Blend doors won't move - stuck on "vent"

[Alex V.]

The climate control on my '85 C3500 (454/TH400 w/A/C) has been inoperable for the last 5 years - ever since I got the truck. The fan and temperature control works fine, but it only blows on vent - won't route the air up to the defroster, etc. I've never been into the dash on one of these trucks, so I don't know where to start. I'm under the impression that the valving/actuating mechanism for the blend doors is run off of vacuum - am I right? Is there a main vacuum supply line from the engine I should check before I gut the dash to get to the switch? Also, I'm assuming the switch isn't a breeze to get to, the way the dash is designed.

[rs74]

I had somewhat the same thing happen. I pulled the glovebox door out and looked back towards where the blend door is. What I found on mine is that a white plastic clip that attaches to a piece of all-thread was missing not letting the door to function properly.

Remove the glovebox door and look back in there. Start the truck and while looking back in there start moving the vent selector back and forth to different positions. This will let you see if anything is trying to move. I have also seen the plastic piece on the blend door itself be broken. You can see in the pic that the right hand white plastic clip that is attached to the allthread that was missing(already replaced in photo) and the white plastic hinge on the blend door that is mentioned.

[Alex V.]

Thanks! That helps a lot.