I have this engine... somewhere. The parts list and exploded diagram is in Supplement 15.
Have you checked the connections between the engine and tender?
Per the diagram the front tender truck is 8000-T80 Plain Truck with Hall Effect Sensor. Not much detail on the diagram but there is a rotating magnet with a sensor wired to the circuit board. Check to see if the sensor is in place and the wires are intact.
Looks like this: