Every boater needs a multimeter!
That’s because today’s boats are stocked like a RadioShack, with a huge variety of electronics. All of it looks complicated, but with a good multimeter and careful study of the accompanying manual, as a refresher in basic electricity, it is possible to maintain most electronics. The multimeter (Figure 1) qualifies for a spot in any tool kit for tracing equipment failure back to its source. Depending on the brand and price, multimeters, which are also known as volt-meters, and Fluke meters, have a range of usefulness that we’ll categorize into three main groups:
This discussion focuses on digital multimeters, however, there are helpful links at the end of the article. These links will take you to Fluke, Simpson Electric Company (a source for more on analog meters), and other manufacturers of good equipment. Just reference Figure 2 for specifics as you dive into this important subject. We’ll begin with a word on multimeters in general.
First, note that there are many brands and configurations of meters available. We will reference the Fluke 78 (Figure 2) shown below, which, by the way, is one of the meters used by our techs every day. No matter which meter is used be sure to keep the operating manual nearby for quick reference.
Mode switch-This switch turns the meter off and on and also selects the kind of testing desired. Pressing the mode button in the center of the switch knob shifts operation to the alternate function for the mode selected.
Digital display-Provides not only numerical readings but also has icons that appear as different modes are selected. We’re all getting used to touch screens, but these are not touch screens. They can be damaged by sharp objects or even by pressing on the screen too hard.
Plugging in the test leads-For this meter, most tests are done with the leads plugged in on the right side of the meter as shown above. The exceptions are “Amps”, which use both the bottom right “common” jack and the bottom left “Amps” receptacle. Note: standard leads can test up to 10 amps, while amp-clamp leads are used above this level.
A.C. and D.C. Volts-Simply stated, “Is it electrically hot?” This is the most important thing to know when working with electrical items. Use caution here: According to MER Service Manager Herb Knight, 120 Volts Alternating Current (A.C.) kills more people every year than all other voltage levels combined. Beyond self-preservation, the multimeter will help determine if an electrical component or circuit has the voltage it is supposed to have.
The Fluke 78 has the A.C. Volts/Frequency together as shown above. Switching to this mode enables the testing of A.C. voltage until the alternate function key is pressed. When pressed, the meter displays a readout of the frequency of the A.C. power being tested. The units of measurement for frequency are in “Hertz” which is abbreviated as Hz. Alternating Current is found in generators, utility power and some engine sensing and control circuits. A measurement of alternating current, this test mode checks for the number of cycles per second (frequency). Household power in the United States is 60Hz, while many other areas use 50Hz power.
D.C. Volts/Frequency is for testing direct current (D.C.) voltage, or alternately D.C. frequency. D.C. frequency tests are done on specialized control systems and certain D.C. motors. DC voltage readings are taken on engine starting and charging systems, as well as some control circuits.
However, if no voltage is present it is time to find out why. That’s where resistance testing comes into play.
Resistance/Continuity Beeper-In layman’s terms, “Does the juice have a clear path to travel?” One of the most common marine electrical failures results from corroded electrical connections, whether at splices, or terminal connections. Resistance testing measures electrical resistance in Ohms, and alternately the meter’s beeper will sound when there is continuity in a system. A zero reading (0.00) means the circuit is open. Readings of 0.1-.3 ohms can indicate either a short or complete circuit. Like the diode check function, the continuity circuit sends current through the leads and the horn will beep if there is a complete circuit.
D.C. Amperes/A.C. Amps-Very low ampere readings (less than 10 amps) may be done with both direct and alternating current using the standard leads, however, read the operators manual to learn when to use “amp clamp” style leads with a clamp-on end on the red test lead. The unit of measurement for current flow is the Ampere (amps).
Diode Check/Temperature-Diodes are tested in this mode. The meter applies a small voltage through the leads to check the direction of flow through a diode. A reading of 0.3-.5 Volts in only one direction means the diode is good. Alternately, when fitted with thermocouple leads this meter will measure temperature.
RPM/Speed-is a tachometer function for measuring rotational speed.
Frequency-This function is bundled with the voltage on the meter we’re showing above.
Duty Cycle/Percent-is used with position sensors. Engine control systems use current with pulse width modulation, and this meter function measures the percent of time “on”.
Temperature/Diode Check-When fitted with thermocouple leads this meter will measure temperature. Alternately, diodes may be tested in this mode. In this mode, the meter applies a small voltage through the leads to check the direction of flow through a diode. A reading of 0.3-.5 Volts in only one direction means the diode is good.
“RPM” testing-is done with special leads and uses both of the left-hand receptacles.
Auto Mode-Most meters are in auto-mode by default, and this enables near instant range changes for measuring widely varying voltage. Consult your manual to learn how and when to use manual mode.