After the head is put back on and the harnesses reconnected, the MPU stops at 3 flashes, even with a known good MPU. It boots with only some MPU plugs connected, and in a game without lamps or displays, hitting a slingshot fires the knocker and coil lockout.

MPU gives 3 flashes after reassembly: two 15-pin plugs swapped

Cause: The plug meant for the auxiliary lamp board had been fitted to J5 on the solenoid driver board. Both are 15-pin .156 connectors keyed at pin 6 and both reach, so it fits without force. The miswiring effectively shorted the 5 V and upset the MPU's start-up test.

Checks

• Check each backbox plug against the manual's wire colours and pin list, not just by whether it fits.
• Unplug solenoid driver board J5 and see whether the MPU then boots normally.

Fix

• Put the auxiliary lamp board plug and the solenoid driver board J5 plug on their correct headers.
• Label the plugs before taking the head off in future.

Systems

• Bally MPU AS-2518-35

Games

• Black Pyramid

Related tips

• Pop bumpers, slings and post weak, flippers fine: bad coil ground return plug
• Coils go weak or the wrong coil fires mid-game: reseat the solenoid driver plugs
• Three flashes and 8s or 9s in the hundreds digit
• Knocker fires instead of the outhole kicker: bad solenoid driver J4 connector
• Flickering displays and flipper drop-outs from a poor backbox ground
• Drop targets score but no longer light features: cracked switch matrix connector
• Rectifier board J3 (to backbox boards) pins
• 12 V at the rectifier board but not at the MPU

Source: Pinball Repair help group!! (Facebook group)