defroster vents

[Sprinkler-Man]

I can't seem to get any air out of my defroster vents I am not sure why...any help?  My blower sometimes doesn't work when I switch the selector from heater to vent.  Do I need a new selector panel or a new blower switch?  My truck has factory air.
Thanks

[bake74]

      I assume it is an 80 truck that we are talking about since you did not say and that is what is in your screen sign in ?
      Have you checked your cables that go from your dash mounted selector to the heater/AC box under the dash to verify they are working and moving ?
      If so then you will need to move in to the inside of the box to see if the flap door is moving.  Also I do believe, I might be wrong though, that your truck should have both mechanical and vacuum controls for the heater/AC.

[Sprinkler-Man]

1980 GMC Sierra Classic.  I have taken out the main ducts to the dash vents to get a better view.  There does seem to be 4 vacume lines running to the vents but i cannot find the one that controls the defrosters.  I will look more once it heats back up and I run the truck again.

[bake74]

     Go to the tech section and look up the service manual for your truck, it should contain a breakdown and appropriate diagrams.