Mercury - SmartTow Engine-to-Helm Harness 20-Foot Connection Cable 84-8M0101642. Overview. SmartTow and Cruise only work as well as the connection between the engine and the helm, and this harness is the link that makes the whole system communicate. The Mercury 84-8M0101642 Engine-to-Helm Harness provides the clean, reliable pathway needed for SmartTow functions on mechanical-control MerCruiser gas engines. Once installed, the helm kit and engine stay in sync, giving the driver smooth pull-ups,