At the last train show I attended, I bought a Marx locomotive that lacked a reverse unit. I found a mess of wires in its place, and of course it didn’t run. So I had to figure out how to wire a Marx motor without a reverse unit. When I wired it up to run correctly, it surprised me. It ran really nicely.
Reasons to run without a reverse unit
First, it was nearly as quiet as a modern can motor. Second, it could pull five scale cars at a mere 7 volts–and this was with a single-reduction motor. Suddenly I understood the modern O gauge guys who say a postwar motor can really creep with a modern reverse unit. Wire a Marx up without the old-school reverse unit and it can creep too.
There are any number of reasons to want to wire a Marx motor without a reverse unit. Maybe you want to quiet down a Marx engine and don’t mind losing reverse. Maybe, like me, you found a Marx engine with a missing reverse unit and you want to get it running again. Or maybe you want to run two diesels back-to-back, which means running one in forward and one in reverse all the time.
Finally, even though Marx’s reverse unit is pretty reliable, it’s less reliable than the motor. A Marx engine is more reliable without a reverse unit than with one.
Whatever your reasons, here’s how to wire a Marx motor without a reverse unit.
Testing without a reverse unit
The coil of wires at the top of the motor is called the field. Normally, those wires run to the reverse unit. When you don’t have a reverse unit, you have to wire them differently.
One of the field wires runs to the top motor brush. If the wire coming off that brush is there, try splicing it to the wire coming off the field toward the front of the train. Just twist the wires together for now until you test the motor. If the wires are missing, refer to the pictures here to see where the wires go. You can also run new wires if the old wires are in bad shape.
The other field wire runs to the electrical pickup shoe that contacts the track’s middle rail. Try splicing the wire from the pickup shoe to the wire coming off the field toward the back of the train.
Test the motor to see what direction it runs in. If it runs in the wrong direction, reverse the wires.
Insulating the wires and finishing up
Once you’re sure the motor runs in forward, untwist the wires. Slide a piece of heat shrink tubing over the wire, then twist the wires and fold them over. Slide the heat shrink tubing over the splice and shrink it with a hair dryer. Now the splice is properly and safely insulated.
If you’d rather fix a reverse unit than remove it entirely, I have a separate post on fixing Marx reverse units.