heater on 1990 Buick LeSabre

hello my heater only blows out the bottom but when i put the ac on it will blow out the bottom vent's and the defrost what do i do? thanks

Asked by for the 1990 Buick LeSabre
2 answers
Check the three way tee on the firewall. You will find it following the vac hose from the cruise servo check for leaks.
Take a look underneath your dash for any disconnected or damaged vacuum hoses. You likely have a vacuum leak that is keeping the vacuum driven motors that turn the doors inside your dash for ventilation control from working.