peterbilt 378 engine fan questions

so here's the deal i have a peterbilt 378 the engine fan stays on all the time
need some opinions on where to look or some troubleshoot help in the right direction.

i replaced the clutch, checked if power is going to the solenoid which with the key in the acc position and the switch is in the on position it gets power which cuts the air, but when i switch it to auto it supplies the air. and does not have any power. with the key in the off position no power but still supplies air. could it be that the solenoid only cuts the air with power? and if so why does it only work with the key on and power supplied. but when i switch it to auto it gets no power and supplies air