A tachometer is one of the most important instruments on your helm panel. It tells you what the Revolutions Per Minute (RPMs) of your engine are at any speed. You need to know your RPMs in order to judge the accuracy of your engine's performance, to determine whether you are overstressing your motor, or to detect other mechanical problems on the horizon. Fortunately, replacing or repairing a tachometer is not difficult if you have even a modest amount of mechanical ability and a few common tools. Catching problems early by watching your engine's performance is a big step toward making repairs faster and easier, and saving money on your boat maintenance.
Buy the proper service manual for your engine's make and model before doing any engine, electrical, or instrument work. While all boats have relatively similar layouts, wiring schematics and instruments, how your boat reacts to throttle movement, its maximum RPMs and the wiring pattern unique to your boat is explained in intricate detail in its service manual.
Use another boat's working tachometer or a new tachometer instrument wired in place to compare performance to the old unit while your boat is in motion. This is a great way to tell if the old tachometer is actually damaged, broken completely, or working fine.
Use a Philips head screwdriver to remove the screws from the helm panel and lift the cover off the instrument panel. Unscrew the tachometer from its bracket and examine it for wire connection breaks or other signs of damage such as fraying, cracks or burn marks. Replace the unit if damage is present or if it doesn't work at all.
Insert a Philips head screwdriver into the setting screw at the centre of the back of the tachometer if the setting is present and the tachometer isn't accurate (but otherwise works fine and no other signs of damage exist). With the boat hooked up to the alternate tachometer, turn the adjustment screw until both units agree with each other.
Check the wiring from the tachometer with a volt-ohm meter. Wire the meter to ground, connect the boat's battery, turn on the engine and touch the purple wire on the tachometer to the positive prob. Using the volt-ohm meter tester is easy. If the charge is appropriate, the light will come on. If the light does not come on, check the wires for breaks, corrosion or connection at the battery end. Replace any problem wiring.
Check the power circuit board for adequate power to the station. If all else fails, check the battery switch at the back of the boat (if you have a dual battery system).
Know what is "close enough" when testing a tachometer. An older unit may read as much as 500 RPMs different than a brand new tachometer. This is acceptable as long as the old tachometer is within a 500 RPM range up or down, is steady, and moves up or down at the same time in response to acceleration and deceleration. Testing and comparison between tachometers is done at high speeds on open water. All tachometers are more accurate at higher speeds and lose accuracy at lower speeds.
To avoid shock, disconnect all boat batteries before working on any electrical wiring.