Mercury - Diesel Vessel Sensor Extension Harness 20 Foot Sensor Wiring Extension 84-899887T17. The Mercury Diesel Vessel Sensor Extension Harness (20 Foot) is designed for Mercury diesel installations where additional distance is required between vessel sensors and the primary sensor harness. It provides a factory-correct signal extension without cutting, splicing, or modifying existing wiring.. The 20-foot length is ideal for larger hulls, wide helm layouts, and complex routing paths where stan