Mercury - Troll Control Dash Mounted J Box SmartCraft RPM Control 8M0083438. The Mercury Troll Control Dash Mounted J Box is designed for boaters who need precise engine speed control during slow-speed operation. It is commonly used by anglers and operators who want consistent, repeatable RPM adjustments without constant throttle input. Mounted at the dash, the control provides convenient access while integrating cleanly with SmartCraft-equipped engine systems.. Maintaining stable RPM control at