Switches register wrongly even though the playfield wiring is good.

Switch faults from corrosion under MPU J2 or J3

Cause: Battery corrosion under the J2 switch header, or corrosion shorting J3 pins 2 and 3 (which carry strobes shared with J2).

Checks

• Inspect the board under J2 and J3 for corrosion and check those pins for shorts.

Fix

• Repair the corrosion damage and replace the headers.

Systems

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

Related tips

• Corrosion under MPU J4 can destroy every chip
• Repairing battery corrosion on the MPU
• Repair battery corrosion before chasing MPU flash codes
• Knocker fires 'randomly' in play: no memory battery, so every game sets replays
• U8 (5101) connections for ringing out corrosion damage
• Cut the original MPU battery off right away
• Bally-35 MPU: remove the on-board rechargeable battery before it leaks
• Reset section parts typically ruined by battery corrosion

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