Repair a Marx reverse unit

Last Updated on January 9, 2018 by Dave Farquhar

When it comes to Marx repairs, the reverse unit is the end of the innocence. Motor repairs are rather easy; reverse unit repair can be as hard as you want it to be.

I’ll share some things I do that seem to make it go easier.

I’ve seen Marx motors stored for decades in harsh conditions and still run afterward with nothing more than cleaning the wheels and adding some oil. Reverse units are another story. I once took apart a reverse unit from a locomotive that had obviously been stored in a high-humidity garage, and it looked like the Statue of Liberty inside. Green copper oxide isn’t a conductor, in case you’re wondering. But I did get it working again, and without having to replace any parts.

Unlike Marx’s competitors, you almost never need parts.

The first thing you have to do is get the reverse unit out of the motor assembly. It is held in place by four metal tabs, but you can remove it without disassembling the motor. Remove the brush plate, then flip the locomotive over. Directly underneath the studs that the brushplate bolts onto, hiding underneath the wheels, there are two additional studs with nuts on them. Reach under the wheel with a small, thin tool (an old Erector wrench works well–especially if you can find a hex-headed Erector wrench) and spin the nuts to loosen them a bit. Don’t spin them all the way off the studs; you’ll have to take the wheels off to replace them. Just spin the nuts enough that there’s a visible gap. Usually you only have to loosen the nut underneath the front wheel.

Next, remove the front motor mount, which is usually held on by two tabs. Gently twist the tabs with a pair of needle-nose pliers until the front lifts out.

After gently prying the motor sides, you can pop out the reverse unit for repair.
After gently prying the motor sides, you can pop out the reverse unit for repair.

The next order of business is to spread the frame enough to pry the reverse unit out. I use screwdrivers but have heard it’s easier with an old butter knife. You’ll have to work it out in stages.

Closeup of the Marx reverse unit
This is the Marx reverse unit before disassembly

Now that the reverse unit is free, you’re in for some more prying, because the lower plastic or fiber housing is held to the top metal housing with another set of tabs and slots. I find this part goes a bit easier. Lift the top off, and the solenoid plunger will come out with it.

Marx reverse unit disassembly
Aftter you gently pry the sides, the reverse unit pops open fairly easily.
The reverse unit works like a two-position switch. It's reliable as long as the contacts are clean
The reverse unit works like a two-position switch. It’s reliable as long as the contacts are clean

The Marx reverser works on the same principle as its postwar competitors but the design is simpler since they omited the neutral position. The fingers and drum are usually dirty from electrical arcing, as well as whatever people have squirted into it over the years trying to make it work again. Carefully spray the internals down with a bit of CRC contact cleaner, then work the drum back and forth to distribute the cleaner, and carefully scrub the exposed copper with cotton swabs, being very careful not to bend the fingers.

Cleaning a reverse unit with a dollar bill
Yes, a dollar bill, of all things, is ideal for cleaning the contacts in a reverse unit

You’ll have to humor me on the next step. For this step, you need a U.S. dollar bill. It just happens that U.S. currency is just the right abrasiveness to remove corrosion and contamination from electrical arcing from copper without harming the copper. Bend the dollar bill into a U shape, then slide it under the finger. You should feel considerable resistance, but with patience you can get it done. Once the bill is under the finger, slide it back and forth about 100 times. Chances are you’ll see some dirt on the bill when you’re done. Repeat on the other side. If it’s easy, chances are the fingers aren’t making good electrical contact anymore. If it wasn’t easy, further disassembly probably won’t be necessary.

Clean the plunger and the hole in the solenoid that the plunger goes into with more contact cleaner and cotton swabs. There must be no oil whatsoever in the solenoid for it to work at its best. A frequent cause of sticky reverse units is oil in the solenoid.

A dirty Marx reverse pivot
This reverse unit didn’t work because the pivot was in bad shape. Some of it was dirt and some was melted plastic. The melted plastic on the contact to the right was really fouling this one up. It is very unusual for the pivot to be in this bad of shape; usually a bit of cleaning is all they need.
Marx reverse unit pivot cleaned up
After cleaning the dirt up and scraping the plastic away with a razor blade, the pivot looks like this. This is clean enough to work well for years.

If the fingers aren’t tight on the drum, or if you see green corrosion, or if you tested it and it didn’t work, straighten the tabs on one side of the plastic housing and lift that side off. Sometimes one side comes off more easily than the other, so feel free to try the other side if one is stubborn. The fingers should be completely straight. If they don’t make contact with the drum perfectly straight, bend one or both of them downward slightly until they make good contact. Clean off any corrosion you find with metal polish, or apply a bit of Rail-Zip to the corroded surfaces and let it sit overnight. After sitting overnight, any remaining corrosion should easily scrub off with a cotton swab. It’s not necessary to remove the Rail-Zip that remains after scrubbing with the swab; it will help inhibit future corrosion.

I’ve also seen drums where the plastic melted a bit and some of the plastic got onto the copper. When this happens, I’ve been able to scrape the plastic away with a razor blade and get the drum working again.

Reassembly is tricky. Replace the fingers if you removed them, then press the drum into place. Next, press the side into place. It’s possible the drum will come out of place a bit, but you can snap it back in. Once all of the slots are lined up and the drum is in its holes, twist the tabs slightly to secure it. Next, slide the brass piece on the underside of the plunger into the unit, then work it into place.

By all means test the reverser before putting it back in the motor. Attach a test lead to the frame and to the pickup shoe. Clip the other ends of those leads to a transformer, hold the motor upright, then power up and down. The reverser should sequence without hesitation. If it works, clip another test lead between the frame of the reverser and the motor, then try applying power again. The motor should run in both directions. If it doesn’t, the fingers either aren’t putting enough pressure on the drum or aren’t clean enough. If everything works, spread the frame enough to drop the reverser back into place. Replacing it is much easier than getting it out was. Replace the front, and enjoy your rebuilt locomotive. After this treatment it should be good for a couple of decades before needing it again.

The first reverse unit I fixed took me several hours, but that was partly because I disassembled it a lot more than I needed to. My third took less than an hour.

And if you can’t get the reverse unit working again, you can wire a Marx motor to not use one.

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One thought on “Repair a Marx reverse unit

  • March 17, 2016 at 6:51 pm

    Is the currency commutative? Could you use a hundred dollar bill, and only go back and forth once?

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