I have read all the problems the blend door can cause, and a bad actuator . I did not see my problem, but it may be the Same. My AC works great, and so dose the heater. The defroster works fine, floor vent works fine, so does the center vents. But, when I have the AC on, and hit the max button all the vents close. I turn on defrost and the cold air will come out the defrost, I turn on the floor vent the AC comes out the floor vent but when I turn on the center vent all the vents close. I have to turn the truck off, and AC. When I restart I will turn center vent on first, then all will work ok until I hit the max button.
Is this too a sign of the actuator ?
92 Full Size Blazer ATM 4X4.
Thanks for any help.

 

Well,,,,I crawled under the drivers side looked above the gas pedal at the heater case and saw two rectangle metal clips hanging loose, when I pulled one straight down, the cold air started blowing through the center vents, oh yes cold air,,,I think there is supposed to be a clip that holds these two together,
still looking for a diagram of Heater AC case,