JLC GG1 6-18372 Unresponsive

Just a quickie, but with everything off, have you then tried turning on the legacy base first, then turn the remote on and bring up that engine and then wait 30 seconds and then turn track power on, does it still not respond ?

Sounds like it may not be seeing the command signal.

I haven't done that. I'll give it a try. I still can't figure out from one day to the next what changed that it doesn't respond normally like it did the day before.

This is a classic symptom of either a dead r2lc (radio board) or, as mentioned above, its not detecting the command signal and its coming up in conventional mode.  Sometimes the radio boards just die.  I would first remove the shell, and re-seat the radio board (6 screws and the shell pops right off).  Sometimes it's just a bad connection in the socket.  The radio board on the JLCs is buried a bit so its a little work to get to it.  Also try the loc with the shell off to see if it detects the command signal.  There is a wire nut connecting the antenna to radio board.  Make sure that is tight.  The antenna is the top two cover plates on the JLC.  If they are making connection to the frame that could also kill the signal. I am very familiar with this because I have a JLC GG1 and its radio board also died.

The good news is that Lionel's half price is coming up.  You can get a new radio board at the sale.  I believe that loc uses revision c13 or higher.