Great job, Tim!
This may be wrong, but what I understand 98bonnie'* message to say is that under the dash, he found that there are indeed two air passageways on the main air duct. One is for the passenger side and one is for the driver side. He removed a bolted-on piece that separates the two passageways and found that each passageway has its own vacuum-controlled door. Both doors did open when he moved the selector switch.
He let the car get hot and stuck his hand into each passageway. The passenger side was very hot, but the driver side was only warm. Up by the gas pedal he saw a plunger-type diaphragm that was not opening the big air door (shown in Randman1'* picture) as he tried all the settings on the selector switch. There was no vacuum going to the plunger at all.
He traced the vacuum hose back to the selector switch and found 4 or 5 tiny color-coded vacuum lines on the switch. He'* planning to get a selector switch at a junkyard and will let us know if it works.
Based on Tim'* input, I just talked with our local Pontiac service man. Why was I not surprised when he said that "we" had never heard of this problem, that it could be one or more of two dozen things like vent-doors, switches, actuators, wiring, etc? Of course I would have to leave the car with them forever so that they could go through it and track down the problem (@ $65 an hour?). He also said that the heating duct system was electrically controlled, not vacuum
I talked with the parts man and he said that the selector switch (for a 95 model) is about $22, but that the vacuum lines don't go to the switch. They go to a separate vacuum controller (forgot what he called it), which sells for about $13. Neither part is stocked and are order-only.
Tim, if a new switch doesn't solve the problem, I have a few questions if/when you have time. If it does work, the questions are pointless
You said that there was no vacuum at the plunger-type diaphragm when you tried different selections on the selector switch. Does this plunger/diaphragm control a door that opens only on the driver side? Can the door be manually manuevered so that we can, at least, get some heat 'til we solve this thing?
Does the vacuum line run to the selector switch or does it go to a separate vacuum control unit that is controlled by the selector switch?
Is there source vacuum at the switch or the vacuum-control unit?
What color-code is the diaphragm line?
Thanks so much, Tim, for taking the time and effort to share your experience with us. And thanks everybody for helping us get this resolved.
Bill, I checked the heater hoses after driving 3 miles to work. The temp gauge showed 150? degrees at the moment. Both of the hoses were about the same temperature and felt pretty warm, but not like HOT.
It was 16 degrees this morning when I cranked the car and turned on the defrost for the iced-over windshield. After about 10 minutes of running, I walked out of the house and immediately saw that the windshield was beginning to thaw on the passenger side but was still solid on the driver side :( How unusual!
I surely look forward to hearing what you learn today, Tim!