The Begin Test or Enter button does nothing.

Pressing the coin-door buttons does not enter test mode

Cause: A corroded or broken coin door switch, a break in the single black ground wire shared by all four coin door switches, or connector J205 (the lower of the pair at the far bottom left of the CPU board) unplugged.

Checks

• Short the two wires of the switch together briefly with a small screwdriver: if test mode starts, the switch is bad.
• Check the common black ground wire.
• Check J205 is connected.

Fix

• Replace the switch, repair the ground wire, or reconnect J205.

Systems

• WPC Alpha-Numeric
• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Game will not enter test mode
• A dedicated (direct) switch does not work
• Game accepts no coins and will not start
• Many switch errors and odd behaviour after fitting a color DMD: check grounds
• Switch rows 1-3 show shorted to ground: opto board missing its column strobe
• One switch also triggers another: corroded switch connector on the CPU board
• A whole switch column stops working: replace the ULN2803 column driver at U20
• Phantom switch closure that survives a new CPU board: bad 4N25 on the driver board

Source: Repairing Williams/Bally WPC Pinball Games from 1990 to 1999, Part Two, Clay Harrell (pinrepair.com)