Yall may or may not believe this. Now I'm not a novice at decoder installs. I have installed them for myself, many friends and I even do them for the LHS when asked. I guess that I've installed several hundred over the years.
Never had this happen before. I was installing a decoder in an older Bachmann N&W J 4-8-4 in N scale. I had a N-scale decoder and went about making sure the motor was isolated from the frame, and the split frame that provided pickup to the motor was also isolated from the wipers on the drivers. Double checked every connection, made sure there were no cross wiring and there was no shorts according to my meter.
Put it on the track, turned on the power, short. Had JMRI read it, and it said no decoder. To prevent damage to the decoder, I took one of mine, repeated the process and again, no joy. Direct short when power was turned on. Told the owner to come over as I hadn't been able to install the decoder without a short. The owner comes over, and I install, again a HO decoder to show the owner whats going on.
Well guess what, it worked! I then removed that decoder and installed the N-scale decoder, and nothing. No short, no movement nothing! Put the HO decoder back in, and again it worked perfectly. Even installed a new light in the smokebox. Put it on JMRI, where the decoder was read properly, and I then programmed it with no problems. We tested it, and although it ran a little rough, due to the small piece of N-scale track I have, and I knew that it was dirty. The owner went home with the loco and some instructions on getting it to run on his layout. (Mostly to clean the track). I then took the N-scale decoder and put it into a switcher that had another N-scale decoder in it. Worked perfectly!
I've never run across an install that was this strange or funky. How about yall? Yall ever have a decoder install like this?