Single fan low/high mod.
 

For those who don't know, I'm doing an engine swap which uses a custom radiator. That's how I got to addressing this mod. I'll be using a single 16" fan and wanted to make use of the low/high speed control that the RX8 has but using only a single single-speed fan. I will add a .5~1r resistor to slow the fan.

No chopping or splicing of the original wiring is needed.

First, pull cooling fan relay #2. Will it throw a code? If so I'll put a 1k resistor across the coil terminals. I'll assume the ECU is just looking for a 12v return signal.

Second, connect the new fan to the RED and BLACK fan wires (lower fan in the drawing).

Third, connect the .5~1r resistor to the BLUE and GREEN fan wires.

That's it. When relay one (low) is on, power flows through the resistor to the fan via. relay 3. When relay 3 energizes, 12v will be connected to the fan. If you don't forget to pull relay 2, you'll burn out your resistor.

Here's a 0.5r resistor that's easy to mount and suitable for the job. Ford uses it in a few cars. With my fan, 0.5r may not be enough to keep the fan quite. Dorman part number 902-219 You can also use the old fuel pump resistor.

https://cimg1.ibsrv.net/gimg/www.rx8...e435cfa32e.jpg

This is what your left with. You do not need to physically remove the wires. I just edited it to show Relay 2 removed from the circuit. It won't throw a code because Relay 2 and Relay 3 had their coils in parallel anyways and I highly doubt the ECU is measuring current draw of the coils.

https://cimg1.ibsrv.net/gimg/www.rx8...d8474e8002.jpg

Here's another trick you can do if you want. Leave relay 2 installed but disconnect the ground point "G2". Run that wire to a light on the dash. It's now a current limited 12v supply when the fan is in high speed. This might be handy for track days to let you know things are toasty.