99 Deville A/C

[Jack]

The A/C cools fine but the (underdash) controls change the air to the
defrost position instead of out the dash vents. It makes "groaning"
noises like it is trying to switch back to the dash vents but it
rarely does. I've looked for a vacuum leak under the hood and had an
A/C shop check too. Found no leaks. I wasn't able to check up under
the dash though.

Any ideas?

[Certainly]

Post by Jack
The A/C cools fine but the (underdash) controls change the air to the
defrost position instead of out the dash vents.  It makes "groaning"
noises like it is trying to switch back to the dash vents but it
rarely does.  I've looked for a vacuum leak under the hood and had an
A/C shop check too.  Found no leaks.  I wasn't able to check up under
the dash though.
Any ideas?

Is the mode being manually selected, If it happens in 'auto' it will
blow where it feels it needs to. It will typically come out the dash
vents on auto when the temperature selected is at, or lower(cooling)
than the in-car temperature. Did the a/c shop check for any codes in
the a/c programmer(ACM). As far as the groaning noises, I am not sure.

[p***@webtv.net]

Post by Jack
the (underdash) controls change the air
to the defrost position instead of out the
dash vents.

Defrost position is the default air-direction when there is a vacuum
motor failure. Check for a loose/broken vacuum hose to the motor and/or
check the vacuum motor.

Dave S (Texas)