08 Prostar. on my screen it says the front hvac is stuck or in wrong position. it has said that since October and I just couldn't use the 2 highest settings but now it crapped out. Of course on a day that it's pouring rain for 3 days, cold ass ice out and I was rolling in the mud fixing my truck. ...... Murphys law.
I'm getting ready to open the dash up and thought I'd ask. in your experience with these, what tends to fail?
If you still have the breaker in your fuse block, pull it and replace with a standard 30 amp auto fuse. That should solve your problem. They get old and overheat. The amp draw from the blower is over the top.
Is the blower motor working? And when was the last time you pulled the in cab air filter and changed it? And if you are in the dash, you may as well replace the two door actuators.
As far as the blower motor, not much to go wrong. There is a resistor in their, that may be your problem. It is the bed of nails looking thing. You can pull that out and check the connections for corrosion. If it looks iffy, you may want to change that out. I do not believe there is any way to test it. And of course, it ain't cheap if you get it from the dealer. Get the part number and check Fleet Pride if you have one in the area, or an AC repair shop should stock them or can get you one at a much better price. Same with the blower motor, that is available as an aftermarket item too.
One other thing you may want to try is to reset your cab controller. I have mine on a dash switch specifically for the AC, as it is buggy as hell. You can reset it by pulling the fuse with the motor running. Give it a minute and plug it back in. That may solve your problem too... Or, with the motor off, disconnect the battery cables, so that there is no power getting to the cab. Give it a minute and reattach. That solves some other cab/body controller issues that I have ran into... Buggy ass trucks, there is an engineer at Navistar that needs to be in the stockade.