Big Blue,
Since this issue is happening to two locomoties from different manufacturers, this is not a programming issue or locomotive issue. I suspect that it may be wiring...
Most DCC decoders are 'dual mode' and can operate on either DC or DCC. They 'sniff' the rail and look for a DCC signal - it they do not find it, or lose the signal - they revert to DC mode. They may take off at high speed! This appears what is happening to you. A loose wiring connection(noisy) can cause this. I have not had this happen on my layout(wired with a Digitrax DCS100), but I have a 'test track' in my study that uses some Bachmann EZ Track and a Digitrax DCS050 unit. Right after I set this up, I ran into some issues with existing engines 'moving' when on the test track or just taking off at high speed. I thought I had a defective Zephyr command station. I spoke with 'AJ' at a NMRA national about this and he said to replace all the rail joiners and make sure the EZ Track plug connectors were good. He also mentioned that Kato Uni-Track has better rail connections, but I thought I would try to 'fix' the existing Bachmann track.
I replaced the rail joiners(some were loose) and even soldered them - No more problems! I have seen this happen on at least two other 'home' layouts that were converted from DC to DCC. The existing wiring was OK for DC operation, but 'garbled' the DCC signal and caused issues.
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
Hi BigBlueConrail,
Is this a stealth engine (item number ends in -0) with an aftermarket decoder, or a factory DCC engine with a Protosound 3 decoder (item number ends in -1)?
Elmer could be correct a short is causing the behavior. If it's an aftermarket decoder that is far more likely. The PS3 decoders are quite good at distinguishing their operating mode because they are actually tri-mode decoders. They run on DC, DCC, or DCS. If it is a factory DCC engine is does not have a CV to lock the operating mode.
If it is a factory DCC engine another possible cause is a faulty tach reader. The PS3 decoders use an optical tach reader to measure actual motor speed rather than inferring motor speed from BEMF. This method is far more accurate and consistent as operating conditions change. If a tach reader fails or a wire to the tach reader comes disconnected the decoder thinks the motor isn't turning and it applies max pulse width to get it moving. Hence, the engine takes off at full speed. You can test this by putting the engine on the track by itself. Command the engine to run at speed step 1. If it takes off at full speed, throttle back to zero. Do that a couple times. If the behavior is consistent it's highly likely the tach reader is bad. If the behavior is inconsistent it's more likely a loose wire that's intermittently making contact.
If your engine needs to be repaired and bad tach reader is a fairly inexpensive repair.
Please don't be insulted, but you DID program the decoder to a unique address, right? I know that sounds pretty basic, but when I added the 3rd cab to my Prodigy Wireless setup, the 1st one stopped working. I finally fixed it when I remembered that you had to set each cab to a unique ID. Duh!
Connecticut Valley Railroad A Branch of the New York, New Haven, and Hartford
"If you think you can do a thing or think you can't do a thing, you're right." -- Henry Ford
Most DCC decoders are 'dual' type decoders. They can run on DCC power and DC power. After a track short, sometimes the decoder has a problem recognizing which type of power is being supplied. If the decoder thinks it is being powered by DC on the track, it just takes off. There is a CV that you can turn off so the loco will only run on DCC power. You will have to look in the decoder manual to find out which one it is. If the paperwork that came with your loco doesn't show it, you will need to go on-line and download the instructions for the particular decoder you have.
Now, I'm not saying that is your problem, but this is a common occurrence with dual mode decoders, and is one possibility.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
When running trains on my layout (prodigy 1 DCC) my MTH SD70M-2 will for no apparent reason take off at full throttle uncommanded. I just got a BLI AC6000 and it started doing this after I got my new locomotive so it may be the cause? Has anyone seen anything like this?