So I went and looked at this again tonight since I did some maintenance, figured why not have another look.

Actuator with the red/blue vac wire seems to work. I disconnected the actuator - it opens and closes depending on where I turn the dial, and is consistent in it's movements. nice loud vac hiss if I disconnect it.

I can't move the white nub around other unnaturally, I can only rotate it back and forth in it's natural motion. I can hear the door inside opening and closing, no binding at all. his door is good.

The actuator directly above it secured to the upper hvac door, that one is brand new AC Delco actuator that I replaced within the last few weeks, thinking it was my problem. that doesn't appear to move much at all as you turn the dial, very minimal and sometimes not at all. not consistent in it's movements.

So I was going to disconnect the arm and try the white nub there and see how well it moves. Isn't this the door and pushes air our the dash vents? It would make sense since I almost always got air coming out the dash vents no matter the setting.

 

no one changed that upper door hey?

 

Your symptoms sound like you have something going on with the vacuum to that upper actuator, if it doesnt make any definite movements when changing the direction ****, and you replaced the actuator already then you need to pull your control switch out of the dash, and test for a leak in the vac hose to that actuator, plug the hose on the actuator end and see if it holds vacuum, all the hoses come to a connector on the back of the direction ****.
Not sure what else to try, have you tried to actuate the upper door by hand without the actuator hooked up and see if it changes where the air comes out? Only other thing you can do is cut an access hole and do some exploratory diagnosis. Maybe something fell down the defroster vent at some point and is jamming the door? I know ive seen pens and pencils hanging out the floor vents on other cars before.
Good luck.

 

So I got energetic tonight and decided to take another stab at it. Evene with the truck off, there's plenty vacuum on that upper actuator - it's hard to extend out by hand.

I disconnected it from the white door level - that door is intact. I can move it with a little effort since I left the spring on it. I can hear the door opening and closing - no binding.

So good news for me is the doors are intact. What now...this thing maybe?

 

unpredictable1: where is the photo above taken from? What year S10 Blazer? What does the HVAC dash controls look like?

 

That's a pic from my 01 Blazer LT with the auto climate HVAC.

That there is what's behind the pull-out ashtray and is some sort of master vacuum control unit.

Not sure my HVAC controls could be leaking, there's just an electric harness that goes to that.

EDIT: With all due respect to TheJuicer, I'm not meaning to hijack but there seems to be tons of knowledge in this thread and you just might have the best collection of info in here. Cheers!

 

Ive repaired that electronic vacuum manifold on other vehicles before, in Grand Prix's they would get water in them somehow, theres a tiny rubber valve stuck to the end of a steel rod that slides through a coil, basically a bunch of solenoids controlling vacuum valves, the rods corrode and rust and would get stuck. I dont think water would be a huge problem in the location its in in the blazer, but maybe smoke tar and ashes could cause problems?.....if the vehicles been smoked in? Otherwise im sure those solenoids go bad too.
This is the big difference between manual and auto climate control, instead of a manual switch that physically routes the vacuum where it needs to go.
You can find the vacuum diagram and Connect a peice of hose to each port one ata time and put pressure or suction to it with your mouth and select different directions and see if the solenoids are doing thier job.
If you decide to dismantle the valves, be warned, you should do it in the bottom of a plastic tote or a box or somewhere safe, each valve has a very small lightweight spring in it and they wont work properly without, they dont come flying out or anything but you dont want to be searching for them on the carpet...trust me...or you can NOT be a cheapskate like me and just buy a new one, its like $60 from rockauto.

 

What is that part actually called? If I can see how to get it out - I'd just replace it. I'm grasping at straws now that the doors are intact and my actuators open and close/extend. (edit: I think I found it, HVAC Control Vacuum Control Valve ACDelco GM Original Equipment 15-72182)

Other than that piece, or the HVAC controls itself ...I'm not sure.

When I removed my HVAC control panel, there was black electrical tape along the seam at the bottom. Where the front black plastic meets the white plastic - almost like it was opened before...weird. But I'm not sure how that piece could be at fault.

 

My Blazer did this yesterday :-( When I put it on floor, it goes
to the windshield.

It has auto climate, but I noticed my feet were cold so I checked all
modes. Everything else works fine. I'll have to stick my head up there
and see whats going on.

 

Before I start replacing anything else, I think I might put my Blazer on level ground a pull the transfer case fill plug after reading this link...

Maybe my 01 has this problem?

GMC Sonoma & Chevy S-10 Transfer Case Vacuum Switch