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No the knob is not broken.
However I did find the problem. The actuator that moves the air mix door from the heat to cool side goes into a mode where it will not move. This can be caused my the door sticking. Under the hood on the housing to the heater/ ac unit there is an area marked. "Cut for service". After removing the area marked there is an access panel with 3 screws. Remove this panel and you will see the air mix door. I manually moved the door from heat to cool a couple times until it didn't feel like it was binding. Do this slowly because the actuator is an electric motor to gear arrangement. GM wants about $120 for the actuator. They are available on ebay for around 35 plus shipping.
Anyway, after moving the door I started the Bravada and selected a mid temp. The mix door moved to a mid way position. I then selected all the way cold and the door moved to block off the heater core. Selecting all the way hot and the door moved to block off the AC side.
If you do have to change the actuator it can be gotten to by opening the glove box all the way. There is a metal loop on the upper left of the opening that prevents the box from swinging all the way out. The loop will swing out to allow the box to open all the way (and dumping its contents on the passenger floor).
You then can look on top of the heat unit and see the actuator. It is on top back by the firewall. It is held in place by 3 screws and has the control wire plug attached. Removig the screws and moving the actuator up will release it from the air mix door. Then just unplug the control wire plug.
I hope this helps someone else.
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