'04 2500 Cummins. heat works good, I can change the heat temp. It blows out the vents of course. I can get it to switch to floor and defrost by switching the flow control switch back and forth along with the temp control. It will finally start coming out the correct vents. I am wondering, since this thing has solid state drive DC motors, if there is a common ground for the motors. I did swap out the controller, but no help. I worked avionics for the USAF, so I know bad grounds can cause problems with stepped DC digital signals. Does anyone know if all the vent system motors to control the flaps use a common ground? And if so, where is it located?

 

Never mind, the ground for the motors comes from the controller. Does anyone know where the defrost flap motor is? I hate to take everything apart if i don't have to.

 

Nevermind, I just found all the info I need on this piece of crap heater box they put into the truck. I seriously doubt I will take it all apart to fix it. I can rig a removable duct to blow from the center vent onto the windshield when it needs it. The only good thing in the truck is the engine.

 

Your truck is 14 years old, it's going to have problems.