interior Blower only Blows to Defroster

[CGCWO]

Anyone have an idea where I should start? AC compressor is disconnected, but would still like to get some air circulation in the cab, but no matter what position the switch is in I only get air from the defroster vents.

[Eagle]

The flappers are controlled by vacuum. You have a vacuum leak.

 

First place to check is where the vacuum hose to the reservoir behind the front bumper passes by the battery.

[CGCWO]

Thanks, much easier than disassembling the dash.

[sloride]

I recently had same problem.  During the reinstall of heater box, I unknowingly popped a vacuum hose 'splice' upon reassembly.  It was the factory one-into-two vacuum hose splice bundled with the hoses wrapped in a loom leading up to heater/ac control on dash.  To determine my problem was in the cab and not under the hood, I disconnected the vacuum hose firewall side just below the pressure bottle going to the reservoir behind front bumper and tried to pull a vacuum on hose leading into the cab.  After discovering no vacuum, I broke out the cigar and blew smoke until the leak was located.  That's only the second time someone has smoked in the truck since it was new.   

[reubj]

Mine will revert to defrost when going up a long incline. I must have a small leak somewhere.

[CGCWO]

Problem fixed, took about 15 mins to track down a vacuum line that was disconnected at the switch on the back of the control unit inside the dash. Thanks for giving me some direction.