Mercury - Tachometer Harness Engine Gauge Wiring 84-827593A2. OverviewThis bullet-pin tachometer harness is the simple plug-in lead used on many mid-size Mercury and Mariner tiller engines. It ties the tach straight into the engine's signal wire without extra adapters or guesswork. On the water, it helps the tach read clean and steady so you can keep an eye on RPM while running a tiller rig. It's a small part, but it makes a big difference when your gauge depends on a solid signal.. Key Features