A different coil fires from the one expected.

The wrong coil fires (check the ROMs and the expander)

Cause: Wrong game ROMs fitted, or on games with a solenoid expander board, the expander relay not switching.

Checks

• Confirm the MPU ROMs match the game.
• On SEB games, check the expander relay in lamp test.

Fix

• Fit the correct ROMs or repair the SEB.

Systems

• Bally MPU AS-2518-35

Related tips

• No playfield coils work
• Use the solenoid test to find a coil that does not fire
• Check for power at a coil and fire it by hand
• How the solenoid expander board doubles up coils
• Test the coils behind the solenoid expander
• No coils work at all
• Solenoid expander relay buzzes or does not switch
• Drop target coils misfire after LEDs or a new lamp driver: expander boards need bulbs

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