Auto Air Conditioning & Heater Repair: A/C NOT BLOWING OUT VENTS, chevrolet silverado, air doors

Question
QUESTION: I have a 2003 Chevrolet Silverado 1500.... The a/c blows out of the defrost, but will not blow out of the vents for very long, before it cycles through to the floor and then back to the defrost.... Also after starting the motor, there seems to be a tapping or knocking noise from under the drivers side dash for about 20 seconds and then the defrost begins working.... PLEASE HELP!!! Where do I start????

ANSWER: It sounds like your actuator is bad.

---------- FOLLOW-UP ----------

QUESTION: I dont suppose you could elaborate a little more??

Answer
Actuators are little motors inside the dash board that move the air doors.  From what you are saying the mode door actuator is what is bad.  Unfortunately you may have to remove the dash to get to it.  It is located on the drivers side under the dash on the black ac case.  You can move your control from panel to defrost and see which one is moving.  That will be the one that is causing the problem.