Auto Air Conditioning & Heater Repair: 97 blazer a/c heater, vacuum gauge, chevy blazer

Question
I have a 97 Chevy blazer, 4 wheel drive with a 4.3. I have a problem with the vents. When I turn the knob to operate the vents, it turns them on, but only the defrost. I believe the knob works because it turns the a/c or heater on but no matter what setting I have it on head or feet it only comes out of the defrost vents.  I recently had a new motor installed, and the problem didn't happen till after. Any help on this matter or how to fix it would be appreciated, thank you in advance.

Answer
I belive the system is vacuum controlled and if there isn't any vacuum the vent doors won't change the direction of air flow to the other vents. Check to see if there is any vacuum at the switch if there is then proceed to check the door actuator for vacuum. For these tests you'll need to a vacuum gauge.