I have a NOS Spectrum 4-8-2,DCC ready, that I wanted to install an LM-2s in. It seemed to run fine in DC when i first checked it, and it even has a firebox light. When I plugged in the module, it started to run, but stalled, and I couldn't make contact again. That night, I brought it to the club (Digitrax) and we put the dummy plug back in. It shorted out the system. We dismantled the entire locomotive but couldn't find the problem. Yesterday, I removed the circuit board and hard wired it but still could not get a connection between the HC-2 and the module. I spent quite a bit of time checking the continuity again,and discovered that the factory had wired the 2-pin plug on the locomotive backwards, so the feed to the tender was opposite of what it should have been. I switched the connections in the tender, making sure the module was getting the right polarity, and now it is running as it should, except I didn't wire the firebox light so that is not working. I can't explain why it initially ran on DC.
I am posting this in case one of you has a similar problem. I did some reading and found that others have had similar problems with Spectrum steam engines. I have several of them but with no issues previously.