The blend door selects between heated and non-heated air for the desired temperature of the incoming air. The actuator depends on the type of control system you have. If you have the common sense manual levers, k n obs, and switches, there will be cables that could become disconnected or out of adjustment. If you have computer controls, the actuators are usually electric motors with position sensors. Either system could use vacuum motors to move the various doors around, but those are typically used for things that are turned either fully on or fully off. A door to the defroster would be fully open to defrost the windshield, or be fully off when you want toasty feet. Blend doors aren't usually set for full hot or full cold all of the time so vacuum motors don't work well for that function.
If you have a manual slide lever for temperature, you might be able to hear the blend door hitting its stops when you move it back and forth. Some cables, such as on Dodge Caravans, are self-adjusting by means of the cable sliding through fingers of a clip on the actuator lever. If the door becomes rusted tight in the cold position during the summer, it won't move in the winter, but the cable will slide giving the impression the door is working. When that happens, you won't hear the door move when you slide the lever back and forth.
If you have vacuum controls, you should be able to hear some of the doors moving when you switch to different functions, and you will often hear something moving around when you stop the engine and vacuum goes away. The systems are spring-loaded to go to the defrost position when there's no vacuum to insure the windshield can be cleared. When none of these controls work, a leak in the vacuum hose can be suspected, or the hose could be disconnected.
Computer controls often have self test / calibration procedures that can get pretty complicated. Problems with these controllers often leads to flashing display lights or the system is dead and has no lights turned on. It's common for them to be intermittent too. If you get hot air sometimes and not others, suspect the computer. Otherwise they typically do not go out of calibration unless the battery was disconnected or run dead.
The two most important clues are whether the two heater hoses are hot and if you can hear something moving when you adjust the temperature control. Beyond that, it becomes a process of eliminating all other possible causes before condemning the computer. Most manual controls can be accessed under the dash or often by removing the glove box liner, radio, or heater controls. Computer controlled actuators typically require removing the steering column and dash assembly to remove the heater box. That gets real involved and is definitely not a do-it-yourselfer project. Experienced mechanics know how to diagnose the most likely cause of the problem before removing anything, and they know what to look for when they get the heater box on the workbench.
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Tuesday, December 14th, 2010 AT 12:37 AM