Why is the air coming out through the bottom vent?

Question

I have a 93 Buick Regal. When I turn on the AC or window defroster, the air come out through the bottom vent. It is as if the middle and top vents are blocked.

Answer 1

The blend door or actuator for that mode is bad or the control switch valve is bad or has a vacuum leak....does the air come out of different vents when you change the mode like from heater to defrost? If not, it's usually a vacuum leak or a bad mode selector switch/valve. Each mode has a door or flapper inside the a/c ductwork to direct the airflow to various vents...be it on top of the dash, in the middle or on the floor....these doors or flappers are controlled by vacuum or electrical actuators and are operated by the control head switch or valve when you select a mode. There are several reasons for them to not work...
No vacuum from the engine or vacuum storage tank....possible broken or cracked vacuum line.
The selector switch/valve is bad or leaking vacuum.
The vacuum actuator is bad or leaking vacuum...electric actuator types may be bad if so equipped.
The actual door or flapper is bad, bent or hanging up.
If the air coming out of the vents does not change when you select various modes, it's usually a vacuum leak or bad selector switch/valve.