Mercury - Troll Control Y Harness Engine Speed Control Wiring 8M0042976. The Mercury Troll Control Y Harness is designed for connecting a Troll Control system directly into the main engine wiring harness without the use of a separate junction box. It provides a clean, streamlined installation that helps reduce clutter behind the dash and keeps the wiring layout organized. This harness is commonly used when space is limited or when a simplified Troll Control installation is preferred.. Reducing u