A boat owner running a boat without a tachometer handicaps himself significantly. This small instrument dial, which details the revolutions per minute of the engine's combustion, provides a critical window into the boat engine’s operation. It can provide an early warning to potential engine problems, and it provides an easy-to-understand metric on engine performance. Simple mistakes can goof up a tachometer, however, especially in its installation.
Start the boat engine and test the tachometer and its ability to receive a signal. Run the engine at a known speed in neutral and compare to the tachometer reading. Turn off the engine and prepare to remove the tachometer if the reading is non-existent or clearly not accurate.
Disconnect the boat’s battery from the boat wiring if the boat runs with one, using a crescent wrench to remove power wires from the battery terminals. Use a screwdriver to unscrew the tachometer from the dashboard it is attached to. Gently pull the instrument forward by hand and outward from the dashboard. Carefully pull its wiring out by hand from behind so that the wires are exposed as well.
Examine each wire attached to the tachometer and record its color. Look for any fraying, broken wires or exposed insulation. Patch up any minor damage by cleaning the area and wrapping electrical black tape over exposed wires completely.
Match up the colored wires from the tachometer to the wiring used on your boat. Confirm that each wire is connected to the right boat harness wire via wire connectors. Connect by hand the red wire for power feed to the power red wire on your boat harness. Connect the black tachometer wire to the black ground wire. Connect the yellow wire to your lighting wiring. Connect the remaining wire (usually green) from the tach to the wiring for the engine per your boat’s wiring scheme. Label each wire with masking tape and a permanent marker.
Turn the tachometer over gently so that the backside is exposed. Check that the back of the tachometer has a switch set to 12 (standard on all 12-volt tachometers), matching the boat’s 12 pole system. Check your boat wiring manual to confirm your voltage if your not sure what it is.
Examine each of the connections (plastic male and female connectors at wiring ends) to the tachometer backside, if applicable. Repair any bad or loose connectors identified. Cut the old connector off with wire snippers, strip 1/2 inch of fresh wire with a wire stripper/crimper tool, and crimp on a new connector using the same stripper/crimper tool. Reconnect the repaired wire connector to the tachometer backside.
Connect the boat harness wiring to the tachometer, if it was disconnected in the repair process, and reinsert the connected wires back into the tachometer dashboard hole followed by the tach itself. Seat the wider face rim (the front edge) of the tachometer's exposed side (the part that sits outside the dashboard) against the hole edge and use a screwdriver to secure it tight with wood screws into the dashboard face.
Reconnect the boat battery to the boat wiring system. Turn on the boat and check if the tachometer receives a reading of revolutions per minute from the engine wiring. Give the boat some throttle and confirm the tachometer reads the increase correctly.
Use the tachometer once installed to get a feel for the “full open throttle” level of your boat per your boat engine's manual. Once practiced, you will then know the upper limit of your boat engine to avoid exceeding it, which can result in expensive repairs.
Do not perform electrical work without first disconnecting the boat's battery. Boat batteries are much larger than automotive batteries and can pack a significant jolt to an unsuspecting home mechanic if a metal tool hits a live wire.