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When there is a problem with the vent system it dfaults to the defrost setting. Sounds like you have a problem in the doors that direct the air flow.
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
Cold air from the defrost indicates that the air doors are in the wrong position. The doors are vacuum controlled in most vehicles. check the vacuum system for a leak.
Air flow is controlled by plastic doors which are controlled by vacume motors. Could be they're stuck, or a vacume leak.
It does not take freon. The 1995 GMC Vandura uses the new R134a.
likely thermostat is stuck. The temp blend door actuator has failed.
vent valve in the dash bad or vacum leak leading to it. What would your recommend on how to proceed to correct the problem? If the vent valve is bad does that have any adverse impact on the engine's performance? How can the valve be tested?
The moveable "baffle" inside the dashboard is not redirecting the air flow as it should. It's operated either by vacuum or an electric motor, in either case, repair is going to require the removal of the dashboard.
Yes
its hydraulic with abs
Check all of your vacuum lines from the ac control panel and under your hood. You have lost vacuum somewhere witch controls your vents. There is one of them loose, disconnected or broken. Should be easy to trace. Good Luck!!
I have a 3500 and mind is under the driver seat