Just to add some info about the IRQ: The pulse is continually generated on the MPU board (by an independent circuit). It should always be pulsing and has nothing to do with switch signals to or from any of the switches. It is simply used by the CPU chip (via software on the ROMs) for internal housekeeping etc.

It also (and still) amazes me to hear of pinball machines being rewired incorrectly, and then being handed off that way to someone else. Like, did they think they actually fixed it when they did that?? grr.

Having said that, hopefully yours is just a matter of correcting the wiring on that switch