my truck won't produce air from the floor or defroster, It puts it out on vent but as soon as you push floor or defrost its like you turned the blower off. Any ideas? Birdtracker

There are known problems with the air passage doors inside the HVAC unit that fail, all the air doors are controlled by vacuum, so the place to start is at the vacuum switch, actuators and the vacuum lines and be sure everything with them is working as it should.

A diesel may have a vacuum pump, probably should start there. low vacuum will mess up the door function fast.

the first thing i would do, is use a hand pump vacuum source to check the blend door[s] function. if they work as they should, i would take a good look at the hoses, then the dash control.
all checks would be done using the hand pump vacuum source.

Iirc on most systems regardless of mfr, defrost is the default no vacuum mode.

You're problem sounds like the control head is failing, especially if losing blower motor operation.

I agree, the push button switch is where I would go first. I bet there is a crispy connector on the back and some overheated wires.
Have you checked anything before asking questions?

I have had the glove box opened up and can get to the vaccum hoses. It appears to switch from defrost to vent and floor. Birdtracker

Does the blower motor actually stop running? Or is it simply the airflow getting cut off?

When checking for vacuum behind the glove box, can you see if the can is moving the air door?

the blower motor actually stops as I go out under the hood and its not running. The odd thing is when I move the mode it changes but I don't see anything moving. Unfortunately the weather has been pure crap and now the temps are going back down into the teens. My son loaned me a Jeep to drive so its not a crisis. Guess I will have to take the cluster bezel off and look behind the switches. Birdtracker

I see Ohio plates. What region?

just north of Dayton. Birdtracker

Too far to stop by and check it out.