After a run of resets the game reported switch rows 1, 2 and 3 shorted to ground, though nothing had been changed; the messages went away with the switch connectors unplugged from the CPU board.

Switch rows 1-3 show shorted to ground: opto board missing its column strobe

Cause: The switch column wire that feeds the strobe to an opto driver board was broken further up the playfield chain. Without the strobe, the opto board holds its row lines low, which looks like a row short.

Checks

• Check the unregulated 12V that powers the optos, at the CPU test point and at the opto board; missing opto power also causes row shorts.
• With all switch plugs off the CPU board, use a jumper in switch edge test to prove the board itself reads each row and column correctly.
• Put a logic probe on the column strobe pin at the opto board; it should pulse, not sit steady. Or, with the game off, check continuity from that pin back to the CPU header.

Fix

• Find and repair the broken column wire. Playfield switch wiring is daisy-chained from switch to switch, so one break near the back stops everything after it.

Systems

• WPC Fliptronics

Games

• Bram Stoker's Dracula

Related tips

• Row or column "ground short" errors
• Testing CPU switch rows and columns with a diode and clip lead
• "Ground Short Row" message
• A whole switch row or column is dead
• Testing CPU switch columns with the switch levels test
• Testing CPU switch rows with the switch levels test
• Finding a switch column shorted to a power circuit
• Narrowing a switch matrix fault to the CPU board, wiring or opto boards

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