Mercury - Diesel Engine Unswitched Power Harness 25 Foot Constant Power Harness 84-8M0150524. The Mercury Diesel Engine Unswitched Power Harness (25 ft) is designed for large diesel vessels where batteries or power distribution points are mounted far from the engine. It delivers continuous, constant battery power without relying on ignition-switched circuits, ensuring critical engine and support systems remain energized at all times.. The 25-foot length supports extended routing through large en