Firing one output also fires a coil or flasher in the other bank.

Wrong coils fire together (A/C bank game)

Cause: A shorted bank isolation diode.

Checks

• Test diodes D31-D46 for a short. Later games carry them on the aux board in the backbox; early games have them on a diode board beneath the playfield.

Fix

• Replace the shorted diode.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C

Related tips

• C-bank coil or flasher not working
• A-bank coil not working
• Whole groups of coils or all flashers dead: A/C select relay stuck
• How the System 11 A/C bank select works
• Where the coil diodes are on System 11
• Isolating the 50 V flasher circuits for LED flashers
• Testing coil diodes on the auxiliary power driver board
• LED flashers blink when the flipper buttons are pressed

Source: Pinball Machines: How They Work & Troubleshooting, Norbert Snicer