Richard this is a vacuum leak.The default position for no vacuum is defrost.
On this vehicle the vacuum reservoir is located behind the battery.Here is a pic of it.
You have to remove the battery and battery tray to remove it. There will be a vacuum line coming from the intake manifold to it and one going from it to the inside of the truck. Maybe one of the lines are broken and have come un-done.
I replaced the vacuum line from the reservoir & the blue check valve, Ford part. I took a vacuum reading from the main line coming from the intake manifold (18 lbs.) The problem is still there. If I take the a/c controller out of dash, can I see the vacuum line running to it? How much of the dash do I have to take apart to get the temp. controller out? Do you have any pictures of the vacuum line coming inside the truck & where does it go? Do you have written procederes how the temp. controller comes out of the dash?
Thank You! Richard
Yes you can see it.The black line is the supply line.
Here are the instructions on removing it,these are for the electronic controls
Detach the center instrument panel finish panel.
Disconnect the passenger air bag deactivation (PAD) switch electrical connector
If equipped, disconnect the 4WD control switch connector. Remove the center instrument panel finish panel
Remove the screws
Remove the EATC module.
The manual controls are pretty much the same.
Here are the vacuum schematics
The first ones are the manual system
VACUUM APPLICATION CHART-MANUAL CLIMATE CONTROL
V = Vacuum
NV = No Vacuum
Here are the ones for the electronic
Vacuum Applications Chart-Electronic Automatic Temperature Control
V = vacuum
NV = no vacuum