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I have a MX645 installed in a 121.? Up to today the loco was running fine.? A problem has suddenly appeared.? All functions are working as should but on speed step one the the loco runs away, i.e. no slow speed control.? I have reset CV8 to eight making no difference.? Reprogrammed the address to 3 still runs away.? Any suggestion as to what might be the cause and a possible fix.

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Thanks

?

Mike

Mike,

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? One of the classic causes of full speed runaway is when the loco sees the power to the

track but isn't seeing the DCC signal -and- DC operations are not disabled..

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? The first thing to do is to disable DC (for now) using the programming track.? Then

test to see if the runaway is gone.? If it is you are "part way".

? The next thing to do is to figure out why it isn't seeing the DCC signal - this could

be due to dirty wheels (most likely).? It sounds like other locos are running correctly?

? Could be dirty track?? Clean the track where it is sitting - very carefully and slowly - for

at least 3 feet (longer is better).? Does that fix the problem?

? Try swapping out the MX645 with another decoder in the same loco.? Does it still have

no slow speed control?? If not, it is the loco - if changing decoders fixes the problem

then it is the decoder.

? Finally, it is possible you have a wiring problem -or- the command station.? We'll

address those when we find that "actually no locos are running correctly".

?

? Decoders do fail.? Wiring can fail.? Even your command station/booster can fail.

--

? Jim in the PNW