nf6x
Feral Engineer
- 1,630
- 50
- 48
- Location
- Riverside, CA, USA
I've studied the M939-series wiring diagram and P2P program some more, and it appears that the 12V tap is only used to power the cab heater fan when in low speed mode. I think that a single battery disconnect switch between the negative battery terminals and the chassis should work just fine in this application.