'80 AC Vents. Only blowing out the bottom ducts.

[Ken K.]

This is on a '80 Corvette. The vent controlls are not working. The fan only blows out the bottom ducts. (Feet area) I switch to the "vents" or "defroster", and it still just blows out the bottom ducts. (nothing out the other areas) What I'm I looking for? Is the problem in the vacuum diaphrams, or vacuum lines? Is there a easy way to test this system?
Thanks Ken

[Jack H.]

Re: '80 AC Vents. Only blowing out the bottom duct

There should be a vacuum attachment on the side of the center console mounted heater/AC control switch that directs vac to open/close baffles in the inner duct work to move air from floor vents to upper vents and windshield (defrost) as you dial different heating cooling settings. On earlier Shark cars, the C60 option section of the AIM (assembly instruction manual) provides a vacuum 'truth table' (vac vs. vent) for the status of each hose line in relation to the position of the selector switch. Also, check your copy of the owner's manual because there's typically a series of manual shut off knobs associated with the upper duct outlets allowing driver/passenger to turn air flow on/off as desired.