Why on a 1999 Chevy astro van not switch from defrost to heater or to air?
Check for a vacuum leak either under the hood or under the dash
Check the operation of the vacuum actuated motors under the dash
Check the operation of the selector switch
Defrost is the default position for this system
lose of vacuum to heater deverter valve.follow heater hoses from engine to heater core valve has two hoses in and two hoses out and one vacuum hose.vacuum hose gose to tee then back to engine other vacuum from tee controls heater switch defrost to floor heat knob. replace all of these lines about 6 feet will do it.now the bad, you must pull back the dog house about a foot divers side to remove one…