Heater stuck on Defrost

Hello all,
My heater on an 86 Mercury Cougar is stuck on Defrost. I can go through all of the positions, but it does not move. It will turn off though. Please
advise.
Scott
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You have a vacuum leak.
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Vacuum problem or vacuum motor problem. Defrost is the 'default' position when there is a vacuum problem. There is a 2-position vacuum motor that controls an air door-flap for both the defrost and floor air direction. One position on the vacuum motor closes off air to the defrost outlets and directs all heated air to the floor outlets. When the selector is in 'mix' position, air is partially directed to both the floor outlets and the defrost outlets. If defrost is selected or there is no vacuum controlling that 2-position motor, then all air is directed to the defrost outlets. Check for bad/loose/unconnected hoses to the vacuum motor....there are 2 and each is colored, red and blue I think. Check for vacuum to the selector switch. (A small vacuum pump is mighty handy for this) If you can hear any hissing sound in the dash area, that's a sure sign of vacuum being lost.
Hope this helps.....
Dave S(Texas)
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