I recently put a decoder in my Athearn GP-50, and when I put it back together, it worked extremely poorly. It sounds like something is rubbing, but I can figure out what it is. Maybe the motor? Any suggestions?
Try repositioning the decoder location. My decoder would rub slightly against the shaft after putting the shell back on. Ran fine until I put the shell on. Just had to keep moving the decoder around until the rubbing stopped. If I recall, I wound up putting the decoder up front near the cab. Sorry, no quick answer for you…