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.....
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