Runaway Loco

[TrainBoyZ-5]

A freind asked me to insert a DCC decoder into his Athrean DCC-ready loco - call this one Loco A.  I did and it worked fine.  He subsequently purchased a very similar Athearn engine, call this one Loco B, and asked me to swap the decoder.  I did that and as soon as power camne on, it took off - runaway!

Checking the internet this seems to be a frequent issue and the solution is to turn off the analog/DC bit in CV29.  We did - didn't help.  So we've swapped the decoder back and forth about 5 or so times and the result is the same: In Loco A everything runs perfectly, In Loco B it is a runaway as soon as it's on the track.  We reset the decoder and still the same problem. 

So here's what testing has shown:

1. When decoder is in Loco A it runs perfectly

2. When decoder is in Loco B it is a runaway

3. When decoder is NOT in Loco B it runs on DC track just fine

4. When decoder is in Loco B and on DC track, It goes forward when power applied and when reversed it still goes forward!  It shouldn't budge at all since analog/DC mode is turned off.

5. When decoder is in Loco A and on DC track it doesn't budge - exactly correct.

 

Any wild ideas as to what is going on here?

Thanks

 

 

[gregc]

are you sure the motor in loco B is not connected to the frame in any way?  

can you remove the motor from the loco and test it with it connected to the decoder?

[TrainBoyZ-5]

Hmmmm . . .

Not supposed to be.  It is a DCC-ready loco just like the other one.
That said, I don't know the history of this thing and that bears checking.

I'll have a look at that.  "...Remove the motor..."  Don't want to try that - yet - may change my mind.

In the mean time there are a lot of wires I can trace looking for a errant frame connection.

Thanks

[gregc]

use a meter to check resistance between each motor terminal and both rails.   Should be infinite.

or disconnect decoder motor leads in loco-A and connect them to motor in loco-B with clip leads.

[wjstix]

There were some Athearn "blue box" engines that were supposed to be "DCC Ready" that actually didn't have the motor isolated from the frame. I had one, it was a mess before I figured out what was wrong!

The motor pops right out, it's easy to remove it and test it. Isolating it, if necessary, is just running one of the wires from the decoder to bottom of the motor and soldering the wire in place, then put down a piece of black electrical tape before putting the motor back in it's cradle.

[Trainman440]

Are you running loco B on a straight piece of track or a curved piece?

I had a similar problem. I solved it. Everytime the loco went on a curve, the brake cylinder's metal wire touched the metal frame on my Athearn Genesis F7a. That caused a wierd phenomenon and the loco would make a wierd sound and run uncontrollably.

Charles