Troubleshooting Tachometers

Age and electrical problems caused by vibration, moisture and corrosion all take a toll on a tachometer’s function and reliability. Here’s how to verify your tachometer’s accuracy and troubleshoot a non-functioning tachometer.

— Story and photos by Steve Auger

Ensure tachometer switch settings match your engine configuration.

An electronic engine tachometer measures your engine’s crankshaft speed in revolutions per minute (rpm). As the engine rpm increases, power and fuel consumption rise along with boat speed. Engine longevity, boat speed and fuel economy depend on operating the engine at the most advantageous rpm for the prevailing conditions of its duty cycle.

When correlated with a speedometer, a tachometer also helps you recognize engine or hull performance problems. For instance, if under normal conditions your boat makes 20 mph at 3,500 rpm and then, on your next trip you had values outside that range, such as only 15 mph at 4,000 rpm, you can presume you have a problem.

Verify that you have the required operating voltage.

Standard analog tach Modern digital tach Smartcraft digital-driven tachometer with analog readout Tach with both analog and digital displays

Basic Set up
An electronic tachometer is simply a pulse counter. When the tachometer on an eight cylinder inboard engine receives four pulses from the ignition coil, it registers one rpm on the tachometer. On a six cylinder, it’s three pulses, a four cylinder, two pulses and so on. On an outboard, the tachometer counts the pulses that come from the charging stator. Because different outboards also have different numbers of bobbins on the stator, there are different switch settings for different engine applications. The tachometer signal is transmitted from the engine to the instrument at the helm panel via the gray colored wire in the instrument harness on inboards and the remote control harness on most outboards. The power supply to the tachometer comes from the “I” (ignition) terminal of the key switch and should have over 12 volts with the key in the on position. (ABYC guidelines for wiring states this wire should be purple). The ground for the tachometer is battery negative and is usually a black wire that connects to the instrument harness. Use the tachometer ground wire as the negative connection when making your voltage checks. Power supply for the tachometer can be checked with a standard volt-ohm-amp (VOA) multimeter. Verify that all electrical connections are clean and tight. In short, there are three wires that have to be connected correctly and a switch must be set in the correct location.

Tach Types
Electronic tachometers used on inboard and outboard engines for the past 40 years have traditionally been analog-type gauges with readings seen as a needle position on a scale. These days, the computer driven, digital tach has become popular. Instead of using the signal from an ignition coil or alternator, a crankshaft position sensor sends a digital signal to the engine management computer, which provides a digital display of the computer’s rpm information. The problem with a digital tachometer for old-school boaters like me is that, when idling, the display number never sits at exactly one number, say 600. Instead, it continually scrolls up and down as an obsessively accurate readout is displayed. I prefer the analog needle and gauge type of readout that sits steady at 600 rpm. Various tachometer manufacturers offer something for everyone, which allows the use of analog or digital computer-driven display and digital-driven analog displays or a combination of both.

Most modern engines have a shop tachometer lead.

How Accurate Is Your Tach?
Tachometers are typically designed as reference type instruments. By this I mean that, at 3,500 rpm, my boat will typically perform the same on each outing. Any change in that status is a clue to a problem with the boat. The tach does not have to be extremely accurate, which is another reason why an analog type instrument is adequate to the task. To find out if your tachometer is providing an accurate readout, you’ll need access to a digital shop tach. This is the same shop tach the professionals use, though some modern VOA multimeters have this shop tach feature built-in and most modern marine engines have a gray-colored test lead on the engine wiring harness that allows a direct connection with the shop tachometer.

Caution: Safety First
Electrical test procedures are provided in detail in your engine’s service manual. Comprehending the wiring diagrams and carefully following these test procedures are essential to conducting these tests safely. The instructions contained within this article have been generalized to apply to all marine outboard and inboard products. Your personal safety requires that you always refer to your original equipment manufacturer’s (OEM) service manual for the special precautions applicable to your engine. Energizing an incorrectly wired electrical circuit can cause damage to the electrical systems of the engine and/or boat and can be a fire and/or explosion hazard.

Verifying onboard tach with a shop tach. This particular instrument is a Mercury Marine combined DMT 2000 digital multimeter and tachometer.

Shop Tach Settings
Before continuing, set the shop tach for the correct number of cylinders and the correct frequency cycle setting. On four-cycle carbureted engines, the red lead goes to the shop tach test lead on the engine, if so equipped, or to the negative terminal of the ignition coil. The black lead from the shop tach goes to engine ground. Start the engine, turn on the meter, advance the throttle to 1,000 rpm on the shop tach and compare the readout to your boat’s tach. It would be considered acceptable if the display on the boat tach was within 10% of the readout on the shop tach display. This means that if the shop tachometer is displaying 1,000 rpm, your boat tachometer could show between 950 to 1,050 rpm and still be considered accurate. If the display is outside of this range, verify that all your tachometer electrical connections are clean and tight and the switches on the boat tach are correctly set. If the boat tach is set as a six cylinder and it’s connected to an eight-cylinder engine, you’ll get an inaccurate readout.
Most two-cycle carbureted engines use the charging system to supply the tach with an engine speed signal. Modern carbureted and electronic fuel-injected outboards also have a gray tachometer test lead wired the same way for testing. If there is no signal, the outboard’s charging system (carbureted models) needs checking. Modern EFI outboards use the engine computer to supply the tachometer signal.

Shop tach settings differ depending on what engine you're testing. Switches must be set specifically for each engine type and number of cylinders for the shop tach to function correctly. These instructions are generalized. Refer to your shop tachometer instructions for correct tool switch settings.

Inductive Clamp Usage
Some engines don’t allow for the use of the frequency function and it’s in these cases only that you would use an inductive clamp that measures engine rpm off the number one spark plug lead. With the engine off, set the shop tachometer to the inductive clamp tachometer position and install the clamp over number one spark plug lead and plug lead into the shop tach. Ensure the shop tach is correctly set on two or four cycle and the correct number of cylinders.

EFI engines require a scan tool to verify tach accuracy.

EFI Scan Tool
On electronic fuel-injected engines (EFI), a scan tool can also be used for tachometer accuracy tests. Start the engine, advance the throttle to 1,000 rpm and compare the reading on the shop tach to your boat tach readout. As with carbureted engines, a 10% difference is acceptable. Any higher and you’ll need to check electrical connections and switch settings. If your tachometer is outside of the acceptable range, check the gray wire with your VOA meter for a high resistance between the tach and the engine. There should be less than 1.0 OHM resistance.