I recently ran into a frustrating problem where my DCC locomotives just ran away by themselves. After a bit of head scratching I found an answer.
I won’t go into too detail but I recently wired up a stretch of track and some sidings with DCC. The locomotives ran smoothly but I noticed that on two straights the locomotives ignored changes of command from the control unit and just ran at their previously assigned speed and direction.
I’d send them into the siding and turn the dial to bring them to a stop but they’d carry on, hurtling off the track into the scenery. And that’s not something you want to happen with expensive DCC locos!
Very frustrating as otherwise the railway worked perfectly but obviously I couldn’t do any more work on the layout until I’d got to the bottom of problem.
The track was cleaned (see clean your tracks) without rendering any improvement. I also tried different locomotives so it wasn’t a faulty decoder. As the trains followed instructions perfectly on other parts of the layout I was also confident it wasn’t a damaged control unit.
I considered it maybe a case of DCC/DC-mode runaway.
This happens when the digital signals are getting corrupted, perhaps due to a poor track wiring, and the onboard decoder assumes it’s working on a DC rather than DCC circuit. In this case, the decoder switches to DC-compatibility mode and sends the loco shooting off at full speed. (There’s a great discussion of this here).
But the behaviour didn’t match my understanding of this.
If I took a train off the track and reduced the speed before putting it back the loco would correctly move off at the slower rate and not go into full-speed as I’d expect if they were operating in DC-compatibility mode.
I spoke to some friends and did the usual search of the web but no answers were forthcoming.
I had me foxed and frustrated for a while.
Eventually, I fixed the problem by attaching more feeds to the rails with the problem.
I’m not sure what the problem is but this solved it and if you’re experiencing this problem it’s worth trying too.
When I have more time I’ll do some further investigation. My suspicion is it’s a variant of DC-compatibility mode kicking in. Adding more track connections would help, which it did, if this was the cause but the behaviour of the locos doesn’t match completely. Disabling DC-compatibility on the decoders will confirm or deny this as the problem but if you have any other ideas in the meantime I’d love to hear your thoughts.
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