AC Blend door "clip"?

[Jeepwm69]

Originally Posted by hatzie

It's controlled by the COLD <----> HOT lever.
Vacuum on newer machines and cable on the older machines.

This is an 83. I have the cable for hot/cold working well, but the blend door (assuming I'm using the proper term for the door that the clip attaches to) just seems to flap back and forth.

Guessing the engine has to be running (to produce vacuum) but what else makes it move back and forth? I want to make sure everything is working as it should before I put all the ductwork etc back in place.