I've always wondered a couple of things about this steam wiring too. The diesels are direct wiring. Some times I've used those boards in steam and don't have the MUX boards. I just steal some wires for things like headlight power, etc.
So the tach blue wire I have connected on the Williams engine right now. It appears that the tach grey wire is positive voltage and is shared with smoke fan positive pin 4. The orange wire must be connected thru so I'm guessing that's the tach's output for the board to read?
So where can I jump the blue tach wire to, to gain another wire on the tether? Did I see it correctly that it's the tach's LED output?
I've always connected it thinking it should be, yet I know on stock steam wiring, only the orange goes straight thru.