lionel electro couplers that open unexpectedly

If this loco is equipped with TMCC or Legacy (you didn't give a catalog number, so this is a guess) There is a standard way of determining the problem.

Disconnect the plug/wires going to the coupler and run the loco.

If the coupler no longer open spuriously, the problem is electrical and can be fixed by installing a 1mfd non-polarized capacitor across the leads.

If the coupler still opens spuriously, the problem is mechanical and the coupler must be disassembled and repaired, or replaced.

I just fixed a friends engine with the same problem. There are a few causes but I believe your engine has Liondrive. Report back with the stock number to verify but most all Legacy diesels are now Liondrive. What that means is you can twist the trucks 90 degrees and they will come off. Do it slowly over a table as there is a coupling than can fall off. Pull away carefully as there is a three pin connector that connects the truck to the engine and carries center rail, outside rail and coupler hot. Make sure there is no bare wire beyond the connector pins. If there is you have to insulate them otherwise if the center rail wire touches the coupler wire it will open it.

Pete

Let's slow down on the caps!  If these are newer Legacy with the RCMC, do NOT install any caps!  Those are DC, and the caps will create issues with the drivers on the RCMC.  If it's older Legacy with the modular boards, the caps are OK.  I don't think the RCMC has the same electrical triggering issue that the old R2LC/R4LC had, so electrical probably isn't the issue unless you have a chaffed wire that's shorting to something.

Thanks for your info. The Reading SD70 were purchased new in Sept 2013 and have no more than 20 hours running time total. Lionel numbers are 39633 and 39634 but only one has the "opening" issue.

It does have Liondrive but no evidence of a three pin connector, just screw terminals. Still a chafed wire is the most likely issue. Worth pulling off the offending truck before removing the shell. The likely spot is where the coupler wire enters through the frame.

Pete

Don't be sticking caps on that one, it has the RCMC.

Not a fix, but a low-brow MU ("lash-up") work-around:

If the offending coupler is on the rear or front of #1 loco, or front of #2, and it has been determined that the coupler has no internal mechanical problem (it latches), disconnect it and treat it as a dummy.

All of us want everything to work all the time, but I try to no longer spend time, effort and maybe money on something that I finally realize that I never, or sometimes, almost never, use. If these locos are always MU'd, two of their E'couplers are really kinda wasted.