My A/C works pretty well except that the center vents are erratic -sometimes work, sometimes don't.

Yesterday, I took the glove box liner out to see if outside air control was functional. The actuator behind the box engaged (opened outside air valve) when A/C turned own but won't disengage (to close outside air) when temp wheel is placed in "min" position. Disengages immediately when A/C turned completely off.

If the "min" position isn't working (which is sending an electric signal I think) there may be some component in the A/C system that is causing my erratic vent operation other than vacuum. Is that possible? What could it be other than vacuum? Could the A/C switch itself (third button from right) not be sending right signal?

Thanks,

Nic
'85 300 CD