Need to rewire some aftermarket two wire fans up. I am running a microtech ecu and trying to use as much of the stock wiring harness as I can to save running new wires unnecessarily. I am part way there with what I need to do but not Sure on a couple of things and hope someone can help.

I managed to run a thermoswitch plug and pick up the original harness plug under the dash. So have found that at the solenoid. Now with two fans must I run each fan off it's own relay? Seems like it from most schematics I can find? But why? Is that to not over load the solenoid? Also, does it matter if the solenoid is completing a live circuit or earth? I can see the no.3 solenoid completes the earth for one speed of the stock fans and it looks like the no.1 completes a live to feed the other speed? So I could use either relay 1 or 3 to power both fans with a little switch on some wiring. Or if I need a relay per fan I might need to use both or one of the others as well. Either way I think I need to loop a ground or a live somewhere to get the necessary earth and live feeds.
Mainly I just need to know how many relays I require at minimum and I can probably figure the rest!
Thank you
Lee

 

The issue you are going to have with the stock wiring it that it may put a lot of load on your ignition switch and burn it out, make it miss contacts and also overload the wiring its self if the fans pull more amps then the oem relays can handle, from what I have seen on my own car the oem relays get issues around 20amps and like every other major wire in the FD is suffering voltage drop.

Stock fans are 3 speed and have 4 relays and use 1 relay per speed and they come on in different situations. Aftermarket are normally 1 speed I would recommend 3 relays but you could do it with 2 if your brave.


I gutted my stock fan relays and oem thermos switch about 3 months after I got my car because I found it lacking

 

Interesting.. That puts a different spin on it! How would I go about running 3 relays? I think I could figure out 3 relays. Would I be using no.3 to complete the earth for one relay and say number one to power it and then come off this other relay to power both fans? Or would I be using one to complete earth and then 1 per fan. Can I check what amps my fans draw easily? I assume if I put a 20 amp fuse or what ever is recommended in the ignition live that will protect the ignition switch? That said the wiring diagram shows it fused at 15amps at the joint box?

 

I have gone over the wiring diagrams today and compared to what's on the car and come up with the following plan. My fans are 80W each which as far as I worked out is just under 7 amps each. So should not cause an issue with the relays?
The relays 2 and 4 power each fan on the stock but are earthed via a heater control of ECU. So instead I will use these relays and use the wire from the thermo switch to earth each of these. The fans will then earth direct to chassis via the earth wire on the OEM fan plugs.
I can see how you could incorporate no.3 relay as a third unit to earth both fans via but don't see the benefit in doing so. You would them need to use the thermo switch to earth all 3 relays and it is only a small wire so I would imagine that might not be the best idea?
Unless there is something I am missing I think the 2 relay option looks most suitable and gives comparable wire gauges on each side of the fan and relays. That said I am no electrician and learning as I go!

 

 

Are you going to use the ECU you control the fan's or are you going to use an aftermarket fan controller?

Have a read In here to Davies Craig
also most of the diagrams found on the daviescraig web site can be adapted to your needs.

I am not affiliated with daviescraig in anyway they just have the best information I have found.

3rd relay would be for aircon if you run it

Fans should be pulling up to 30 amps

 

Thanks for the posts. I will check out the video and Davies Craig. I was just planning on having the fans on or off controlled by the thermo switch. Will get an fc switch eventually so they come on at a lower temp. I am not running a/c not worth the extra weight and mess in the engine bay here in the UK!
I had an additional thought about loading of the ignition circuit as well. The ignition side is only powering the relay. The direct 12v is what will power the fans via the 60amp fuse with the 2 relay setup I am planning.

Cheers
Lee