I've searched the Forum diligently and haven't found this answer, so I called Ken at Electric RR Co this morning and he kindly provided the details missing from the Lionel-authored installation sheet that comes with the ERR Co conversion when adding an ERR/Lionel Railsounds board to the basic conversion board (the AC Commander in my current conversion.)
When you do the engine number setting in the last stage of the conversion with the sound board, you will also set the name for the feature you chose (a cab light, feature code 6, in my case.) I have searched for a complete list of the feature codes, 1 - 9, but still haven't found the list.
When setting the engine after the installation: With power off, move PG-RUN switch to Program. Turn on power to engine on track. On the CAB-1 (or CAB-2 I assume) press ENG>your chosen 2 digit engine number>SET>AUX1>6 (for me with #6 cab light as feature) or your chosen feature number 1 - 9, and you're done programming. Power off, set switch to RUN, and you're done. This avoids the problems mentioned that happen when the feature code is not "properly set", like a runaway engine at 18 volts etc.
Now, with the feature code chosen, AUX1>9 turns it on, and AUX1>8 turns it off, whatever feature you've chosen or installed.
If someone can provide the list of TMCC Feature Codes 1 thru 9 here, that would be appreciated. And if anyone has details to add or suggested changes to these instructions, please post.
Thanks again, Ken, for your help this morning. (BTW Ken commented he doesn't control the editing of these instructions, so we just have to hope Lionel will add these details at some point.)
Don
