Wiring up a new EMS in the car. My question is regarding the cooling circuit. I no longer have the stock relays/sensors and plan to trigger the fans off of temperatures read by the computer.
The computer has two outputs; each one I can assign to the coolant temp sensor and setup a temperature level trigger on it. That being said I plan to run both fans on low using one output at a lower temp level and then both fans on high using a higher temp level. I'm aware that essentially the fans have two parallel 12V circuits that run through them. Switching on just one circuit allows for the operation at low speed. Switching on the second along with the first kicks the fans up to high speed correct?
If this assumption is correct then using one relay for each output... should I be using a 30A for each or 60A... Looking at the stock diagram I'm a bit confused.
Thanks.

 

If the fans have two circuits one will have lower current for the low speed and one will be higher for high speed. When the second one engages the first one should shut off. I would use a 30 amp fuse for the first one and a 60 for the second one.

 

I did a bit of research and testing the OEM fan control setup, even made a couple of diagrams and tables in this old thread:
https://www.rx7club.com/3rd-generation-specific-1993-2002-16/2-trigger-cooling-fans-462760/




To simplify things, you could use two relays and wire the "switched ground" directly to ground (eliminating Relay 3 from the OEM setup). When the first relay (replaces Relay 1) is switched on, you will be at low speed. When the first relay AND the second relay (replaces Relays 2 and 4) are switched on the fans will be at high speed.



I don't recall the exact amount of current the fans are supposed to draw, but the rule of thumb is to use a relay that is rated for at least twice the normal operating current (because fans require about double the current when they are first switched on). The fans pull power through a 60A fuse so it would be safe to use relays that are rated for at least 60A.