Switch 51 (left ramp) always reads closed, even with the switch replaced or disconnected at the switch, and in lamp test some lamps light along with others.

One switch stuck on and lamps lighting in pairs: swapped diode board plugs

Cause: A previous owner had swapped two plugs under the playfield: the switch harness was in the lamp connector on a diode board and the lamp harness in the switch connector.

Checks

• Unplug the under-playfield diode board connectors one at a time while watching switch test.
• Compare the connector wire colours with the manual's wiring chart.

Fix

• Put the switch and lamp connectors back on their correct headers.

Systems

• Gottlieb System 3

Games

• Class of 1812

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