a/c vent problem

[mygreen98]

i have only floor vent operation no matter what position control knob is turned to, a/c and recirc buttons won't light up. blower works on all speeds and defrost button works. what i thought was the common bad transistor on the climate control assembly, is not.i replaced the blown transistor, no change in problem. tried another control unit, all vent positions worked as did the recirc button, then i pressed the a/c button and instantly back to same problem, just floor vent no lites on a/c or recirc.