Switch test reports switch 3 closed although every playfield switch is open, and spinner hits make a bonus count down several times.

Switch 3 shows closed and scoring goes haywire: check the coin switches

Cause: Switch 3 is a coin chute switch in the cabinet switch matrix. The coin switches have no isolation diodes, so one stuck closed upsets other switches in the matrix.

Checks

• In switch test, unplug the coin door connector; if the closed switch clears, the fault is on the coin door.
• Inspect the coin chute switches for bent or stuck blades and for wires pinched in the door hinge.
• If the game has a replacement MPU, clear its memory as its instructions describe.

Fix

• Repair, re-gap or replace the stuck coin switch (the owner disconnected it to clear the fault).

Systems

• Stern MPU-200

Games

• Meteor

Related tips

• One drop target only registers if it is the first one down: reseat and clean the PIAs
• Switch reads closed even with switch connectors unplugged: shorted resistor on the MPU
• One target also scores its neighbour: a stray switch jumper elsewhere
• Test a playfield switch diode
• Spinner hits garble the displays and trigger other switches: post screw touching GI

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