The defroster in my Duster blows all of its air out of the air conditioner vents instead of the defroster vents. The heater does the same thing. I pulled the hose off of the little vacum control thingy on the side of the air box( the one closest to the floor) and it pulls a vacum only when the heater is on. I tried opening and shutting the damper by hand but it changes nothing. Any suggestions?

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

There are actually three pancakes on the box. Sounds like one of the others isn't working. IIRC, the one that quit on my Duster was the hardest one to get to.

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Official Cookie and Mater Tormentor.

It could be one of the other actuators. It might also be the check valve missing or inop or the vacuum reservoir. My Duster had no check valve or reservoir so the doors would open or close depending on the vacuum status. Now it works fine.

where is the check valve at, and how do you test it?

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82 D150-225/727
02 Dakota-3.9/5 speed
87 GMC C7000-8.2 Detroit Diesel/5+2

I don't know where it is supposed to be because mine didn't have one. If you trace the lines you can see if one is on yours or not. Start from the source (manifold or carb) and follow into the under dash area. To check it, I just blow through each end. You should be able to blow one direction but not the other. Since it is for vacuum, you have to think backwards when hooking up. The end that you can blow through should be the end away from the source. so it allows vacuum to pull but when vacuum stops, it retains a force on the actuator. You can get check valves in the bone yard or the "Help" section of parts stores.

This makes me appreciate the cable-operated heater/defrost system in my '64! I've always had problems with vacuum operated this and that.