1999 GMC Yukon Denali Base 4. Cyl 1.8L
When a/c blower is on, you can hear it really blowing in the dash, but not much air comes out vents.
The Mode Door Actuator controls the direction of airflow. It selects which way air flows through the different ducts. If your climate control system has lost the ability to control which vents the air comes out of, the MDA has probably failed. Both the ATC and MTC systems use this device.