2009 Mercury Grand Marquis - AC issues

.when ac is on blower cycles from vent to defrost. then stays on defrost.

Okay, non mechanic making a wild guess. AFAIK, your blend doors are actuated by vacuum. A leak somewhere might produce the results you’re describing.

But if it has electronic controls, there is a fault between the climate control module and actuator. The default position is defrost if a fault is present.

The HVAC in your vehicle is both electronic and vacuum operated by the climate control module.


The defrost mode is vacuum operated while all other modes are electronically operated.

Inside the climate control module is a vacuum manifold that controls the operation of defrost door vacuum motor.

I would guess there’s a problem with the vacuum manifold inside the climate control module.


1 Like

This kind of shows what is involved with the EATC repairs. If you are unable to do this work yourself then there are some outfits out there who rebuild and sell these on an exchange basis.