I would like to have my fan turn on by the ECU, and AEM says it's possible, but I have been advised not to do this. So where does a rad fan switch go then? I used to control it manually, but his has dangers obviously. Thanks!

[Edited on 16/7/11 by RK]

Soren S2

[Edited on 16/7/11 by sorens2]

Sorens answer looks good to me. It is essential to have the relay between ECU and fan.

The other relay is optional in my mind and you could just have a switch (depending on fan size and switch size). Motors have tricky start-up loads, but you will use the switch very little so its unlikely to wear out.

I thought the ecu would normaly switch the negative to the relay

If it's a seperate fan switch (ie not ECU or manual controled) you mean...

It'd normally be screwed into a boss on the rad' and wired... Fused 12v supply > switch > fan > earth. The rad' switch is made to take the fan load so no need for a relay.

If you have no boss on the rad' then it can go in the rad' side of the thermostat housing or in the top hose (adapters available) but these aren't the best positions. The 12v supply is normally ignition swtched but a few tin-tops use perminent supply, allowing cooling to continue after engine off. Most fan switches are just 2-wire type but there are some 3-wire ones, that switch one wire at a lower temp and then the second at a higher temp for a second fan.
You could even wire it as Sorens diagram but with a rad switch as well for "belt, braces and hands in pockets" approch.

Thanks everyone. I am still confused however. I want it to come on automatically, and NOT from the AEM ECU. I have too much money and time into that to screw it all up not knowing what I am doing.

I don't see a spot to put a thermo switch. I DO have an overheat switch I installed on the head that closes at 110 c. Could this go to the relay and then the fan? When the car was on the rolling road a few weeks ago, temps were very low, so I don't foresee a problem coming on when things are very hot.

you will need a radiator thermal switch as fitted to 90% of nrmal road cars. Get one that suits tour required temp ( there was a list of part nos and temps on here somewhere). Then wire it as per sorens diagram using the switch wire from the thermoswitch instead of the ecu.
so +ve to thermo switch then from the other side of the switch to the relay and the rest as per sorens diagram with the manual overide option IF you want it.

OK that's better - thanks!

Check out this link it gives you everything you want.......i hope.

http://www.godspeed.me/forums/viewtopic.php?f=14&t=1289