Coil plungers drag or stick, with black sludge in the sleeve.

Sticky coils from old oil in the sleeves

Cause: Someone oiled the mechanism and the oil has mixed with playfield dust.

Checks

• Pull the plunger and look for a black, tacky film on it and in the sleeve.

Fix

• Replace the coil sleeve, which cannot be cleaned, and clean the plunger.
• Do not re-oil.

Systems

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

Related tips

• How Bally coils are switched
• Check every coil's resistance before first power-up
• Coil diode and wire orientation when replacing a coil
• 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

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