Hi Juicer,
Your diverter gate axle is broken.It is inside the plenum and snapped because it is plastic. Get any notion of malfunctioning controls out of your head!
I am still driving my Blazer in a cold Canada climate but fixed issue in the summer when I cut a hole in the plenum allowing me to remove the two broken halves and reinstall an un broken one, then patch the hole. I actually never even re hooked the vacuum actuator linkage or motor. I live with one setting with equal heat on floor and defrost and have been doing so for years. I simply could not find a way of removing the vacuum actuator without busting the plastic anchor which will now require re gluing. I'm not even going to bother with this control anyhow and I don't trust glue. If you want insight and practice, as I've said go to a Pick and Pull with a hammer and smash a brittle plastic plenum to bits until you are able to extract a diverter in tact. You will need this diverter unbroken and re installed if you want floor heat.

 

I was right there where you are at about to cut into the hvac system. I took the panels down and with the truck running would turn the hvac to floor and watch the actuators try to move. And when I knew how they moved I reached up into the dash and helped them one by one. When I did that I realized it was the actuators because the doors worked when I assisted the actuator. It's actually pretty impressive how much force those actuators put out to move the doors. The doors don't move very easy there is a decent amount of spring tension on them. so i would assume if the doors can be moved to the correct position manually they are fine and you need to really look at the actuators and or other vacuum leaks.

The main vacuum line that goes to the hvac is super tiny hard plastic line so if there is the slightest leak at the controller, actuator, etc it's not going to work. That's what,was happening with mine but if you just looked at the actuators it looks like they are getting stuck when actually there is a tear in the bellow and they can't do their job properly. In my case 2 out of 3 had tears so there just wasn't enough vacuum volume to get anything to shut fully.

 

I got the heater/defrost actuator off and tested it while unhooked from the door pivot. It pulls all the way in, and pushes all the way out, when using full defrost or full floor. But when I set it to defrost/floor mix it doesn't move at all. Also when it does move in and out under vacuum, it wobbles all over, doesn't travel in a straight line. I unhooked from the vacuum lines and can easily push it in and pull it out. I read somewhere that there's a spring in there, so there should be some resistance when I push in. There is no resistance whatsover. I don't have vacuum tester tools to do any further testing at the moment. Can you confirm that the rod should travel straight, and that there should be some resistance? Unfortunately my parts store could not find the part even available in their inventory, so I wasn't able to look at a new actuator to compare.

 

Forgot to mention, with the actuator unhooked from the pivot arm, I tried turning to operate the door. The arm seems a little wobbly, hard to turn but I'm assuming that's because of the spring, and I couldn't get any air flow change by operating it manually. So I'm assuming the door is definitely broken inside. Just trying to decide if I should order the actuator also. Thanks so far for your help!

 

Like I said if it was me I would replace all 3 if they are still oem. One of them comes with a replacement gray plastic piece and spring. I didn't need to use it since mone wasn't damaged.

 

I went ahead and ordered the heater/defrost actuator and the door. Ran out of money to order the other actuator on the driver's side, and RockAuto didn't show the actuator that's on the passenger side. They showed the flat black actuator for blend door, but not the one that's similar to the 2 on drivers side (GM calls it a slave door actuator). I will hope for the best and will update when the parts arrive.

 

I had to pull all 3 and get the part numbers off them. 2 of them are still current part numbers one had an updated part number. I had to order one from the dealer for like $18 and that was for sure the one behind the glovebox. the other two I found new on ebay for cheaper then any place else. I probably can look up at least 2 of them still see attached images.

 

Been following this thread and found some great info. Thanks for that. I do have a ? on how to remove piece with spring from door arm. Should I use a screw driver and pop it off? Also does that floor setting door control airflow to the window defrost register on top of dash? With door in full up position does it cut airflow to defrost?

 

I read something about putting a small screwdriver into the end of the link, but haven't gotten far enough to worry about it yet. When my parts arrive, I will question further. Meanwhile, anyone with detailed pics and answers would be appreciated!

 

I will say that right now I have the wire shoved up the drivers side floor vent, so at least my feet don't freeze. Still seems like there's a little air coming out of the dash. Maybe when there is an actual non-broken door, and a good actuator, it will seal it off completely. But I am definitely, obviously no expert....just playing it one day at a time.