Motor contacts and rail contacts are arranged symmetrically in the NEM plug, so rotating it shouldn't affect behaviour in any way. Only, if you use a decoder that is sensitive to polarity, which some old Märklind decoders were (and maybe new, I don't know). DCC decoders should have this. So there are two alternatives: bad contact cured by rotating the plug, or wrong polarity in track. You could check the latter by switching cables to track, red and brown, instead of rotating the plug. But this will probably cause your other Märklin locos to be standing still...
If this is so, the loco is wrongly wired. Switch cables between slider/wheels and nem contact then. Or change the decoder to a non polarity sensitive decoder.

Regards,
Lars Westerlind.

Bart,
I agree with almoste everything you say. Except:
1. Brake section (Märklin/Lenz style) problems, could occur in the same situation as polarity senitive decoders might have problems. So most probably, if you get the correct polarity for the digital signal, brake sections will almost certainly function too.
2. There might be problems to switch cables between slider/wheel and soceket; easier done for the decoder. For the setup, it doesn't matter where the changing is done, BUT, the dayMcLae would like to change decoder it does. It seems most unlikely that the decoder is wrongly wired, but not impossible that the loco is; as an improperly wired loco with work with many decoders it could also pass tests...

But there is something fishy here in McLaes description. McLae, could you describe more exactly your setup: what actions did you do on the Programming Track and what on main track, Could you change the plug back and still get the original behaviour, what happens when you switch track cables... I assume 3 rail, right or wrong? And what decoder do you use?

Regards,
Lars Westerlind.

McLae,
1. When speaking about polarity, I was speaking about the digital signal. The digital signal changes between + and -; for the loco to get power, this is rectified inside the loco in a rather normal way. So, it's true that the polarity in that sence doesn't matter. But I was speaking about another aspect - Decoders also listen for signal info - in order to changes speed, light etc. And by Märklin decoders, (becuaus the risk of having changed red and brown cables is little), this is sometimes, maybe always done by only accepting the signals to have a certain polarity, and not the mirror of it. A DCC decoder, which is intended for 2 rail, where one never knows what is "red" and "brown" so to say, it's compulsory to accept both patterns.
2. The not about Märklin locos is important - it tells me that you most probably have the right wiring (sorry for suspecting this error, but it is easy to do it wrong under a C-track, and also at the stupid IB connector). Nevertheless, it would be a good and easy test if you versed the cables with the plug in the position where the light works.
3. You don't tell us if you use the Programming track, and if you use CV programming of the decoder. I assume you don't? You say changed adress? By CV programming, or the button on decoder, or what? And please note, (sorry to say this), that the cables to the PT might be reversed as well...
4. You say you have the lila f2 cable. In last years Roco catalogue, the 10738 doesn't have f2, only f1. Are you sure you have the correct decoder? I wouldn't be surprised, such matters change fast.
5. I guess this decoder is made by ESU, which is possible to run on both Märklin and DCC protocol. But I really am not sure. So it might be important what protocol you use: I assume Märklin new is used for this, the IB should show an m etc. But please tell. And if DCC, it's different with DCC 14 and DCC 28+...
6. Would it be difficult to reverse the plug again and see? If it is bad contact or something...

So, it's me who is sorry,
I'm a software engineer too, so I would like even more info before trying to give advice.

Regards,
Lars

to answer questions:
Programming was done with CV; push button on decoder, select lok address, send reverse code. Address change tested by setting new Lok number, selecting new address with IB, and reversing. Lights would change when Lok was reversed, but Lok did not move.

When decoder inserted 'backward', Lok ran in both directions, run, stop, reverse, run, stop, etc. (just no lights)

On Lila cable, it just hangs off the side, not mentioned in documentation.

On a side note, the documentation talks about electrical isolation, not putting tape over parts of decoder, etc., but this decoder is covered in plastic. Do I need to remove this plastic?

As to part numbers, I ordered this from Reynauld's; this is what they have listed to modify the engines they sell to digital... It did not occur to me until reading your posts that there might be more than one type of decoder from Roco. It is possible I have the wrong type of decoder, but how do I tell?

BTW, does Uhlenbrock make NEM Plug decoders?