International 8100 fan solenoid

Maybe @Heavyd knows the original location. The supply line would have to be capped or the port it goes to plugged.

 

If it's got a Horton like this one check if it has bolts where the red circles are.
Those holes are for bolting the clutch solid and on all the time.
If it does then you probably have a bad fan clutch.
Put 12 volts and ground to that solenoid while the truck is aired up and it will tell you whether the supply line is hooked up or the line to the fan is.

 

I would imagine the solenoid went out. Instead of replacing it they just took the supply line off so the fan would engage. So when you hook the supply line up, it will probably disengage the fan if it's working properly. You may just need a new solenoid. You can run a supply line from anywhere as long as you have pressure.

 

If he has the older horton fan clutch like the one in the pic I posted, it took air to engage them. That's why they had the tapped holes to bolt them solid if they failed.

 

You are right. I hated Hortons.

 

Air supply for fans is usually secondary air, or the orange circuits. You can get it from any secondary supply line, it really doesn't matter.