Mercury - Diesel Engine Switched Power Harness 3 Ft Power Harness 8M0150521. The Mercury diesel engine switched power harness is designed for diesel installations where a clean, compact power connection is required. This harness supplies engine-switched power, ensuring connected components receive voltage only when the engine is running. It helps protect onboard electronics from unnecessary power draw when the engine is off.. The 3-foot length is ideal for tight engine compartments and installat