Hi! I tried all the connection options and it did not work. (((
Hey, outstanding work! Here is an observation I wanted to share with you with my fan speeds on my Mac since my power cable mod (Pixlas mod on Mac Pro 5.1)
It seems that apple regulates fan speeds based on power draw, (either voltage or amps) and not really based on temperature sensors. Let’s assume for a moment this is the same case with your iMac, -your fan would stay on Full blast because of twice the power draw because of the second added fan.
If you want to have the original fan speed situation I think you have two Options:
#1
The power for the second fan has to come from a different location to run, but gets regulated by the first fan sensor cable (maybe the third cable off the fan plug) This way the combined Sensor cable would only get the power read of the original fan, because the second add on fan gets the voltage / amps from a different Outlet on the mainboard.
Question remains wher to find an alternative spot for getting power to the second fan that is not monitored on power draw. Maybe someone else here with more iMac Know-How can chip in.
#2
Slow down or throttle down both fan speeds because of an additional adjustable resistor / potentiometer / poti to get more grip on this issue and burn up excess power on that resistor to get slow dual fan speeds.
Just my 0.02 cent ideas..