ANSWER:---
The defrost setting is a default. When the system looses vacuum it will send the air to the defrost. You either have a vacuum leak to the actuator or the actuator itself is bad. You will need to inspect the lines for a leak and put vacuum to the actuator to make sure it moves.
If air not blowing out any other vents in any other modes. If it continues blowing out of the defrost vent , suspect a broken vacuum hose between the engine and firewall where it goes to the heater controls. Most systems that use vacuum motors to move the mode doors around are spring-loaded to go to defrost mode when there is a failure. That insures that even though your feet might freeze in winter, you will have a clear windshield. There is only one vacuum hose and check valve so there is nothing to mix up under the hood.
Besides that vacuum supply, it is also possible to have a defective vacuum motor under the dash. The clue to finding that is that some functions will change but some things will work improperly. It's when nothing changes at all when you switch modes that the vacuum supply hose should be suspect.------- --------------- Your vent controller is not working. Is the air coming out the defrost vents? If yes, this is the default when there is no vacuum to the switch. In this case, check the small plastic vacuum line that is connected to the engine intake manifold and going into the firewall. Make sure this line is connected to the manifold and the hose is not cracked. If this is good, go inside the car and see if you can trace that line to the control switch. If the line is good to the switch, see if you can pull the vacuum connector from the switch for troubleshooting. with the engine on, check for vacuum on the line from the engine with your finger. If the vacuum is there, your switch is probably leaking and needs to be replace.------------
Besides that vacuum supply, it is also possible to have a defective vacuum motor under the dash. The clue to finding that is that some functions will change but some things will work improperly. It's when nothing changes at all when you switch modes that the vacuum supply hose should be suspect.------- --------------- Your vent controller is not working. Is the air coming out the defrost vents? If yes, this is the default when there is no vacuum to the switch. In this case, check the small plastic vacuum line that is connected to the engine intake manifold and going into the firewall. Make sure this line is connected to the manifold and the hose is not cracked. If this is good, go inside the car and see if you can trace that line to the control switch. If the line is good to the switch, see if you can pull the vacuum connector from the switch for troubleshooting. with the engine on, check for vacuum on the line from the engine with your finger. If the vacuum is there, your switch is probably leaking and needs to be replace.------------
