I have a 97 Chevy full-size van, 117K miles. When the AC/vent/heat/defrost knob is in the off position, the Max AC is running. All of the other systems seem to be off by one position also, meaning that when the knob is in the vent position, the heat is on. What is it that is supposed to actully shuts off the systems when the knob is in the off position?