Here is a link to the service manual. Look for the circuit board. Sometimes they get cannibalized over the years. http://www.lionel.com/media/servicedocuments/18213CompleteAll.pdf
Does the circuit board in the locomotive look like this? It is the standard electronic reverse unit used in many can motor locomotives. Look at the upper left corner of the picture and you will see a blue and white wire. These would go to a switch that is needed to reverse the loco. If there are no wires on your board, or were cut off, that explains why you cannot reverse the loco. You need those 2 wires connected to a switch to enable the board to operate. Without the wires and the switch, the board is locked in one mode - forward.
Larry
Modeling the "Fargo Area Rapid Transit" in O scale 3 rail.
There should be a small switch at the bottom of the cab with 2 (usually blue and white) wires going to it. That switch enables the e-unit to cycle, or locks it in one position. Switch is part # 3664-303 and (2) screws are #8400-5 if you need them. Brasseurs has them in stock.
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