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Re: heat and ac switches between floor, dash, and...
The door for the air flow is controled by vacuum operated motor. there is a leak in that vacuum circuit. when accelerating, engine vacuum drops until the rpm levels off. when there is no vacuum to the motor, the default postion directs air flow to the defrost vents to make sure you're able to clear the windshield.
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its called the blend door actuator its either stuck or or broken or shorted out it is deep in behind the dash located inside the air box where the heater core is and the ac condenser is its a fun job as for you have to rip your dash apart
The 1992 was prior to Buick putting cabin air filters in so there is not a possibility it is clogged. So more than likely your blower motor is going bad or your fan control switch is bad. Does this problem occur with heating as well as AC? Does it change if you switch between blowing on floor, face, window(defrost) If it doesnt change based on location of blow and type of air temp then I would start by turning fan on high and placing head (ear ) against dash in center of console area. Have someone turn fan knob from low to high with engine OFF...After goign from low to high and back a few times you can see if the blower motor is responding to signal from switch. Listen for a whine, squeal, rattle, or humm. A whine or Humm would indicate a bad blower motor... Squeal or rattle a bad squirrel cage/fan blower blades. If you hear no noise and no perceivable change in blower speed/noise then its possible your switch is bad. Unfortunately the only way to definitely diagnose either is to pull the switch from dash and use multimeter to test for voltage change and for blower motor to pull dash. There is a digital control unit for your car as well. Although rare sometimes the wires on this can loosen or it can go bad. But since you didnt state teh fan wasnt working at all and you are getting cool air, that woudl be teh last thing to check
what position is the control switch in? does this occur when accelerating?
the door that directs air flow is controled by a vacuum motor. the default position for the door directs air flow to the defrost vents in the event of vacuum loss to ensure the windsheild will be clear of mist. if there is a small leak, vacuum will temporarily be removed when accellerating and the motor will revert to the default position. there are vacuum lines from the control knob to the motor you must check these lines for leaks. the diaphram in the motor may be bad. also check the main vacuum supply line to the control knob. usually red in color. if there is a vacuum storage cannister. it may be leaking. this cannister will maintain vacuum for the circuit when accellerating. engine vacuum decreases when accellerating.
that switch controls a vacuum operated door that lets a/c or heat thru.when accelerating,the vacuum in the engine changes so it sound like a vacuum issue.take apart the center stack and there is a vacuum actuator in there connected to the flap door .i have seen those have issues with the metal tab broken away from the plastic that moves it.is the a/c piping cold under the hood where it goes into the heater a/c assembly?the compressor should be cycling on and off and the pressure tested.
You have either lost vacuum to the mode control switch or the switch leaks, this switch is in the ac conrol head in the dash. When you select dash, floor etc. this what changes where the air comes out.