Auto Air Conditioning & Heater Repair: a/c diverter door, gm systems, john bowles

Question
1980 corvette
I am puzzled by how to repair my a/c diverter door or how it functions. The diverter will send all air to floor vents or defrost vents as per factory setting but not to a/c vents. What is happening here? Is there just one door in this system? The factory manual seems to indicate that there is only one for this funtion.
Many thanks,
John Bowles

Answer
HI JOHN,EALRY GM SYSTEMS USE VACCUM OPERATED UNITS TO CONTROL THE SELECTOR DOORS IN DASH. THE MAIN CONTROL DIAL IS BEHIND THE DISPLAY,WHICH INVOLVES REMOVING THAT FRONT SELECTOR UNIT.THAT DIAL SELECTOR DRIES OUT AND LOSES VACCUM.IN TURN CERTAIN DOORS WON`T OPERATE.

GOOD LUCK,DAVIDT