Chevrolet Repair: 1988 Chevy Suburban vaccuum, suburban 4x4, chevy suburban

Question
I have a 1988 Chevy Suburban, 4x4, 350 carburated engine.  It will not allow me to select between the floor and the dash/defroster registers.  It will also not allow me to change from heat to AC either.  I have checked and I do not have any vaccuum pressure that I can find on the system.  Unfortunately the auto repair manual that I have (Haynes)does not address the heating and A/C system.  How do I fix my problem?

Thank you,
Richard

Answer
Hello Richard, This is all from memory so I hope it helps. There is a linkage that moves the mode doors above the gas pedal. I have seen these come apart for who knows why, put them back and they work fine. If your missing vacuum, I would start by checking the line that comes off the intake manifold. You need to have vacuum on the purple plastic line at the control head. If you trace the lines you should locate the problem.

Let me know

Mark