Compressor Inlet Temp sensor

[Chamberpains]

Unplug the fan solenoid with the key on, engine running, after the air is built up in the truck and the fan releases. I dont know where they are on a prostar but the majority are on the driverside fire wall. As soon as you unplug the solenoid the fan should engage. On a CM871 I believe this will throw a dash code (CEL). If it does not engage the fan then the solenoid is shot. If it doesn't do anything and/or doesn't throw a CEL then your probably looking at bad ECM or the programming.

Knowing how to diagnose things is a big time saver. So I figured I'd share why a couple of us are only thinking the solenoid is stuck open or the ecm or programming is wrong.

When the OP 1st stated that his fan is engaged when he starts the truck to build air and disengages when it gets to pressure throughout the day, that tells us that with no air the hub is locking in as it should and unlocking once air is built up. So the hub works mechanically. On a CM871 and newer, if the solenoid had bad wiring it would throw a CEL. He doesn't have one. So that eliminates most of that. So that leaves limited possibilities.

Now to narrow it down further for the OP, the reason you don't need to monitor other sensors is because you also say the truck overheats and throws check engine lights but the fan doesn't kick on. If you had said that it overheated and no lights came on you would be looking at bad programming or ECM because overheating is one of the strongest overriding codes in the engine. Once that code is triggered the engine automatically overrides everything and starts to protect itself. It turns the fan on, starts to derate (limp mode) then ultimately it'll shutdown. (If its still programmed to do so)

Being that you get a check engine light for overheating that tells us that the ECM recognizes there is a problem but the fan is not reacting. That points directly to the fan solenoid or in rare cases, crazy or wrong programming or occasionally a guy might see an ECM wire put in the wrong slot on the main plugs (this only happens when someone wrongfully rewires the harness, sometimes after engine swaps or harness replacement, but even then it should have a CEL)

What is the maximum temperature the truck gets to? Has it actually shutdown on you because it overheated?