I originally did a PM (my bad, I did not read the rules), so this reads like it is to one person, but really it is a general question for anyone willing to help out.

I have an 88 K5 4WD with a stock 350 (TBI). I've owned it for about 5 years now, but of that I have been gone about 4 (military). So, although I have owned it a long time, I have really not been able to mess with it.

The A/C-Heater controls have always been messed up (will not switch when you move from A/C-Vent-Heat-Def). After getting caught in the rain and having the windows fogging up bad the other day without a defroster, I decided to investigate. I broke the entire dash down to parade rest. I found one problem, the door that controls the main air movement has an arm that comes out and then a little piece of plastic which attaches it to the door. That piece of plastic is broken, and I assume I will have to change the entire heater core, or box, or whatever the outer covering is called, to fix that. So, I used cable ties to hold that main door closed and direct the air towards the heater/def side, and ignored the rest of that side to focus on getting the defroster to work. I learned that the little actuator on the end (driver's side) of the heater core box is what directs the hot air either down (heater) or up (def.). I took the vacuum line off and with the truck running and me moving the selector switch, I could not feel any vacuum. I took that weird thing that all of the vacuum lines come in and out of (back of A/C-Heater control switch) out and checked it all out. No visible blockage/broken lines. I did notice that when I changed settings, the little arm I spoke of earlier (with the broken plastic) does move. So, I assume there is sufficient vacuum coming in. Also, I followed the vacuum line to the engine compartment and it tees off to (what looks like) the transmission and somewhere else. I figured if there was a problem it would have shown up in other places.

My question is what you think could be the cause of the unit not switching from heat/def (moving that door). Also, any ideas on that broken plastic thing I mentioned? What is the least expensive way to repair/replace it?

I really appreciate any assistance you can provide.

 

The hvac system in your truck is controlled by engine vacuum. If hoses were disconnected and/or connected incorrectly, it will cause the system to not operate as designed.

The diagram below shows the vacuum system used to control the HVAC modes in your truck: