Interlock circuit on WPC pinball machines

Somewhere during the production run of the Twilight Zone pinball machine, Bally/Williams introduced an 'interlock circuit' to their pinball machines. It consists of extra wires that plug into the wiring which goes towards the playfield, and a switch near the coin door. When the coindoor is open, solenoids and flashers do not receive power anymore.

You know it's installed on your WPC game when you get this message when you open the coin door:
'COIN DOOR IS OPEN COILS AND FLASHERS ARE DISABLED'.

You can also see it's installed, the difference is that now there are 2 switches left inside the cabinet that the coin door closes. Older games only had one switch.

So what does it do ? Exactly like it says, it disables power physically to the flashers and coils. This is to prevent accidents from happening, as often someone wants to do an adjustment to something like a switch, slips and shorts 60 volt into the switch matrix.. Or you just stick your hand under the playfield without watching and touch the coil lugs, sending 60 volts into your hand..

OK now you understand why it's a good idea to disable this power. The result however is that coils and flashers don't work in the self-tests. To solve this, either push in the top button by hand, or use this little tool:

This yellow, weirdly shaped tool was delivered together with the new pinball machine.

Its task is to slide over the top power switch, so it's pushed in all the time and you can run selftests with power to the flashers and coils.

Note that this circuit needs to be plugged in ! Many operators just unplug it !
If it's loose like in the picture and not connected into the wiring between the transformer and backbox, the power will not be cut when you open the coin door, even though it does say so on the display !!
So always double check if it's plugged in or not and don't trust what the display says.

Note some older games like Stern (like my Orbitor One) and Gottlieb games have a different type of cutoff switch.
They will cut off the power to the game completely (including pcbs) when you open the coin door.
To run tests, make adjustments, .. with the coin door open, you need to pull the switch out for the machine to power up again.