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Wire a Marx motor without a reverse unit

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. If you have a broken or suspect reverse unit, you might want to bypass it to get the motor working again quickly. If you do operations on your layout and need the ability to reverse, you can come back and fix it later.

Whatever your reasons, here’s how to wire a Marx motor without a reverse unit, or bypass it.

Testing without a reverse unit

The coil of wires at the top of the motor is called the field.  Normally, the two wires coming off the field run to the reverse unit. When you don’t have a reverse unit, you have to wire them differently.

Wire a Marx motor without a reverse unit

In this picture, you can see how the wire from the front of the field is running to the top brush holder, and the one from the back of the field is running to the pickup shoe under the motor.

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. If the wire to the pickup shoe is missing, just solder a new wire in. The pickup shoe usually has a hole in it to make connecting a wire easier, but you can solder a wire to anywhere on the shoe. Alternatively, if your motor still has the light bulb socket in it, you can pick up power from the socket instead. As you can see, my motor didn’t.

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

Wire a Marx motor without a reverse unit

In this picture, you can get a different view of how the wires run through the motor. The slightly discolored and oversized insulation is black heat shrink tubing.

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. Hit them with a touch of solder to hold them together, then 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 solder better than me and you’re careful how you fold the wires, you can barely even notice the shrink tubing is there.

If you’d rather fix a reverse unit than remove it entirely, I have a separate post on fixing Marx reverse units. Fixing a reverse unit takes an hour the first time you do it. Bypassing a reverse unit only takes a few minutes, so I don’t blame you if you go for the quick fix first. Just be sure not to cut wires too short, so as to not limit your options in the future. After you fix it, you can always go back in and add a reverse lockout switch if you want.

If you choose to bypass the reverse unit, keep in mind they aren’t difficult to remove. My post on repairing reverse units shows how to remove them. Even a non-functioning reverse unit sells for a few dollars on Ebay, so selling unwanted reverse units isn’t a bad way to bolster your train budget a bit.

You can also do a similar process on Lionel motors to run them without an e-unit.

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