[Parisjet]

I learned a lot about this system yesterday while fault tracing my compressor not coming on. I had checked everything i could think of. I had line voltage at the control box in the compressor bay as well as low voltage (34vdc) but no power to the compressors. My coach is plugged into shore power 50Amp and all of the other electrics were working normal. My switch in the galley will turn off the power to the compressor control box (where the #1 & #2 priority switch is located)
I then noticed that the screen on the Siemens small green box was blank. Just on a lark, since i had spent hours checking everything else, I thought I will try starting the coach and just see what happens. When I turned the ignition key I got nothing, chassis batteries were totally discharged. I then flipped the emergency switch on and the lights came on the dash and to my surprise the compressors started running. I looked in the compressor bay and the screen on the Siemens box was lit. so here is the takeaway on how the system is wired.

The Siemens box controls the compressor circuit and according to the factory it controls the pressure low-on / high-off . The Siemens box gets its power from the chassis battery and not the coach battery or the inverter or the shore power.
You must have a good chassis battery for the compressor (control circuit) to operate.
The power for the compressor can come from the shore power or the coach batteries via the inverter.
My compressor 120v switch in the galley turns the compressor on/off, it will not run with the switch turned off.
I am adding a battery minder charger today to make sure that the chassis battery is fully charged. I can keep it charged with the emergency switch but have heard that this isn't a great idea long term on battery life.
usually there is a fault trace diagram that would have made this easy to diagnose, I may have to draw one up and post here. It would have saved me at least 3 hours of fault tracing without a diagram. The top line would go like this.