1992 Chevy C Pickup No Defrost

This pickup does not have A/C. Heat blows out of the middle and lower vent fine, but nothing blows out of the top defrost vent to the windshield. What should I look for? Is there a baffle somewhere that might be stuck? Any advice would be great. Thanks!

Look for an obstruction on the defrost duct. Than check the mode door.(left end of the plenium)

Is the "mode door" on the left end near the floor or closer to the windshield? There is a working valve near the floor in the center of the console that moves back and forth to switch between the floor and middle dash vents. What is the best way to get to the mode door? Thanks again.

First thing.
Check for good vacuum from engine to vacuum tank and from tank to heater controls.

Near the transmission hump.

The mode door works fine. We are referring to the same thing. There is something else wrong. I'll check for a working vaccuum and obstruction next. Thanks.

I have had mode doors with a pen stuck in them holding them open.

I could see that happening. I'll check things out later tonight and see if I can pinpoint a possible problem, now that I have some ideas of what to look for.

Ok. I've looked things over. I do not see any vaccuum lines for the heater controls. Everything appears to work with a lever and a cable. There must be some sort of an interior valve that I haven't found yet that is stuck?

!!! PROBLEM SOLVED !!! The linkage that operates the valves was gummed up with old lubrcant. It was old and hard, not allowing on of the valves to move at all. A little squirt of WD40 and working them back and forth has them freed up and working again. There is no vaccuum for this system it is all mechanical.