Only three LED flashes, sometimes with an 8 or 9 showing in the hundreds place of the score displays.

Three flashes and 8s or 9s in the hundreds digit

Cause: The MPU J1 and lamp driver J1 plugs swapped; they are about 4 inches apart and keyed alike.

Checks

• Follow both J1 cables to confirm which board each belongs on.

Fix

• Swap the plugs back.

Systems

• Bally MPU AS-2518-17
• Bally MPU AS-2518-35
• Stern MPU-100
• Stern MPU-200

Related tips

• MPU gives 3 flashes after reassembly: two 15-pin plugs swapped
• Flickering displays and flipper drop-outs from a poor backbox ground
• Drop targets score but no longer light features: cracked switch matrix connector
• 12 V at the rectifier board but not at the MPU
• Rectifier board J3 (to backbox boards) pins
• Knocker fires instead of the outhole kicker: bad solenoid driver J4 connector
• Game starts but won't fire any solenoid except flippers: repin J4
• All digits on all displays show 2s or 7s

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