Normally vacuum-operated systems have mode doors that are spring-loaded to go to the defrost position when there's a problem with the vacuum supply. That's done for safety. Your feet might freeze, but the windshield will be clear.
First, find the vacuum hose that leads from the engine through the firewall. It will be shown on the vacuum routing diagram under the hood. There is usually a check valve or storage canister in that line. Check if you can blow through it one way and not the other way. Alternately, find that hose inside the car and check that there's vacuum there when the engine is running.
Next, remove a vacuum hose from one of the motors on the heater box. With the engine running, work the heater controls to see if vacuum appears on that hose at some point. If it never does, try a second motor for good measure, but most likely the mode switch is broken internally and is not switching the vacuum to the different circuits.
If you DO get vacuum on that hose, reconnect it and listen if that motor is moving. If it doesn't, the vacuum may be insufficient to overcome spring pressure to move the door. That can be the result of a tired engine, a vacuum leak in another hose, or a restricted vacuum hose leading to the heater switch. You might also suspect that hose has a leak where it goes through the firewall, from rubbing on the rubber grommet it goes through.
If you hear or feel a mode motor moving but nothing related to airflow changes, the door may be broken or the linkage from the motor could be broken or disconnected. Typically though, at least a few functions should still work.
Tuesday, September 16th, 2014 AT 5:23 PM