It sounds to me like you have a bad vacuum leak, although not as bad as the one I had in my Volvo. From your description, your car is generating some engine vacuum for the vent controls, but only under certain, "high vacuum" conditions. I don't know if you are a DIY (do-it-yourself) person or reliant on mechanics. But I suggest either you or a trusted mechanic performing a vacuum load test on your vent controls.
You should be able to find two, small-diameter plastic tubes passing through the firewall behind the engine. One will go to the heater control valve. The other will tee into a 3/8-inch diameter vacuum line. Disconnect the small vacuum line at the large tee. Connect a hand held vacuum pump to this line and pump up a vacuum in the line. The system should be able to maintain a constant vacuum once the controls have adjusted to the setting selected on the dashboard. If not, the leak is under the dashboard. You should draw a vacuum into the system for each vent control setting. Then, for each setting, turn the ignition to "on" and turn your fan switch on "4". The air should blow from the vents selected. If not, then the problem is with the vent controls under the dashboard.
Below is my experience, posted here previously. My problem was in the engine compartment and not under the dashboard. I hope this helps. If you have further questions, please reply. I can supply digital pictures of what to do if you like.
I don't need help with this one because I figured it out myself. However, others may learn from my experience.
Shortly after I purchased my car, I discovered that the A/C vent controls did not function. The fan switch worked well, but I would only get air flow through the Defroster vents no matter what vent position I selected. Also, air would be slightly warm no matter the position of temperature control. I also discovered the A/C belt removed from the compressor and the A/C clutch wiring purposely disconnected.
The previous owner had the car serviced at an independent Volvo garage in the "People's Republic of Berkeley", California. Two years ago, the shop discovered that the controls did not work and quoted $250 to repair, though they didn't state why they would not function. On the same repair bill, they replaced the temperature control valve and charged a fee to remove the A/C belt. Then, ten months ago, they replaced the temperature control valve, again, for whatever reason.
I quickly discovered that these controls required engine vacuum to operate. I found two vacuum tubes that passed through the firewall. One went to the heater control valve. The other to a tee in a large diameter vacuum line. One end of the large line attached to the intake manifold. The other end went somewhere under the left side of the car. I looked under the car and found the line attached to a long, cylindrically shaped, plastic tank, mounted forward of the front crossmember.
I disconnected the main vacuum line from the intake manifold and plugged the tap. Then I disconnected the vent control vacuum line from the tee, attached a hand-held vacuum pump, and pumped down a vacuum in the vent controls. I quickly found that the vent controls operated normally as long as I kept a vacuum on the controls. This told me that I had a vacuum leak somewhere on the main vacuum line.
I went back under the car and unbolted this plastic tank from the undercarriage. It is held on by three bolts. Once I got it out from under the car, I easily saw the large crack that traveled half way around the far end of the tank. No wonder the vent controls did not work, there was no vacuum in the line. Not only that, the car had a major intake vacuum leak that went undiagnosed for over two years, and the independent mechanic that had serviced the car all that time, saw the clues, but was too blind to figure this out!!
This tank is positioned in such a manner under the car, that it is the first thing to be hit by large debris on the roadway. It appeared that the crack was cause by blunt force trauma from road debris.
The purchased a new vacuum tank from my local Volvo dealer for $47. Once installed, the vent controls worked like new. I then replaced the A/C compressor belt, reconnected the clutch wiring, and now the A/C blows 40 F air in the coldest position. I don't think I would recommend that independent Volvo garage to anyone I know.
So, if your vent controls stop working, be sure to check for vacuum and the condition of this tank before trying to take apart your dashboard controls.
Chris the "K MANIAC"
1986 740 GLE
(5) 1964 Chrysler 300-K's