Mercury - Diesel Engine Unswitched Power Harness 10 Ft Constant Power Harness 84-899887T13. The Mercury diesel engine unswitched power harness (10 ft) is designed for diesel installations that require continuous battery power to the engine or related electrical components. Unlike switched harnesses, this circuit remains live at all times, making it essential for systems that must stay powered regardless of engine state. The extended 10-foot length is ideal for boats where batteries or power dist