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This is a common problem on all Astro/Safari vans. The problem is a leak in the vaccum lines that control the vaccum switch responsible for changing air direction from the defrosters, dash vents, floor vents..etc. Many of the problems that plaque the Astro/Sarfari vans and many other "Mini-Vans" is the build up of excess heat in the engine compartment. It\'s a tight place to put a V6-4.3 Vortec engine and the heat generated bounces back and cooks everything on the engine (coils, modules, sensors, wiring, and of course plastic vaccum hoses). No need to mess with your switches or anything behind the dash -- replace those thin plastic vaccum hoses to operate the vaccum switch (located just below the A/C canisters, to the right of the heater/ac blower housing) and your dash switches will magically send heated or cooled air where you want it.
The problem is that the plastic lines that the air flows through get frail and cracks and breaks.What happens is when the hose breaks you loose vacuum and the system reverts to defrost mode.You should be able to see it from under the hood on the passengers side.You will have to find the area that is broken and then go to the parts store and get some rubber hose that will fit over the plastic hose.Take a piece of the plastic hose with you so you can get the right size hose to fit over it.This just happened to my 97 Astro van.I found the broken hose and repaired it with the rubber line and is working normal now.Hope this helps.Let me know what you find out.Good luck.
If it is blowing hot air out of the defroster and not out of the heater vents then the door that directs the heat from the defroster isn't working properly it is blocking the heat from going to the heater vents. Also check the cold air door. It sounds like it isn't closing all the way to shut off the cold air and not letting the hot air pass.
There is a flap in your heater box that is controled by your vent switch by your heater controls. listen for a vacuum leak. that is common and hopfully the problem. there is vacuum lines that control the vent door that redirects the air to your vents.
Has your dash been messed with recently? if so i would check there first. you should hear a hissing if thats the problem.
Otherwise these also are common for that door flap to break and not switch NE more. thats the only 2 things that i can think of off th wtop of my head to be the problem. this would require the whole dash to come apart if its this problem