neepie:
So i want to shutdown or slow down fans even more if not needed or computer is idling. And add some aftercooling to keep fans at high speed after high temps to let all hot air flow away
You might look at PC-utilities that allow you to do the same thing. They have the benefit of not having to change any of your wiring, plus have direct access to the temperature sensors inside your PC. How would your Arduino get access to those temperature sensors?