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This is Mth 20-21309-1 GP38 Monongahela PS3.  It became sluggish on its last run and wouldn't pull any cars. after taking it apart, it seems motor synchronization was off. The video shows what looks like no movement out of the front motor. Thinking possibly the front motor went bad, I replaced it with a spare of the exact same type, but no change. This is at a speed of 7-10 smph with no change in action. If i bump up to 20 smph then both motors kick in and it goes. As you can see, if I press on the rear truck at slow speed, it gains traction. I have also made sure my track is clean and I changed the traction tires as well. No change in action. Any help is appreciated. One of my favorite engines.



Mike

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MTH GP38-2
Last edited by LT1Poncho
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usually it's just a piece of ballast stuck in the side gears. Make sure you fully rotate the flywheel on that truck several times to check for binding

Try running that motor (disconnected) with low voltage DC power. Listen and look for any binding

and check wiring for any shorting! (usually against frame)

Ok, I checked the side gears, they are clean. Rotated the flywheels, no binding. Both motors run under low voltage DC with no binding. Both motors will also run under normal command control but only above 20 smph and only after a little jerky action??? Below 20 and you get what the video shows.???

Closer inspection with trucks detached from the motors and slowly turning the wheels shows that the front trucks's side gears do in fact bind up at one particular point. I think the rear axle on the front truck is slightly bent because at one point in the rotation, the gears do not mesh. Does this mean the entire truck has to be replaced? Can the axle be removed and carefully bent back to true so the side gears mesh for the entire rotation?

If an axle is actually bent or missing a tooth on the gear on the axle, you can easily replace that one axle "IF" you can find MTH Item# 20-89008 which contains four axles that folks use to switch 2-rail engines to 3-rail engines.

The small plastic piece on the bottom of the truck is held on by 2 small screws which are easy to access and remove, and all you have to do is lift the "bad" axle out and put a new good one in there. Usually the axle closest to the fuel tank has a traction tire on it, so you need the same type to replace it. The item 20-89008 comes with four axles, 2 have traction tires and two don't.

Last edited by DG
@DG posted:

If an axle is actually bent or missing a tooth on the gear on the axle, you can easily replace that one axle "IF" you can find MTH Item# 20-89008 which contains four axles that folks use to switch 2-rail engines to 3-rail engines.

The small plastic piece on the bottom of the truck is held on by 2 small screws which are easy to access and remove, and all you have to do is lift the "bad" axle out and put a new good one in there. Usually the axle closest to the fuel tank has a traction tire on it, so you need the same type to replace it. The item 20-89008 comes with four axles, 2 have traction tires and two don't.

Wonderful information @DG thank you. I will look for this part number. I am convinced the axle is bent based on how the gears are interacting. At one point in the rotation of the wheel, the gear will not even touch the other gear next to it. All 4 other gears turn together smoothly. Axle has to be bent. Wheel looks uneven. Thanks again.

Mike

Hi did you do the upgrade or was it a new engine, if you took a part the front motor n forced it together you could have caused your problem. I always rotate the cam motor to make sure it locks in correctly n then install the screw into the truck from the bottom while you tighten the screw. then rotate the cam motor flywheel to make sure it rotates smoothly!

PS make sure you don't pinch the wire between the frame and truck!

Alan

Last edited by Alan Mancus

@LT1Poncho -- since I am the one who sent you on the trail of replacing the axle, one strong word of caution.

Each of the axles have a black plastic piece and a brass piece on them that go into slots on the truck. The axle on the front truck that has traction tires has the brass piece on a different side than the axle on the rear truck that has traction tires. That is because the truck supports 3r vs 2r operations and if the engine is set to two rail operations, the wheels have to be isolated to avoid a dead short. I have attached a picture of the two axles from the replacement set showing the difference. The picture shows the two versions. The top axle has the brass piece next to the gear, the lower axle has the black plastic piece next to the gear.

Why am I pointing this out - you said you might want to take an axle from another truck on a different engine and put it in your problem truck, it is likely since you are dealing with the front truck on your problem engine, you probably should take an axle out of the front truck on different engine.

Hope this makes some sense..

.IMG_1268

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@LT1Poncho posted:

@Mikado I must be lost because I can’t get anything to come up. If you have a suggestion on what exactly to search please share! I have tried all combos I can think of.

Wrong search place. Stop looking in the "Exploded View Parts Diagrams". That's why you got no results. Again you are using the upper search link only for exploded diagrams.

Screen Shot 2023-12-02 at 1.51.36 PM

You need to search using the part number under "Service Parts" https://www.mthpartsandsales.c...h/?type=serviceparts

DC000001

I get 13 results using that part number

Screen Shot 2023-12-02 at 1.54.04 PM

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An update on this loco repair….

Thanks to the generosity of a forum member who sent me replacement axles, I am happy to say this loco is running perfectly again. As was suspected, the rear axle of the front truck was bent ever so slightly. As soon as I put the new axle into place and turned the wheels, I found no binding in the side gears whatsoever. Now the loco will crawl at 3smph and pull a consist without hesitation. This forum is wonderful. I am new at this, I hope to be able to pass on the knowledge I have learned at some point in the future to someone like me!

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@Ted S posted:

@LT1Poncho Glad you got it sorted!  What I would like to know, is how the axle got bent.  Did it fly off the table?  It doesn't seem like something that would happen with normal running.  Thanks.

I generally agree that it would take some force on most MTH axles- especially standard 3 Rail wheelsets because the axle diameter is chunky. However, this was a 2R/3R style truck with insulated wheels meaning they are not as strong as you might first imagine. it's due to being insulated, so there is plastic and turned down axle sizes that contribute to making this easier to bend than a normal 3R high rail wheelset.

One of those things where tradeoffs kind of catch up.

@Ted S posted:

@LT1Poncho Glad you got it sorted!  What I would like to know, is how the axle got bent.  Did it fly off the table?  It doesn't seem like something that would happen with normal running.  Thanks.

@Ted S To my knowledge the loco was never dropped or flew off the rails. However, this loco always had what i would call a “noisy” drivetrain. I felt like I could hear gears grinding more than other locos. I never really thought much of it. I always kept the side gears greased well, and had no issues. My guess is that the axle was bent from the factory or the wheel was not pressed on perfectly true. I also wonder if something like a small ballast pebble didn’t get lodged in the side gears and cause the issue. Really don’t know.

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