Working on a 2.0 3v w/Mux Rail King Big Boy. Everything works except for the smoke when moving. Smoke works in neutral, but as soon as the movement starts no heater smoke - I can feel the fan motor moving. Placing the circuit boards on the board tester works, but isn't a true simulation due to no Mux boards. I've tried replacing the Mux boards as well.
I don't have any other 2.0 steamers with Mux boards in my possession to try a tender/boiler swap (which would narrow it down 50%).
I've seen similar issues (quite often actually) where it is the speed control sensor or wiring - but in all of those cases there usually is no speed control or chuffing sound.
Work on hundreds of train items a year, but every once in a while one stumps me. Ideas?