Closing a switch shows nothing on the display.

A switch does not register in the switch test

Cause: Misadjusted actuator, switch stuck closed, loose wire, bad switch, broken row/column wire, open switch diode, or dirty or badly gapped blade contacts.

Checks

• Check the microswitch actuator arm, and make sure the contacts are not jammed shut.
• Check the wires are soldered firmly.
• Short across the switch with a test lead and watch the display.
• Check wire continuity to another working switch on the same row or column.
• Check the switch diode.
• On a blade switch, check the gap and that the contacts meet.

Fix

• Adjust, clean, resolder or replace the switch or diode as found.

Systems

• Williams System 11
• Williams System 11A
• Williams System 11B
• Williams System 11C
• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Two or more unrelated switches trigger together
• One switch does not work
• One playfield switch does not register
• "Check Switch #" message at start-up
• Reading WPC switch numbers and the switch test display
• Displays show only 0s and Xs at power-on
• How wiring faults show up in switch test
• One switch also triggers another: corroded switch connector on the CPU board

Source: Pinball Machines: How They Work & Troubleshooting, Norbert Snicer