I have a bit of a hybrid project, including my first attempt at adding LED headlights to a dummy diesel. It's an MTH / Railking SD-9, but what I've added to it are most of the parts from a Lionel Deluxe Upgrade kit for non-powered GP-9 B diesels. The first part involved taking the electrocouplers from the supplied trucks and transplanting them on the MTH SD-9 trucks - which by the way were a perfect fit. Then I got to mounting the Lionel LCRX0906 board and getting everything connected. The electrocouplers work perfectly - the LED headlights, not quite...
For each of the headlights I have two 3mm, 3V warm white LED's wired in series with a 300 ohm resistor also in series - and that part seems to be just fine. Reading previously about having problems getting the light outputs on some of these boards to work correctly, I also added a 470 ohm resistor directly across each light output to the common ground.
Setting the engine on the track and testing it out, I can only get either the front headlight or rear headlight to work at a given time. This is where it gets odd... One or the other of the headlights will work, being able to be switched on and off, and going on and off when changing the locomotive direction. The other will always remain dark. However - if I rattle the engine around a bit, the light that previously didn't work will start functioning correctly. And of course, the one that worked previously will now stay dark continuously.
I initially wired up a test with the components listed using test leads for one headlight, and had no problems. Is this odd behavior a possible indication that the 470 ohm resistor across the light output to ground isn't sized correctly? I've checked the wiring and don't see anything loose and all connections seem to be in order. If anyone has seen this kind of behavior before I'd appreciate the insight!