The motor in my 37951 culvert loader quit working. I replaced it with a new motor bought from Lionel Parts, and the loading mechanism now works again. But the "Switch" terminals on the loader no longer control whether it is "on" or "off" -- it seems to be stuck "on". The optical detector for gondola car *does* work -- if a car is not present, the loader won't run. But once a car is in position, the loader runs immediately. The "Switch" terminals are definitely not somehow shorted together, as there's no continuity between them.
I've followed the "Switch" terminals through the internal PCB, but I don't have a schematic so I'm just following the traces on the board. One terminal is effectively connected to the Ground for the electronics on the PCB. The other terminal goes through a diode to a pin on a 14-pin DIP integrated circuit that is surface-soldered to the back of the PCB. (The diode is perhaps to protect the IC against inadvertently applying power to the "Switch" terminals?) So when the switch terminals are connected together, the pin on the IC should be pulled to Ground. Presumably it floats high otherwise. I can also see pins coming out of the IC that control the power transistors that drive the motor. These pins provide signals as expected for when the motor is "off" or is "on" running in either direction.
But it appears that the pin on the IC does *not* float high when the switch terminals are not connected. Continuity testing when the PCB is not powered shows that pin as connected to Ground, which I think explains why the loader is stuck "on" when the optical detector shows a car as present. A separate pin must be the signal from the optical detector, and it must be working, but it didn't follow all the traces for the optical detector to confirm.
The obvious fix is to order a new PCB from Lionel Parts. It's available, but would run $50 with shipping and taxes (after already having spent $50 on the new motor 
). So I thought I'd try a longshot to see if perhaps someone has any previous experience with a problem like this, and has additional insight. I could just power the whole loader on and off to control the mechanism, but then I lose the lighted building and the flashing marker lights. I also may try unsoldering the problematic pin, just to confirm that it is stuck low in the IC even when nothing is connected, but my soldering tools and skills may not be up to working on something that small. If it's really the IC that's bad, ordering a new PCB is probably my only choice.
Thanks for any additional insight,
Michael
