Mercury - SmartTow Engine-to-Helm Harness 25-Foot Connection Cable 84-8M0101643. Overview. SmartTow and Cruise rely on a clean, dependable path between the engine and the helm. The Mercury 84-8M0101643 Engine-to-Helm Harness provides that link for mechanical-control MerCruiser gas engines, giving the helm kit the communication it needs to manage towing profiles and steady-speed operation. With this harness in place, the whole system feels more consistent, and the driver can focus on the rider in