If you're standing in front of the truck looking under the hood, all the way in the back left corner on top of the blower motor is a vacumn actuated valve hooked to a small linkage that goes straight down beside the blower motor. I think this is the valve that switches the air from say defrost to floor or front. Anybody know for sure?
I am having trouble with my a/c just switching itself from front to defrost and while looking for vacumn leaks noticed the valve, which does not hold vacum, the diaphram seems to be bad and I need a new one, whatever it is

 

If memory serves me correct, it controls the flap which diverts air across the evaporator or the heater core depending upon position. When replacing it, be very careful with the thin white plastic vacuum tube that goes to it. They are very fragile and like to break. Mine broke and I had to replace a section of it with small diameter rubber vacuum hose. When it broke I could not select between heat and a/c.

 

if we are thinking of the same thing here, on my truck that thing controls the recirc. function when you switch to MAX AC. the plastic line on mine rotted out and i replaced it with a rubber line.

 

Well I replaced it and also found that the infamous "white" vacumn line was bad also and had to replace the section between the vavle and the harness everything runs into. No prob. so far the easiest fix yet and only a few bucks.

Odd though, I swear my truck runs a little better. Is is possible that the vacumn leak was large enough to affect other functuions, like vacumn advance or anything that would make the engine perform better at low rpm.

Of course I would also swear my truck is faster after I wash it too