Problems with coupler and LCRU

A few things come to mind.

1) The 6-18056 is one of two steam locomotives with an early, non-modular form of the AD20 board in the tender. The system must be reset/restored with the 740 code, and both locomotive and tender should be on the track. This makes sure that the LCRU2 in the locomotive and the AD20 in the tender communicate properly.

2) The AD20 boards (both the modular and non-modular forms) have two varieties that serve two completely different purposes. The AD20A boards decode the serial line from the wireless tether in the tender, and also activate the coupler and back-up light when appropriate. The AD20B boards are for certain diesel installations, and provide the oscillating ditch lights when the horn is blown. These AD20A and AD20B boards are visually identical... the only difference is the code in the microprocessor. The reason I bring all this up is be sure you received the AD20 board with the correct code from Lionel.

3) Even though track power goes through the AD20, it's the RS2.5 sound board that provides the 5 volts DC to the AD20 for the microprocessor. So check to be certain the output voltage from the RS2.5 board is OK. This is necessary for coupler and back-up light operation.

Hope this helps.
TRW

By coincidence, I just had a look at the other locomotive (6-18062 Santa Fe Warhorse Hudson) that has this particular electronics package with the same problem as the OP above -- the tender ElectroCoupler was stuck on.

I was able to repair the AD20 board in the tender, but in doing so, it occurred to me that my original third statement above wasn't completely correct. I had forgotten that these early, non-modular AD20 boards get their +5VDC supply from the RS2.5 board... so the RS2.5 board IS necessary for the operation of the coupler and back-up light. I've corrected my original post above.

TRW