A stepper will not advance, reset or step down cleanly, or its lamps and features do not work.

Game will not work: stepper unit binding

Cause: Factory grease has hardened, grease on the coil levers makes them lazy, or the brass rivets and wiper fingers are oxidised.

Checks

• Work each stepper by hand: it should step up crisply and return to home freely.

Fix

• Clean, lightly lubricate and test every stepper (see the stepper cleaning procedure).

Systems

• Electro-mechanical

Related tips

• Rollover advance unit won't go back to 1 on reset: weak return spring
• Gottlieb credit unit will not step up or down
• Stepper will not step reliably: sticky activator arms
• Kinds of stepper unit
• Bonus lights stop advancing and stay on one value: stepper unit plunger sticking
• Cleaning and aligning an EM stepper unit
• Ball count stepper won't advance: pawl arm frozen in old grease
• Double bonus not collecting: bonus stepper mainspring overwound

Source: Repairing Electro-Mechanical (EM) Coin Operated Games to 1978, Part One, Clay Harrell (pinrepair.com)