Disconnect the 'E' unit wires from the motor. Apply power to the loco, and check for DC voltage at the wires. Cycle the transformer on and off a few times to see if the unit cycles. If there is no voltage, make sure there is AC voltage at the board input, usually the heavy red and black wires.
The parts list for the locomotive is here, and the board, #13 on the list, is 610-0107-100, available from The Train Tender.
Larry
