cycling defroster door

[towin'gmc]

First of all thanks to 89 Suburban for all of the pictures and research. I have the vent/heat door working against that strong spring and the plastic tab is broken that connects the servo to the door…. so it will not close. Both of those doors work on linkage but why does that spring work against the servo action? That tab part is prob not avail anywhere. Also the servo on the defrost seems to cycle back and forth by listening to it over 30 seconds it will move and then back. The vacuum lines are in place OK, could that be a leak?

I just saw AJII post on this door tab and thanks for the part #! I will look for one.

Any ideas on the cycling door?

 

[bucket]

That's odd, I don't even see how it could possibly cycle since it's vacuum. It can lose vacuum at times if you don't have the vacuum tank, but then it would be related to engine speed.

 

[chengny]

Check the rubber hose sections of the vacuum tubing - especially on the engine side of the firewall.

A split in a rubber hose will often open and close with changes in vacuum. It opens (the flap pops inward) as vacuum rises and snaps shut when vacuum is lost. This results in the dampers repeatedly moving back and forth from where you have them set to to their default positions.

 

[HotRodPC]

I think that spring is a pull off of sorts. Actually it's not but it kind of works that way to help the blend door to the floor.

My defrost does not work just right either, and it just might be that spring isn't strong enough. Here's how mine fails. All Vac lines are hooked up. So I have my truck started and running going down the road. I switch over to Defrost, and no Defrost. I noticed one day, when even though it didn't work, I left it in the Defrost position. Next Day, Going down the road, and I realized, WTF? My defrost it working. Played with it til I figured out what the deal is. If I want defrost, I have to turn the motor OFF. Switch to defrost then it works. When I want to go back to floor or vent, I can switch it and it goes just fine to where I want. But then when I want to go back to Defrost again, I have to shut the truck off.

I've not gotten into why it's this way, but it very well might be that spring. It's needed to help overcome vacuum to swing the other way. So what I do, when I start my truck in the morning on a cold day, I switch to defrost before I start the truck. And go my merry way. Sometimes I just leave it there or if I crack a window the windshield will stay clear and I can swap to bi level vent heat.

 

[towin'gmc]

Appreciate responses

Thanks! I have located those tabs on GM parts. I will check out the rubber connectors and stuff: should prob be replaced anyway. After I get the new tab on the door I will start up in defrost and see if that will keep it working. Good ideas from you guys and thanks again. I'll let you know if I find something.

 

[towin'gmc]

Got my issues done

Found vacuum leak bad lines around the distributor from heat. Cycling has stopped. Also replaced the little inline valve at the line splitter which was brittle.
HotrodPC does your truck have air? The swing door from heat to vent below the glove box could be hanging up on the second door. both swing and I noticed that with the truck off, the door sets in between the two openings not in contact. So that means the dashpot pushes and pulls depending on the call control. Yours might be not move the entire range of motion and hanging up on the other door to the right upstream. ??? Thanks for thought.

 

[HotRodPC]

yes, it's an AC truck. Thinking about how that works, you very well could be right. I know exactly what you mean. When I feel like digging into that again, I'll sure check it out. Mine WAS the plastic door type. I put an older heat box in that uses metal doors now, but it could still be the doors hanging up.