North American Anti-runon Valve - 1973-on
Hover over a wire to confirm the colour

12v is only connected through the fuse to the valve when the ignition is off. The oil pressure switch only connects an earth to the other side of the valve when the engine is running and generating oil pressure. So with a running engine the valve only operates when the ignition is switched off. When the engine stops and the oil pressure dies away the valve releases again.

The anti-runon valve (A) and oil pressure switch (B) as fitted to Bill Etter's 76