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you have a sucked shut or broken vacuum line under the hood somewhere. mine was burnt and broke, same problem. it is a little, skinny, hard vacuum line from left side of motor over to right across the front.
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.
There is a vacuum leak under the hood. Find the vacuum line on the passenger side where it comes through the firewall and follow it to the engine. You system needs vacuum to operate and without it the default setting is on defrost for safety reasons.
You more than likely have a vacume hose that has come off at the vent switch or under the hood. Unfortunatly you will have to pull the vet switch out of the dash to see if there are any vac. lines off. You can visibly inspect the rubber and plastic vacume lines under the hood. Just be careful how you handle the plastic lines as they become brittle from the heat under the hood after a period of time.
check your vacuum lines that sevice the plenumm doors that redirect the air. A good kick in one of the foot wells under the dash can dislodge a conection. The other problem may be the main dash selector its self