Surging Locomotive Solved - Now there's something you can't do with DCC!

[William Brillinger]

Way back here in this Keep Alive topic, https://rpug.pdc.ca/index.php/topic,327.msg3467.html#msg3467, I mentioned a possible issue with MU'ing Keep Alive equipped units that use the Full Bridge Rectifier with units that do not.

The reason I was concerned it was an MU'ing issue is that when it ran by itself it all looked good, but when it was MU'ed to another identical unit, the 2 were bucking against each other.

This week I finally got to the bottom of the problem and as I concluded in the Keep Alive post, it was not the KA's or the bridge rectifier that were the issue. I had suggested in the Keep Alive discussion that it was a mechanical issue, but that turns out to be incorrect also.

Given the symptoms of the issue I was not sure which loco was actually having the problem until I pulled them out again this week and tried them again to identify the offender. While running them I noticed that one was flashing a brief warning. So I ran them while watching the LM info screen and low and behold I discovered the issue.

The voltage reading on one of the units was fine for a few moments and then it would drop to about 10V, then it was fine again, and round and round it went. It was surging almost imperceptibly until it was coupled to another loco. Under load it was impossible to see, but when it was MU'ed to another locomotive, the effect really stood out.

Now that I could see the problem on the LM Info screen, I knew what to look for.

I stripped out the control board and hardwired the LM directly to the pickups and motor. It still surged.
I changed the LM. It still surged.
I put the LM into a new identical loco. It worked fine.

It turns out one of the pickups was failing.

Problem solved.

Watching the voltage in the decoder in real time, That's Cool!

[nodcc4me]

Good job troubleshooting, Bill. I have found that almost all of the stalling, rough running problems I have experienced were due to either dirty wheels or bad contact connections. Some manufacturers just stick the tiny wire in a hole on the connector and put a plastic cap on it. Those are the first things I check, and I usually remove the caps and solder them.

If the misbehaving engine had been equipped with a KA, would that have masked the problem?

[William Brillinger]

If the misbehaving engine had been equipped with a KA, would that have masked the problem?

Thanks Al,

It has a KA. Once I took the KA out the problem was much more pronounced. Instead of the power dropping and rising on a curve, it would just bottom out all of a sudden.

I left out a few steps in the above description, before I removed the board, I did remove the clips and solder the pickup wires. Of course that didn't change anything.