breath and say a little prayer before flicking on
the OR switch!