Bill Lester

One other note: The connection to the STV valve is not shown since it is unaffected by this circuit except for a tap into the source line which goes to the large fitting on the modulator valve (mounted on the A/C control head). A blue striped hose goes from the small fitting on the modulator to the STV vacuum port.

As far as the colors go, the yellow is source vacuum, the green is anything triggered by the A/C controls, and the orange anything triggered by the heater controls. It’s not totally clear because of the interaction especially when both heat and A/C are on.

I'm posting the diagrams that I put together to clarify the various vacuum flows between the various heat and A/C conditions. You can see from these flows how the two vacuum diaphragms are reversed in the Service Manual drawing. It is important to note that the vacuum switch entitled "Heater Adapter" is physically located on the A/C control head and the "Air Conditioning Adapter" is physically on the Heater control head. I had installed new hoses based on the color code shown in the original drawing so I will have to make sure the right vacuum motor is engaged by switching them when I install the control heads. Hope this helps.