its a vacuum line it goes from the belt side at the back and runs behind the engine and down the drivers side and under the battery to the pod under the skirt in the front wheel
The heat/ac switch in that new of a car is controlled off of vaccuum. The hose on the back of the temp. control may have a hole in it not allowing the vents to switch over. An auto a/c specialist can check it out anywhere $30 to $50.
My ac blows only in the defroster and not any of the other vents i have 98 ford windstar 3.8 v6
If the climate control on a 2003 Trailblazer will not switch from feet to head vents, try changing it to defroster then back to head. The mode button on the dual climate control model can only be switched from face/feet to feet and then to defrost. The feet and head vents worked at the same time.
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You will need to figure out if the problem is in the dashboard switch, the wiring and connectors or the vent control motors.
This is a vacuum issue, as your demand on the engine increases (power to climb the hill) so does the vacuum created by the engine by increased acceleration...this causes the vacuum operated controls in the airbox to switch position.
your switch cable might be broke you will have to remove your dash
The Heating/ ventilation systems in cars are large structures throughout the dash board, that have many channels of which air can flow through. When you use your face vents, the air is blowing from the upper vent channels. If you use the windshield defroster, the air comes from the very same upper channels, the channels just switch. They have a divider that blocks airflow one way or another. Very much like train tracks, when you change the settings with the switches, the airflow changes because the divider has moved. Therefore you cannot use both at once. The only reason why you can use the foot vents, and face vents at the same time, is because they have a divider that splits the channel that they share, you can use face vents (full force), foot vents (full force) or both split (half force). The Defroster is made to work fast, so if you use the face, and defroster vents at the same time, the defroster would be half as effective. This is why you cant use them both at the same time.
Check for vacuum leak either under the hood or under the dash Check the operation of the vacuum actuators Check the selector switch Defrost is the default setting on this system
There may be something blocking the deflectors from sending air up there.
27Jun09: I had this problem on a 1999 Lumina LS. The answer is a vacuum leak. Mine was near the vacuum canister that controls the vents. The hose was replaced which (with labor) cost about $100. Just got it back and it solved the problem. DRB
the middle vents only supply air from the ac unit using a separate in dash fan. the floor and defrost vents come only from the heater