The HVAC Mode Control Switch (Mode Switch) is the primary control for the Heating, Ventilation, and Air Conditioning system. This is where the operational mode is selected, i.e. Off, Max, Norm, Bi-level, Vent, Heater and Defrost. The mode switch is located in the center of the A/C control panel (item #7 on Section 7-3-0 of the online store). This switch is both mechanical and electrical in operation. The mechanical part is a vacuum control switch which, depending on the mode selected, opens or closes several vent flaps in the HVAC box to route air flow through or around the heater core, to the windshield (for defrost), etc. The electrical portion turns off the blower in the "OFF" position, and engages the A/C compressor in the A/C and MAX modes. The electrical portion of the switch rarely causes issues. Problems with the blower operation are usually caused by the fan speed switch, the blower itself, or the fan blower resistor.
The vacuum portion of the switch, however, does tend to dry out and leak vacuum with age. This is often accompanied by a hissing noise from the center of the dash under the radio. When this happens, air will come out of the wrong vents, or no vents at all, no matter where the switch is set. If you are getting air from the center vents when you select "Defrost" for example, this is probably the cause.
If, after switching between any of the settings you hear a hissing noise, your mode switch should be rebuilt.
New units are no longer available, but they are easy to rebuild and a good confidence builder for performing other tasks on the car. The mode switch rebuild kit PN K114752 comes complete with all parts required and illustrated instructions for removal, rebuild, and replacement of the rubber diaphragm.
Revised 8/4/2016 DAS Minor rewrite/reformat.