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My Legacy SD90MAC has an odd speed consistency issue at speed steps 23-26-ish. It is fine up through step 22 but when I bump it to 23 and then through 25-26 it actually slows slightly and even might slightly surge then slow again. Not a lot but detectable (see the vid). Above 27-28 it smooths back out. It does this with a consist of any number of cars or without. Above or below that range it runs fine.

It does it in a curve as well as on straight track, more noticeable in forward than reverse. I’ve attached a short 55 second video in which you can hear me state each speed step going up from step 21-22 through 28 I think. (Sorry for the engine noise, I forgot to turn it down.)

I’ve checked the trucks and gears and everything is clean and lubed. I pulled the cover off the rear motor and can see the flywheel visibly slow and become a little erratic at 23 but then it picks right back up. It seems like an electrical issue but the light does not flicker and it does not matter what section of the track the engine in. The traction tires are tight but I did buy this unit used so I don’t know how old they are.

It does not appear to be an actual traction issue but when you watch the engine that is how it looks.

My other three legacy diesels do not do this so I’m puzzled by this quirky behavior. Ideas?

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Looks like something going on with the software on the board, hard to pin that on a mechanical issue.

As Marty says, the product number is pretty mandatory when discussing internals of stuff like this.  We could obviously go research the model and road nunber, but it's much more helpful if the person asking for assistance simply provide the necessary information.

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