Trucker Forum - Trucking & Driving Forums - Class A Drivers - Engine fan always stays on when truck is on

Hello everyone, I have an issue that has to be addressed as I only have a month left to do this. I have a 2001 freightliner century with a 12.7 detroit diesel engine in it. The engine fan only runs constantly when the motor is running but not when the key is turned to the first position. I checked the solenoid valve next to the coolant tank and its hissing when I flip the switch on from the cabin so i know its working. The ac system on this truck was taken out by the previous owner I bought it from, so it has no ac system as the hoses are cut off from the ac compressor. Im trying to get some heat here for the winter and also not have my engine freeze up on me since the fan is on all the time. I also tested to see if I can move the fan while the key is only at the first position and it does not budge at all. Then again these engine fans operate only by air but I also knew that so I let the pressure build up and then I tried spinning it.. still didnt spin. The fan clutch looks silverish looking still so I thought the previous owner rebuilt it or put a new one. This is also my first truck so I knew nothing about technical stuff and I purchased it for less because I had to do an engine piston,temp sensors and all sensors overhaul on it which I had a mechanic do it. He says its a stuck clutch. Anymore tips would be good before we put some hands on it. Thanks