Mode door operating backwards

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JohnMark

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85 c10.

My hvac mode door seems to be operating backwards. For ac modes and vent, the mode door flows the air for the floor/defrost side. For off/heat/defrost, it goes directs air to the dash vents.

It seems if it was a lack of vac, the door wouldn’t operate at all. What do I need to check on this?
 

yevgenievich

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Check the physical actuator. On passenger side below glovebox remove the piece that turns up and door is behind. Often I find screws from below put in to hold the door in place when actuator breaks.
 

JohnMark

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Check the physical actuator. On passenger side below glovebox remove the piece that turns up and door is behind. Often I find screws from below put in to hold the door in place when actuator breaks.
The door moves though, meaning the actuator works, right?
 

yevgenievich

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It can partially move with broken actuator piece and maybe someone put a limit on it. Or vacuum lines were hooked up incorrectly. One of the easier things to check.
 

Charlie

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JohnMark

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It can partially move with broken actuator piece and maybe someone put a limit on it. Or vacuum lines were hooked up incorrectly. One of the easier things to check.
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yevgenievich

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Actuator is working correctly. It should move to about middle position for normal air and goes back for defrost.
There is another door right beside it that has adjustment to location it is in. I would need to look at a working one to see where it supposed to be pointing to as it has been couple years sense I had one fully apart.
Check on the sticky for some references as well. There was a diagram posted with all the vacuum connections that could help verify that part
https://www.gmsquarebody.com/threads/inner-cab-heater-ac-housing-reference-pics.4809/
 

JohnMark

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Wouldn’t the big door be open for heat/defrost? The defrost/floor vents are behind that door and are blocked off when the door is closed.
 

yevgenievich

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Heat/cool is controlled by a separate door by the lever. That one is not vacuum, but cable pull.
 

yevgenievich

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Normal should direct mostly to top vents, bi level I believe splits more even between top and bottom, and max directs to the top only. Heat goes to floor and defrost to windshield.
 

JohnMark

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Heat goes to floor and defrost to windshield.

Exactly and from the video, you can see that heat and defrost are pushed to the front vents. This is my problem. The hot/cold wire control works fine.
 

yevgenievich

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My description of close is backwards from yours. Door all the way back should send to defrost. Might need to check vacuum connection on the back of controller.
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On next picture is the firewall side, it shows that when that door is all the way back the flow goes to the long portion of the box where there is a door for defrost and down. The door seen is not the one discussed, that one is heat. When heat is on, defrost door should stay closed.
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JohnMark

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Thanks for the info. I have tried pulling the hvac control unit out to check the vacuum distributor part but it is so tight I can’t pull it out from the dash enough to check it. I must be missing something.
 

yevgenievich

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I have another spare ac box in the parts pile and can probably take better pictures. But would be 2-3 days from now before I get to it. Taking the radio out helps with the HVAC control panel. Can push it back and lower to radio spot. Then work on it from the original opening.

Last time I had one apart, I remember there was one door/tunel that can be put in wrong. If your heater core was replaced at some point of time, I wonder if it was put back together with things not in the correct position
 

ChainBlue

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Did you get this figured out? I am having the same issue. I think maybe I have vacuum lines on wrong. I wanted to ask before I took the heater box loose to check them.
 

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