The VUK under the skull stops firing. There is about 75 V on one coil lug but none on the other.

Skull VUK stops working, voltage on only one coil lug: open coil winding

Cause: The coil winding had broken open. In circuit it read about 2 ohms through other paths, but with its wires lifted it read open.

Checks

• Measure each coil lug to ground with the coin door closed: voltage on one lug and none on the other means the coil is open.
• Lift the wires (and one diode leg) before measuring the coil's resistance, or other circuits give a false reading.
• Note the ground side at the driver connector should not show voltage when the coil is idle.

Fix

• Replace the coil.

Systems

• WPC Dot Matrix

Games

• Terminator 2: Judgment Day

Related tips

• Launch coil locks on and blows its transistor and fuse every time it is tested
• No 50 V coils work
• Firing each coil from the WPC solenoid test
• Firing a coil by grounding it with a clip lead
• Two or more coils fire together

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