Runaway locomotive

[Bart1701]

I swapped decoders in one of my N Gauge diesels today. I installed an NCE N12A0. (FYI - This decoder had been in another locomotive and that locomotive was working fine.) When the loco is on the track and power is turned on, the locomotive starts moving at about 200 mph without even selecting it on a throttle. All other locomotives are still working properly.

I looked up some reasons for this and cleared all the slots on my Digitrax Zephyr DCS50 and also set CV29 to 034 for this loco ( to turn off analog mode ). Nothing has helped - I still have a runaway locomotive.

What else might cause this and what are possible solutions?

Thanks,
Bart

[Steveruger45]

Hmmm. I will resist the temptation to give our IT departments first response to troublesome digital electronics. 😁
But seriously it is as if the decoder is getting a signal to tell the motor to run, so I think I would first double and triple check for shorts from the frame to the motor tabs on the decoder.

[MK]

[Oops - Steve sneaked in but we are both thinking the same with a short somewhere.]

Check your contact points.  Since it's not your decoder because it was working on another loco, there's some kind of short somewhere.  Make sure the underside of your decoder is not touching any part of the frame(s).  A piece of Kapton tape on the underside of the decoder will prevent that.

Also, did you try a decoder reset?

[Bart1701]

Thanks. I'll try the decoder reset first since that's the easiest to do. Then I'll look at my install to see if I can find where it might be shorting out somewhere. I pretty much would bet on there being a short somewhere. We shall see!

Bart

[Bart1701]

[Oops - Steve sneaked in but we are both thinking the same with a short somewhere.]

Check your contact points.  Since it's not your decoder because it was working on another loco, there's some kind of short somewhere.  Make sure the underside of your decoder is not touching any part of the frame(s).  A piece of Kapton tape on the underside of the decoder will prevent that.

Also, did you try a decoder reset?

I did the decoder reset. Address is now 03 so that should confirm a successful reset. However, that did not fix anything.

I did a reinstall of the decoder using new pieces of kapton tape on the sides of the frames to prevent the motor contacts from touching the frame.

Where would the kapton tape on the underside of the decoder need to go? From just outside the decoder pads that the motor contacts touch over to the ends of the decoder, from just outside the 4 pads that connect to the frame over to the ends, or something else?

Thanks,
Bart

[Jbub]

Try removing the decoder and put the engine on the track. If it takes off there's contact between the motor and frame. If it doesn't there could be an issue with the decoder. And always double check your wiring. Red and black are track power while orange and gray are motor leads

[Maletrain]

Try removing the decoder and put the engine on the track. If it takes off there's contact between the motor and frame. If it doesn't there could be an issue with the decoder. And always double check your wiring. Red and black are track power while orange and gray are motor leads

If there was a short between the DCC on the tracks and the motor leads, wouldn't it just buzz with the alternating current input?

[tehachapifan]

Is there more than one throttle on your layout?

[Jbub]

If there was a short between the DCC on the tracks and the motor leads, wouldn't it just buzz with the alternating current input?

Most likely but trying to isolate the chassis and decoder is a starting point. We've seen weird things here especially if it's an old loco that may have had something installed and forgotten about. Heck it could be a light board shorting out against motor and chassis and the diode is letting the one half of the dcc power run the motor.

[MK]

from just outside the 4 pads that connect to the frame over to the ends,

This.

The newer Digitrax decoders (I know yours is NCE) since last year (really approximation here) have the underside of the decoder covered with white tape from the factory to lessen the chances of shorts from underneath.

[Steveruger45]

MK is right except NCE decoders come with yellow tape.
See photo.
You want only the Frame tabs to touch the frame. You can apply the tape to the underside of the decoder but you would have to make a cut out so the motor tabs can contact the motor terminals.  I usually stick the tape to the frame and trim once stuck in place.  [ Guests cannot view attachments ]
Is there a chance of the motor terminals being pressed sideways and touching the frame.  If yes, then wrap some caption tape around the frame in those places too.

[carlso]

Is there more than one throttle on your layout?

I echo this question.

[MK]

I think Bart said he cleared all slots in his Zephyr.

[carlso]

I agree that is what Bart stated. However, that does not answer the question. As you are aware a second throttle that has not been set to speed step "0" and the address not dispatched properly can cause some strange stuff. Just looking for possibilities.

Carl

[Bart1701]

Yes, I have more than one throttle on my layout. Visually, all are throttled down.

As Jbub suggested, I did take the decoder out and put the locomotive on the track. It sat perfectly still.

I don't know if this was a valid test (my electronics knowledge is limited!), but I took the decoder out, carefully attached wires between my track and the pads that would touch the loco frame. I then used my old Life-Like track tester (basically 2 wires, a plastic housing and a lamp) and touched the wires to the pads that would make contact with the motor leads. The lamp illuminated. I then did the same with another decoder I had recently retired. When I touched the tester wires to the motor pads, no light.

Am I right that power should only flow through to the motor pads when a signal is sent to the decoder to allow a certain amount of power to be sent to the motor? If that's correct, doesn't this also suggest that the decoder I am having the issue with is letting full power flow through to the motor pads (which would cause my engine to run wild?) Would this also suggest that the decoder got fried somehow?

Just throwing this out there - After I had swapped decoders between my 2 locos, I put this one on my programming track to read back the loco address. It came up with the address of the loco I removed it from. So I changed the 4 digit address to the new loco it was in. I noticed when it was writing the new address that the motor started for a brief moment and moved the loco maybe 1/4" to 1/2" on the programming track. Not sure I've seen that before. Might that have had something to do with my predicament?

Thanks,
Bart