Mercury - Tachometer Harness Engine Gauge Wiring 84-69108A5. OverviewThis four-pin tachometer harness ties your tach into older Mercury control and wiring setups without any guesswork. It's built for the classic four-pin plug used across several legacy Mercury harness families. Out on the water, it keeps the tach signal clean so the gauge reads steady instead of wandering. It's a simple, reliable way to keep older boats wired the way Mercury intended.. Key Features • Four-Pin Connector - Matches