Sorry I missed this ...

Here's what i would do...

With the coin door open:

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Check 1...Remove J205 from the CPU board and use a ground wire to short J205-7 to ground. If the volume goes down this is a wiring problem, if not its a CPU issue, or you didn't ground the right pin...these are numbered right to left.
NOTES:
if you have mini-grabbers (NOT Alligator Clips) ground is available on J205-10. These wires do not go to the coin door switches directly, they go to the coin interface board at J1, and out of the interface board at J3 to the coin door.

Assuming check 1 was good:
Check 2 ... Ground J3-8 on the coin door interface board, you may have to ground this with the connector still connected as removing this cable may cause errors due to SW 24 (always closed) becoming open and you will not be able to use any coin door switches to clear it. If the volume goes down you have a wiring issue between J3 connector and coin door switch, or a bad switch; if volume doesn't change the problem is between Interface board J3 and CPU board at J205-7.

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NOTES:
This information is only as valid as the technical drawings released by Bally.

I normally track problems from where I know its failed and work backwards, you can just as easily check from the switch itself and work forward to the CPU; the main takeaway here is knowing how the circuit is wired, and what you expect to see with each test.

Good Luck.

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