I have a E250 and which tank is main? For the fuel tank valve is it in main when power is off and when energized it selects AUX tank?

 

Mid-Ship tank is MAIN, rear is AUX. If the selector switch and wiring is original equipment, then when powered up, the valve is moved into position to use the AUX tank. When power is removed, the valve is "Spring" returned to the MAIN tank.

 

agreed with you.