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Discussion Starter #1
Hello,
I installed a digitrax Dh166D decoder in a Bachmann F7a locomotive. My NCE can read and program the decoder. And it will move F/R but when I decrease the throttle to zero the locomotive keeps moving. If I increase and go to zero it will stop on the first, second, third or maybe even more tries.

The only thing I was confused being my first time soldering a decoder harness was there are two sets of L/R power pick ups. I connected both sets to the appropriate red/black harness wires.

Any suggestions on what I did wrong? 馃檪
 

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if it's still moving, that means it's still getting power, and your NCE zero actually isn't zero voltage..
 

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Discussion Starter #5
if it's still moving, that means it's still getting power, and your NCE zero actually isn't zero voltage..
All other locomotives stop at zero voltage. It also doesn't respond to the emergency stop button.

After some more testing feels like maybe a delay in either signal to decoder (although it was working fine in an model power f7a, Walthers I believe). Or the signal being received by the motor.

Because when it won't stop I can increase/decrease throttle, nothing happens and train still moves at whatever speed it stopped responding to the Cab, wait 2 seconds increase/decrease throttle and it stops every time. After that it will stop at zero throttle until I reset the power or the train sits idle for a period of time, then it does it again.

The eBay auction I bought these from was a lot of 2 identical f7a-b unit sets, so I am going to pull the same model decoder from another locomotive and install in the other f7a and see if I get the same results.
 

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Try a factory reset then.
CV8 = 8. Hit it a few times to be sure everything resets.
Dial up 3 for the loco address and see if it still moves on speed step 0.
 

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Do what D&J Railroad said. If that does not work there is a CV for start voltage.
Look on your CV chart that came with decoder for start voltage and back it down some.
I have had the same thing happen.
 

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Discussion Starter #8
Try a factory reset then.
CV8 = 8. Hit it a few times to be sure everything resets.
Dial up 3 for the loco address and see if it still moves on speed step 0.
I tried a few times with same results. I installed the other one decoder I had in the other F7A and it runs smooth. But this one I did all the connections over and soon the trick to get it to stop, stopped working and by the end of my trial and error it would run at whatever start throttle I did and stuck there until I either pulled power or took it off the track. It just stops responding to any commands after it starts running.

So I took the decoder out and installed it one of the B units and it runs smooth as well. Even worked in consist with the working A. So I don't know much about motors but sounds like see something isn't right with it maybe. I'm going to put it back to DC mode and see if it has the same behavior. I don't recall it not stopping when I originally tested them all on DC but I didn't run them long just a quick loop.
 

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If you are plugging the decoder into a board on the F7 it sounds like a bad board. If you are wiring it direct perhaps you have 2 wires wrong on the wiring harness... like the headlight wired to the motor... that would have the locomotive run once movement was commanded and the headlight turned on... then unless you sent a shutdown code the headlight would remain on and still power the motor...

NB... DCC newb, but with a decent amount of wiring history (houses/radio equipment/rc cars...)
 

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Was there a capacitor in the errant loco motor circuit by
any chance? That could interfere with the motor
reactions to the decoder.

Since the decoder works fine in another loco it would
seem that the problem is in the loco wiring. Doubt it would
be the motor at fault. It either runs when fed DC or
does not.

Another thought. Is is possible that one of the motor
terminals was shorted to the frame or another wire?

Don
 

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OP:

Just wondering about something as described in your original post.

When you say, the loco "keeps moving", do you mean that it keeps moving at the same speed?

Or... does it slowly decrease in speed, finally stopping on its own?

And... what happens if you hit the "emergency stop" button?
That should cut power to the rails and it should stop immediately (along with everything else).

If you have an engine that "runs on, but speed gradually decreases to 0" AFTER you turn the throttle off, that might indicate that CV4 (deceleration) is set to an abnormally high value. Try setting CV4 to "0"...

The best thing to do when an engine's decoder won't behave is to completely reset it "to the factory specs" (usually CV8 to 8), and "start over"...
 

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Discussion Starter #12
OP:

Just wondering about something as described in your original post.

When you say, the loco "keeps moving", do you mean that it keeps moving at the same speed?

Or... does it slowly decrease in speed, finally stopping on its own?

And... what happens if you hit the "emergency stop" button?
That should cut power to the rails and it should stop immediately (along with everything else).

If you have an engine that "runs on, but speed gradually decreases to 0" AFTER you turn the throttle off, that might indicate that CV4 (deceleration) is set to an abnormally high value. Try setting CV4 to "0"...

The best thing to do when an engine's decoder won't behave is to completely reset it "to the factory specs" (usually CV8 to 8), and "start over"...
When you say, the loco "keeps moving", do you mean that it keeps moving at the same speed?

Same speed that I start with (usually 10/128, quick increase button or throttle usually 5). It gets stuck at that speed and won't respond to anything, not even the emergency stop. I either have to kill power or remove it from the track for it to stop.

I tried resetting it to default multiple times and even re-wired it a couple of time. I will say that out of the 4 F7A-B units this was the only one that had the orange and gray wires reversed as far as wires go. Orange wire is wired to - and gray is wired to + on the motor. Not that it mattered but to check I hooked up reverse and confirmed all it did was reverse the direction of the loco. So at least one mistake was made during assembly and evidently something motor related.

As stated above I gave up and put the decoder in one of the B units. Over the weekend since the body's of the units are the same I took the shell, front coupler and window engineers from the non working F7A and put them on the working B unit body. So now I have 2 working F7A and a broken B. Like I said, I don't know much about motors but willing to give anything a try. I didn't see any loose connections, strays or any wires touching.

Thinking about taking the shell and weathering it for my layout or if possible remove the motor and turn it into rolling stock??? I imagine I would have to get new trucks??? Or can I use the ones on there?
 
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