Mercury - Tachometer Harness Engine Gauge Wiring 84-86396A8. OverviewThis five-pin tachometer harness gives you a clean, direct connection for systems using Mercury's Commander-style controls. At four feet long, it's sized well for most side-mount layouts without leaving a bunch of excess wire to tame. On the water, it helps keep the tach reading steady instead of bouncing or cutting out. It's a simple, dependable piece meant for boats still running these proven control systems.. Key Features •