A marine tachometer designed for a Johnson outboard lets you know the revolutions per minute, or RPM that the engine is turning. A marine tachometer must be waterproof, and all electrical connections must be waterproof as well. All connections must be made using marine-approved wire no less than 18 AWG. The tachometer cannot be closer than 18 inches from a magnetic compass. Because of the strength of marine radios, some erratic operation may be noticed in the tachometer during transmissions.
Select the location where you will mount the gauge, and drill or cut a hole of the correct size. Ensure that in mounting the gauge it will not interfere with the operation of any of the other gauges or equipment essential to the safe operation of your boat.
Adjust the setting on the back of your gauge to match the number of poles on your alternator. Typically a Johnson outboard alternator has two poles, but check to make sure. An incorrect setting will result in poor operation.
Disconnect the negative terminal of the battery during installation. This will prevent damage to the tachometer and the boat's electrical system.
Connect a red wire from the "BAT" terminal on the back of the tachometer to a power source that is activated by the ignition switch. Consult your boat's wiring diagram if you are unsure which circuits are controlled this way.
Connect a green wire to the tachometer terminal marked "SIG." Connect the other end to the terminal or a wire on the unrectified side of the alternator. Most late-model Johnson outboards have a tachometer hookup wire at the control box, or a tachometer hookup harness can be purchased from your Johnson outboard dealer.
Connect a black wire to the tachometer terminal marked "GND." Connect the other end to a good boat ground. You may use a good engine ground. Consult your boat manual for locations, or connect to the battery negative cable, or a wire originating directly from it.
Connect a white wire from the terminal by the instrument lighting bulb on the tachometer to a wire or terminal in the boat lighting system. This will allow the tachometer to be illuminated when the boat running lights are activated.
Only use a tachometer approved for marine use. Using an automotive-style tachometer will result in inaccurate readings and possible damage. Use only marine-approved wire and connectors.