Ok, I’m going to try to answer this but wait until someone confirms it accurate. I’m not a mechanic.
No matter what, to release the clutch you need both air and power.
No power = locked fan
No air = locked fan
If everything is working properly (temp switch, clutch, solenoid):
1) Start truck and air up system. (air is needed to unlock the clutch)
2) Shut off engine. Only run long enough to fill air tanks, do not run until engine is hot as that might complicate things with temp switch. You want a cold engine. (a hot switch locks fan)
3) Turn on power switch (do not start). (solenoid needs power to unlock clutch)
4) Crawl under and see if the fan will turn. If it turns… it will turn with resistance. It will not just spin with a flick.
If it turns, then great, if not, you have a problem. Don’t fix what aint broke. There’s plenty of other things to spend time and money on with these trucks.