2009 Mercury Grand Marquis: A/C blows Cold From Defrost Vent Only?

Its a Vacuum related Issue.
Inspection is required.


On this system  it uses vacuum to control the air direction. At idle and at lower engine rpm vacuum is at the highest.Defrost and floor is the default position when there is no or little vacuum. So in your case default positions are working.
You  need to make sure there is no  vacuum loss from a hose ,check valve or a vacuum reservoir. The vacuum reservior is under the wiper arm and pivot shaft assembly. Not easy access for vacuum hose under the hood or at the back of the HVAC controller.

Inspecting this parts will help you to confirm,where the vacuum is getting lost.So you can continue your repairs on that part.

This details will help.
Thanks.

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