The coupler DOES open on command, but also when not commanded. Is the coupler to blame, or might it be the control board for it? If the latter, can I buy the board and replace it myself?
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First step would be to replace the coupler, it's $10 as opposed to a couple hundred! 
Thanks, John. I will do that, but I'm also wondering why the fix would be that simple, assuming that the coupler was opened by some [spurious] command. Maybe you're telling me that it may NOT be a voltage signal, but maybe just a mechanical failure?
phil gresho posted:Maybe you're telling me that it may NOT be a voltage signal, but maybe just a mechanical failure?
That's exactly what I'm telling you. If you unplug the coupler from the electronics, and it still opens, it's not an electrical issue.
If the replacement coupler still opens on its own, it probably a grounding issue. I’m not familiar enough with your loco to make a specific determination. Inspect any terminals attached to the chassis or trucks. Look for anything that may prevent PERFECT conductivity. Double check the connection of the coupler wires. Look for corrosion, glue, paint, dirt, or anything that might cause an intermittent electrical signal to the coupler. Good luck.
Actually, if the coupler keeps opening only when connected, but not when disconnected, time to look at wiring. I'd look for chaffed wires that are momentarily being grounded, that's an issue that I see on occasion. If you don't address it, it can do a lot more damage. A high resistance joint won't cause the coupler to open as a rule.
The coupler has PV on one lead and the other is pulled to DC common to fire the coupler. Note that DC common is NOT frame ground. If the coupler control wire is grounded, a ton of current can flow through the coupler, and if continuous it can take out the board.
Mike, that is NOT a fix for the PS/2 or PS/3 electronics! Apples and oranges here.
