In play, locking a ball in the right lock makes the trough serve two balls, and closing the middle lock switch fires the right slingshot. Every switch tests correctly one at a time in switch test and the fault cannot be reproduced there.

Locking a ball serves two balls and fires a sling: faulty coin door interlock switch

Cause: The coin door interlock switch (switch 22) was faulty and stayed closed. With it closed, two lock switches in the same matrix rows and columns complete a 'square', so the CPU sees a fourth switch that is not really closed (switch ghosting).

Checks

• Hold the lock switches closed together in switch test and see whether any other switch appears.
• Look for any switch that is always closed and shares a row and a column with the problem switches.
• Unplug switch groups one connector at a time (most have their own) until the fault stops.

Fix

• Repair or replace the faulty coin door interlock switch so it opens properly.

Systems

• WPC-Security (WPC-S)

Games

• The Shadow

Related tips

• Dedicated (coin door) switches
• Matrix fault remains with a good CPU board
• Sanctum lock magnet throws the ball out instead of locking: adjust the magnet core
• A dedicated (direct) switch does not work
• The Shadow: battlefield misses balls in the centre
• Phantom switch closures: one switch triggers something unrelated
• Fitting a new switch diode the right way round

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