No switches show stuck, but closing switches in one column shows the wrong switch numbers.

Switches register with the wrong numbers

Cause: Strobe or return lines shorted together in the playfield wiring (in the reported case, two strobes touching under a pop bumper bracket).

Checks

• Unplug MPU J2 and J3 and jumper each strobe to each return in turn. Correct numbers mean the MPU is good.
• With every switch open, check continuity at J2/J3 between every strobe and return line, and from each to ground. Any continuity is a short.

Fix

• Find and separate the shorted wires.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35

Related tips

• One switch or a string of switches never registers
• Clear stuck switches using switch test
• Closing one switch kills the whole column: slam tilt switch touching the coin door
• Switch reads stuck closed, or a pop bumper chatters
• Tilt doesn't work and closing it fires a pop bumper: dirty slam tilt switch
• Random game faults when a switch without a diode sticks
• Player 1 display shows credits and ball in play
• Pop bumper and both outkicker switches dead: bare wire touching the coin door

Source: Repairing Bally Electronic Pinball Games from 1977 to 1985, Part Three, Clay Harrell (pinrepair.com)