Troubleshooting a unresponsive DCC engine

I've had an Atlas RS-3 in L&N livery since June 30, 2009.  It is a beautiful engine and ran well.

It came as a DC model but I successfully installed a TCS 1278CN decoder in it and the engine performed flawlessly. It quickly became one of my favorite engines and it would pull 14 cars up my 2% helix. In August 2018 I was wiring some locomotive storage tracks in my Loyall, KY yard with the engine on one of the tracks (I should have known better than this.) Apparently I shorted or otherwise messed up the engine and it would no longer run. I have several other engines that run well, so I sat my RS-3 on my “RIP” track for maintenance.

After 5 years I finally decided today was the day to tackle this problem. First problem was that my test track/program track has rather complicated wiring and I had forgotten how it worked!! It can be used as a DCC programming track, a DCC test track, or a DC test track. I had some connections labeled but not all. I had to trace the wiring and search my memory on how to set it up for use as a DCC programming track.

Underside of the test track showing the maze of wiring
The track is hinged on the wall above my workbench so it can be
swung up out of the way when not in use.

Top of the test track in the down position.  

Finally I got it set up properly and using Decoder Pro I could read the values of the configuration variables (cvs), but it wouldn't run. After a short struggle I managed to get the body off so I could access the motor, decoder, and wiring.

I verified that power was getting to the decoder but it wasn't sending power to the motor. Then I accidentally pressed on the front decoder board and the engine began running full speed in forward and the headlight was on. I turned down the throttle but the engine didn't obey. I also reversed the engine, but again it continued full speed in the forward direction, and then it stopped. Suspecting a short in the board I spent several minutes pushing and probing the board. Occasionally it would again take off full speed in the forward direction, ignoring any commands from the throttle. But there was no consistency in how or where I pressed on the board when the engine ran away.

I suspected internal damage to the decoder and pulled a new one from my stock. I started to pull out my soldering iron and decided to try one other thing first.  Before I undertook changing the decoder I decided to try a factory rest of the decoder. Using Decoder Pro I did a reset but when I read the cvs back, nothing had changed. This seemed to confirm my suspicion that the decoder was damaged. Then in Decoder Pro on the TCS tab, I found that the decoder was locked from change! I had to change cv 15 to match the lock code in order to unlock the decoder. Once this was done, I performed the factory reset again and (trigger the confetti drop and pop the champagne), the engine began operating again!

So apparently my electrical work did cause the decoder to lose something that required a factory reset to correct.  Luckily it wasn't damaged.  I did however waste a lot of time in figuring this problem out.    Learnings;

#1 Remove engines from track when soldering on the track.

#2 When hooking up power feeds to track, ensure the power is off.

#3 Check the instructions for your specific decoder as each brand has its own nuances.

#4 Decoder Pro is a great tool for troubleshooting decoder issues. Be sure to check every tab for decoder specific information.

#5 If a decoder begins acting abnormally, perform a factory reset. This will likely resolve the issue.

#6 Write instructions on the setup & use of complicated equipment that you use infrequently.