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Q: Heat/AC only blows through defrost, then after 10 minutes only blows through vents. on 1997 Oldsmobile LSS

When I start my car, the heat or A/C will only blow through the defroster, then after 10 minutes it gradually switches over to the mid level vents and will not switch back. It does this whether I select "defrost" or "vents." If I select "lower" it will blow on the floor, and it will switch back to either the defrost or vents (regardless of which I select) depending on how long the car has been running and which one it was blowing from before. Any ideas what is going wrong? Is this controlled by vacuum in this car or is it electric? Thank you for your help.
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A loss of vacuum or low vacuum could be the culprit. When all vacuum is lost the climate control defaults to defrost. Check your vacuum line for cracks/loose connections. The line for the climate control runs from the upper intake manifold (rear part) to a tank on the lower front fender and then to the firewall pass thru to the climate control module behind the glovebox.
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