The schematic for this tractor has more twists and loops than what I've seen on most, but working backwards, The brake switch has 2 N/O switches and when depressed closes the start circuit to the start solenoid while the other one, is connected to + 12V of the alternator. I see a green wire connects to the engine. I assume you need to supply power to this lead to keep the engine running since it is fused in the fusebox and seems to be connected to the voltage sensor in the fuse box. I don't have any answers just observations. Have you tried to re-seat the voltage monitor and other connectors?
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