Mercury - Diesel Engine Unswitched Power Harness 15 Ft Constant Power Harness 84-8M0150523. The Mercury diesel engine unswitched power harness (15 ft) is designed for diesel installations that require continuous battery power to the engine or supporting electrical systems. This harness supplies always-on power and does not switch off with the engine, making it essential for diesel components that must remain energized at all times. The extended 15-foot length is ideal for larger engine rooms whe