I tend to agree there was some skullduggery on the wiring. No way should the Systems Monitor be associated with the run and start circuits.Thanks. In my case I'm replacing a BCGTC with a BCGB and they are connected to the boat very differently. The BCGTC was working, but it had a giant hole in the block where a connecting rod tried to escape. Holding the On switch in the ON position allowed it to run, bypassing the lack of oil pressure. So, there is no issue with the boat as wired. (as purchased - seller said he was using it in the past, and knew nothing about a thrown rod - whole other topic!)
If I knew what the Violet wire's purpose was, that would be helpful. Since the 2 halon wires are direct connected to the 2 Generator wires in the System Monitor, and the Violet one can't can found at the genset, it's a quandary. The other wire goes to the aux oil pressure switch, so it's purpose is clear - it signals the System Monitor of Oil Pressure loss (the primary would shutdown the genset at the same time, so a bit redundant), but why it connects to the Halon system makes no sense.
The BCGB control panel is nearly identical to the BCGTC, but the 5 extra wires in the BCGTC don't correlate to any drawing of each generator. The only difference I see is how the choke solenoid is energized.
The later version of our boat, 380AC, has a genset drawing that looks like the BCGTC terminal block connected to the BCGB style 15 pin connector, but it uses 2 switches, and no Monoplex.
I have have decades of electronics & electrical background, and this has me baffled. Mostly due to a lack of correlation between the boat and the published wiring diagrams.
I think getting into the harness, and tracing wires, may be my only option at this point.