The A/C in my 96 sierra will not stop blowing out of my defrost vents. I have tried replacing the mode actuator and it did NOT solve the issue. The mode actuator is rotating correctly and moving freely. The mode will change from one set of vents to the others, but regardless of what it is set to, the A/C will also come out of the defrost vents. Is it the mode flapper itself? If so how do I get to the mode flapper?

350 v8 vortec, 4x4, SLE, Z-71

 

All I know is typicaly, in most vehicles, when their is a failure, the defrost position is the default, so you can always keep your windsheild clear. Unfortunatly I can not think of what could be your specific problem.

 

On some units that are vacuum controlled, a leaking vacuum supply to the control panel can do this
.Due to age there may be a cracked /leaking vacuum hose that supplies vacuum to the control panel.