97 Lumina...Switch stuck on defrost vent and

Most likely either the vent switching mechanism is dirty and won't switch the flaps -or- most likely the vacuum that is used to turn the vent flaps is either leaking and not pulling enough vacuum. You will have to get underneath the dash near the center, to check and see if you can manually move the vent flaps. If so, then most likely it is a vacuum leak in a vacuum line to the flap mechanism.

check the vacuum lines under the hood for emissions and ac controls for broken or leaking hoses

Vacuum supply starts on the drivers side of the engine. There is a vacuum line going into the brake vacuum assist from the manifold. branching off of that line is a double line that snakes under the intake manifold, behind the valve cover, along side the valve cover on the passenger side headed to the front of the engine. Then it curves to the right side of the engine. One set of hoses from this line goes to a valve control (red and green i think). The other line goes to the vacuum storage ball, it should be black. The vacuum circuit continues out of the ball in another black line that snakes into the firewall. It goes under the heater core black box and disappears under it. Now go into the passenger compartment and drop the glovebox. look to the back right you will see a black and a white vacuum line coming out of the firewall. The black line is your vacuum supply from the engine. It goes up to a little rubber manifold. disconnect the lines coming from the dash switch and check for vacuum on the manifold where the black line is going in. The white line controls the cabin air intake under the glovebox and the thermostat shutoff under the hood. It should only have vacuum when the switch is in a certian position, AC probly. The other three lines coming off the rubber manifold snake behind the dash vent door assemble and come out near the gas peddle. The red one controls the dash vent. apply vacuum to the red hose and see if the vents go to the center dash position/AC. The yellow and blue? ones control the floor vent control near the gas peddle. yellow gives you half floor and half defrost. blue gives you all floor. you can use a small vacuum line to connect the black vacuum supply to each individual line on the manifold and check the function of each vacuum control assembly individually. I hope this helps. On my truck each control unit works fine when i apply vacuum to them individually but when I hook the dash switch up to the rubber manifold it goes to defrost. I tried changing the dash switch and the lines running from the switch to the rubber manifold and the vacuum ball under the hood, nothing works. I suspect I may have leak i just haven't found yet. next step is to get a vacuum gage and start checking the amount of vacuum i am getting at each vacuum line. I'm sure it is supposed to be getting a specific amount of vacuum and will not work if the vacuum drops below a certian level due to a leak in a line somewhere? God bless.

I had the same problem and found a broken T fitting in the vacuum line under the hood - right side at the firewall Hope this helps

The HVAC system uses engine vacuum to open and close the blend doors. Check for a broken vacuum line in the engine compartment. It will exits the firewall left (passenger side) of center and will be a hard plastic line. With age and exposure to engine compartment temperatures they become brittle and break. It can be repaired with a rubber vacuum line that fits snugly over the line to reconnect them.

either the AC\Heater panel switch on your center dash or the heater box vent door is stuck or not operating. Most likely panel switch or disconnected \ broken vacumm lines at back of switch

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The HVAC door actuators are powered by vacuum, and the default position is the defrost/floor. Check for a disconnected or broken vacuum line somewhere near the passenger rear of the intake manifold. You should be able to find a line coming out of the firewall in the same area and follow it to the intake. Good luck

There is a vacuum leak under the hood. Find the vacuum line on the passenger side where it comes through the firewall and follow it to the engine. You system needs vacuum to operate and without it the default setting is on defrost for safety reasons.

you have a vacuum leak check the vacuum hoses behind the controll panell

I found a broken vacuum hose on top of the intake which runs the controls