I have a 1989 chevy cheyene. I was wondering if anyone new about the heating controls in it. I have figured out that if I open the bottom flap the heat goes up top but it has to stay open and there is constant heat you can not change it to coldand then no heat goes to the bottom and as soon as I change it to the bottom the heat stops up top and just blows cold air. I am stuck and can't figure it out. PLEASE HELP ME it's going to get cold soon.

 

check the vacuum feed to the controls

 

Could you please explain a little more??
Thanks

 

check for vacuum hoses at the back of the heater controls, and make sure there is vacuum at it.
If theres no vacuum hoses, check the linkage