I'm trying to troubleshoot a Bally pop bumper (LT Bumper on the schematic) that won't fire. It works on the diagnostic solenoid test, but will not fire when I close the pop bumper switch or short it manually bypassing the diode. Looking at the schematic showed me that it goes back to the MPU on J2-5 AND J2-15. Disconnecting the J2 plug and shorting those two pins does work to fire the coil. Also everything else in the row and column on the switch matrix seem to work.

I am able to trace one side of the switch back to J2-5 and the other side back to J2-15 using continuity test on the multi-meter. But if I jumper the switch closed, bypassing the diode, I don't seem to get continuity when I test J2-5 and J2-15 on the harness plug with the multi-meter. Does this make sense?
bally_bumper (resized).JPG

Well I took the playfield out so I could trace wires easier. Everything checked out, and then I put it back in and connected everything and it works again. I have no idea what the problem was. I suspect it will be back, but for now it's fine.

Btw, I did also remove the MPU to look at it. I decided not to reflow any of the connector pins because only one pop bumper was not working. All of the other devices that used the same pins as the pop bumper were fine.

The problem with that theory is that 4 or 5 devices all use the same pins (j2-5 and j2-15) to switch on. There was only a problem with one device, so that leads me to believe it is something else.