For diagnosing electrical system problems, no tool on board is more useful than a portable multimeter.

electrical system

How to Using a Multimeter

For diagnosing electrical system problems, no tool on board is more useful than a portable multimeter.

Like any tool, however, the multimeter is only of value if it is working properly, is calibrated correctly and you know how to use it. Take yours out of its case occasionally and clean it, replacing the batteries so that it will be operational in an emergency and so that leaking batteries don’t ruin the instrument. Be sure that the wire probes are undamaged. Most of all, ensure that the owner’s manual is in the case so that you can refresh your memory of how to set each meter function up and read the results properly.

Better multimeters have protection circuits built in to some functions, but not all. If you are reading voltage, for instance, set the meter to read 120 volts and then plug it into a 240-volt circuit, the meter will likely shut itself down and give no reading. Readings of amperage are more critical and applying too many amps to most instruments will destroy them.

In fact, of all the potential uses of a multimeter, the one you are least likely to need, and one which will not be covered here, is the amperage function. Many small, portable multimeters will read only small amounts of amperage, sometimes measured in milliamps. This can be useful in diagnosing problems with electronics, but very few of us have the skills for repairing radios or radar units anyway. For electrical systems that carry fairly substantial amperage, the amperage side of most multimeters has very little use.


Follow your owner’s manual, usually the black wire probe is inserted in the socket marked Common (often abbreviated as Com.) and the red wire probe is inserted in the Volts socket. Set the dial on the multimeter to the highest voltage you think the circuit might carry - for 12-volt systems this usually is 20 volts. If you’re not sure what the voltage might be, use the highest setting your instrument will allow and work down once you have a reading. Digital voltmeters are much easier to use and read in this regard.

To read voltage, the black probe must make contact on the ground, or negative, side of the circuit and the red must be touched to a point on the positive side. If you reverse the polarity, some digital multimeters will give the proper voltage reading preceded by a minus sign while others will read zero. If you receive a zero reading, check that the batteries in the meter are good (many have a battery test function), ensure the probes are in the right sockets and that the dial is set to volts, and make sure that you are getting contact on the fittings you are testing. The last thing to check is that you may have the polarity reversed.

When working on 12-volt systems, it is good practice to start by checking the voltage directly across the battery terminals as this tells you three things—one, your multimeter is setup correctly, two that a dead battery isn’t the source of your problem, and third, it gives you the base voltage at the battery.

This measurement also tells you a good deal about the state of the battery. A 12-volt battery measuring 12.1 volts or below is totally discharged and incapable of doing any work. A measurement of 12.9 or 13.0 volts indicates a fully-charged battery in good condition. Above 13.0 volts tells you instantly that there is a charging circuit turned on and operational, most alternators and battery charges have a peak charge rate of between 14.0 and 14.3 volts, depending on their settings. A reading over 14.4 volts shows that the regulator on the charging source is not calibrated correctly or is damaged (unless you are purposefully equalizing wet-cell batteries at a higher voltage). Between the normal readings of 12.1 volts (totally discharged) and 12.9 volts (totally charged) the reading is nearly linear, meaning a reading of 12.5 volts would indicate a half-charged battery, while a reading of 12.7 volts would indicate a three-quarter percent of charge.

When looking for a fault in the wiring system, checking voltage around the circuit is a good way to locate the problem. Once the voltage at the battery has been established, move to the next point in the affected circuit and see if the voltage reads the same. For instance, the light over the nav station will not work and the battery reads 12.8 volts.

All the other lights and appliances on the boat work fine. Moving outward from the battery, the first junction in the circuit is the back side of the nav station circuit breaker which also reads 12.8 volts, so the circuit breaker is fine. The next place to check is the light switch. Here you may want to leave the black (also negative) probe of the meter attached to the negative terminal of the battery with a long jumper wire to get the meter near the light. Again, both sides of the switch read 12.8 volts (assuming the switch is turned on). The next junction in the circuit is the bulb socket - here we get a reading of zero. So you know that the break in the circuit is either between the switch and the socket, or we have a defective socket.

Voltage readings above zero, but well below the battery’s base reading, are also valuable clues when troubleshooting an electrical circuit with a voltmeter. Let’s say that at the front side of our imaginary light switch you got a reading of 12.8 volts, but at the back side you get a reading of 8.9 volts.

Some power is getting through the switch, but not enough to light the bulb - something inside the switch is causing a major resistance to the passage of power and the switch should be replaced.

Continue checking along the circuit until you get all the way back to the battery. Many breaks in circuits occur on the ground (negative) side, especially at the ground buss where all the negative wires in the boat are usually tied together. You can check the negative portion of the circuit by leading your jumper wire from the positive battery terminal to the red probe on your voltmeter, using the black probe to touch the ground connections.

Ohmmeter and Continuity

An ohm is a measure of electrical resistance, or the obstruction to the passage of electrons. Almost everything poses some resistance to electricity, but in practice, a properly sized copper wire in good condition has very little.

Ohmeters confuse many people the first few times they use them because they read “upside-down” in a way. An ohm reading of zero means there are no ohms, thus no resistance - and this is a very good thing in electrical systems. A reading of infinity means there is absolutely no electrical connection between the two items you have your probes on. Less than infinity, the bigger the number on your multimeter’s readout, the more resistance there is to the passage of electricity, and this is a bad thing.

Most ohmmeter portions of a multimeter must be calibrated to zero before they will read accurately. Move the probes to the proper sockets and set the instrument to read ohms (often marked by the Greek Omega symbol, ?).

With one probe in each hand, but not touching, the multimeter’s readout should be infinity or a string of 9999999. When you touch the two probes together, the reading should immediately go to zero, indicating no resistance between the probes. If the gauge reads something above zero, consult your multimeter’s owner’s manual to find out how to adjust the reading until it reaches zero.

The best use of an ohmmeter on board the boat is for what is called testing continuity. You can place one probe on each end of a brand new wire of good quality and the ohmmeter reading should be very close to zero. This means the electrical element inside the wire is continuous, without breaks or areas of high resistance. An old, corroded wire with a few slices and kinks, on the other hand, might show several hundred ohms of resistance. A completely broken wire will read at infinity.

As a general rule on boat wiring, you’re looking for the lowest readings of resistance you can find. Checking along the circuit just as you did with the voltmeter, you can locate a break, or open, in it easily, as your readout will jump to infinity or a very high number. There is no better tool than an ohmmeter for checking the operation of fuses, circuit breakers, switches, and yes, even light bulbs. Simply adjust your ohmmeter to zero and put one probe on each contact of a light bulb or fuse, a reading of infinity means the filament inside is broken. Switches and circuit breakers may show high resistance, in which case they should be replaced, make sure they are in the On position while testing.

With a good quality multimeter, a little practice and the patience of an electrical sleuth, you will find that many electrical problems are easily tracked down and corrected. A simple corroded wire, broken terminal or loose connection won’t stand a chance when you drag your valuable little tool out of its case and go fault hunting.

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