random electrocoupler openings on 2 different Legacy diesels

There are other threads on this that identify a) weak springs in the electrocoupler plunger, and b) electrical noise that triggers the electromagnet to open the coupler.

I identified that weak springs were the culprit on my engine by disconnecting the coupler plugs from the board in the engine.  The couplers still opened which indicated weak springs.  I drilled the rivet, removed it and pulled out the plunger and spring.  I stretched the plunger spring a bit, then reassembled the coupler.   Problem solved.  (Take care to capture the springs when disassembling).

If the couplers remain closed when the circuit is disconnected, but open when connected, then electrical noise or a faulty coupler may be a problem.  Although adding a capacitor across the coupler wires was recommended to mitigate noise years ago, Lionel says this is not a good solution with Legacy boards.  I would try a replacement coupler before adding caps.

Correct.  The cap fix is for TMCC.  BTW, it works.  Couplers are $10.00 from Lionel.  I see this often and follow what John outlined to determine if it is an electrical or mechanical issue.

I also had the coupler problem on the AC6000.  I found that I had a very small cut in the wire which was making contact the  body.  Fixed the wire  with a small piece of shrink wrap tubing.

One caveat, early Legacy with modular R4LC are still like TMCC and a capacitor is on the coupler circuit.  Normally these caps are already there on the motherboard.  Make sure still present and not damaged.  The newer RCMC based Legacy is a DC circuit and so the caps are not appropriate nor should any circuit be tested against chassis ground.  G

George, you are correct  the Legacy modular board system has the caps on the motherboard for the coupler.   A 1uF 50V non polarized capacitor is installed on the motherboard connecting to the hot and ground coupler leads.  Again, couplers are cheap.  Every new coupler I install gets a tab of super glue where the wire first comes out from under the black protective covering.  This stops any chance of the wire pulling out and causing a headache.

You made a good point that is not conversed about often using the newer RCMC board.   Chassis ground is never to be used for testing anything with the RCMC board.  Board ground only.  I am a huge fan of the RCMC board system.