I have a 1998 Mercury Montineer which exhibits a decline of AC ouput on any uphill grade. At the benining of the grade every thing is fine but as I go further the air just seems to disappear. Then after the grade I get it back. I first thought that it was the blower motor, but after replacing it the problem continues. I then discovered that for some odd reason the air was being deflected up to the defrost vents. Just as cold as ever the air was directed at my windshiled but not at me. Then after the hill top I noticed the air decreased on the defrost vent and increased back toward me. What the heck could be happening???
The flaps that control airflow within the HVAC system are controlled by vacuum, and when you climb a hill the throttle opens more and less vacuum is available, so the flaps head for their default position, which on this vehicle is obviously “defrost.” There is a vacuum leak somewhere in the system. A loose vacuum hose, perhaps, or a hose with a hole in it, or a check valve that is not functioning. Finding it could be fun, since most of the components are buried deep under the dashboard. A service manual with diagnostic procedures will prove most helpful.