Wow you guys, those diagrams and the explanations are perfect. I was figuring there was a trick in the "5-port" switch that combined vacuum signals in various settings, but couldn't figure it out just from trying to pull test vacuums on various ports.
The colored diagram with the table is the most helpful, but I'm trying to wrap my head around the logic there, which assumes an X means vacuum applied to that circuit in the 5-port switch (3 always "on" since its engine vacuum):
OFF: Just engine vacuum to the switch
De-Ice: Defroster door (4) and 100% outside air (1 + 2) through heater core based on temp door position, blowing to windshield
Heat: 100% outside air (1 + 2) through heater core based on temp door position, blowing to feet
Vent or Out: Diverter + Deflector door combo (5) and 100% outside air (1 + 2) through Evaporator/Heater core based on temp door position, blowing to Vent system
Inside: Diverter + Deflector door combo (5) and 20% outside air (2 only) through Evaporator/Heater core based on temp door position, but this includes De-Ice (4)? I didn't realize it was going to push air to the dash when in Inside air (recirc) mode; How does air get to the Defroster door when the Diverter door is already full open/blocking the path to the Heater floor vents and the Defroster channel? Some still comes through the Heater core and can get there?
Based on this, I can assume I know why I hear a hissing sound when I push my control all the way to Inside: My 1+2 diaphragm is working fine, my 5 diaphragm is working fine, but my 4 (Defrost) diaphragm is dead, so it's causing a vacuum leak there (I hadn't tried running Defrost and hearing it hiss since I'm in Florida, was only checking Vent, Outside AC and Inside AC most of the time!). I was concerned that what I was hearing what a problem with the engine compartment diaphragm and that I may not be getting proper inside/outside air mixing functions, but I was able to pull vacuum on 2 and see the 1+2 diaphragm bar move from inside the dash (with the 1 port plugged).
SO: from what I can see I can't remove/replace the Defrost diaphragm without pulling the entire heater plenum?
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