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I have a 91 ext cab 2wd S10 with a 4.3 and my heat and AC air doesn't come out my vents what could be the problem
 

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Boozebag
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Most likely you have a vacuum hose that is either rotted or disconnected. There is a vacuum hose that goes through the firewall about the middle. It's small.
Once you locate it, follow it. It needs to be connected to a vacuum source. If that's not the issue, check your vacuum reservoir. It's a black plastic ball at the left front lower radiator support. They crack with age (sometimes).
Another issue - could be your HVAC control in the dash. They don't go bad frequently, but that is a possibility.
Listen for a hiss in the cabin. If you hear one, the HVAC control is the problem
 

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There is a small plastic clip on the very bottom actuator closest to the drivers foot, that's what is broken. Good luck finding one, haven't been able to find a new one yet. Found one in a junk yard truck years ago but lost it.
 

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life begins @ 20 psi
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Sounds like the selector switch in the dash for your HVAC control is broken. Take the two screws out from under the controller and pull it out of the dash. Both the electrical connections and cable for the blend door will prevent you from pulling it out much... You will see and circular "octopus" with multiple colored vacuum lines going to it. There is a quarter sized "star" washer that has a small rectangular clip that holds it in the center to the back of that vacuum selector switch. When it breaks, it is usually right below where the controller mounts inside the dash due to the way the dash is made. This results in partial or no HVAC selector control.
 
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