In coil test several coils fire the wrong device (for example the ramp up selection fires the ramp down coil, or both), the select relay pulses when it should not, backbox flashers and the drop target do nothing.

Wrong coils fire and drop target dead: bad PIAs, shorted coil diode and Q14

Cause: Several faults together: the PIA chips U17 and U18 on the CPU board were bad, the drop target coil's diode was shorted, and the shorted diode had damaged the drive transistor Q14 again after it had already been replaced.

Checks

• With coil test stopped, the A/C select relay should hold steady (off for A coils, on for C coils), not pulse.
• Unplug the playfield coil plugs and check each CPU coil output pulses correctly in test, including the select relay driver.
• Check each coil's diode, and recheck any drive transistor that failed before.
• LED flashers on this system can glow all the time from the flasher preheat resistors; test with incandescent bulbs.

Fix

• Replace U17 and U18, replace the shorted coil diode, then replace Q14 again.

Systems

• Williams System 11

Games

• Road Kings

Related tips

• System 9 and System 11 seven-segment LED codes
• Blanking LED wrong, or all solenoids pulse and blow a fuse at power-on
• Right sling locks on at power-up and a pop is dead: bad 7402 at U45
• Special solenoids dead after fitting an early System 11 CPU in a later game
• What each 6821 PIA on the CPU board controls
• Testing the 6821 PIAs in circuit with a meter
• Checking PIA outputs with the test EPROM running
• Find out how the board died: failures cascade to the PIAs

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