When equipment fails, boat owners often say they don’t know anything about electricity. They won’t touch any of the wiring. This is unfortunate because a majority of the time, faults are caused by loose connections, corroded wiring or blown fuses—all things that are simple to troubleshoot.

The trick is being able to “see” why the electricity is not flowing. You have to know where to look. Adding a multimeter to an onboard toolkit is key to understanding what’s happening.

A Look at Electrons

Electricity requires a source of power. This source can be a battery charged with direct-current energy from an alternator, solar panel or charger; or the source can be alternating current from a generator or inverter.

DC energy, unless it’s intended for propulsion, is generally low voltage: less than 24 volts. If it’s less than 50 volts, DC is considered non-lethal, although it can cause welding-level sparks and heat if wires are shorted (if a bare positive wire touches a bare negative wire or other grounded component).

AC, on the other hand, is high voltage, at 120 or 240 volts. The threat of electrocution is much more present, in addition to the danger of sparks and heat from exposed or faulty circuits.

A multimeter is important, first and foremost, to confirm that AC wiring you are working on is not live, even after you have unplugged or turned off switches. You may be reaching into areas with live AC buses and connections. Test leads are insulated so you can hold the plastic part while the metal part is safely touched to the terminals or wire ends—but obviously, you don’t want your hand to touch anything metal. In addition to using a multimeter, take off rings, bracelets and necklaces, and roll up loose sleeves. Get in the habit of not touching anything metal in an AC wiring system without first checking to see if the circuit is live.

Electricity Revealed

A digital volt ohm multimeter comprises several pieces of test equipment. Multimeters can check for AC and DC voltage, as well as millivoltage, AC and DC amperage, and resistance. They can also help with diode testing, AC frequency testing and AC capacitance testing. Specialized clamp meters can show higher amperage without disconnecting the wiring to make the current flow through the meter.

Good electricians and boatbuilders will set up a boat so that it meets European compliance standards, or American Boat and Yacht Council standards. This means most wiring should be neat, color coded and well secured.

However, as boats age and people with a variety of skill levels work on them, there may be a mix of colors, including several on the same run and all butt-
connected. There also may be poorly run and secured spaghetti wiring that complicates troubleshooting.

DC Voltage and Millivoltage

Checking for this is often the most useful first step. To begin, make sure the test leads are plugged into the correct sockets. Many multimeters have various sockets for checking amperage or voltage/resistance. These require you to move the red test lead into a separate socket for the different functions. For a voltage test, you want the red test lead in the voltage/resistance socket, and the black test lead in the common (ground) socket.

Next, set the dial to the correct test. You must set the range of the test to read old-style analog voltmeters with a needle dial. As an example, if you are checking for good battery voltage on a 12-volt DC system, the meter would be set to 0-20 volts. Most digital multimeters are auto-ranging, which means they will figure out what range to use based on the reading they see.

This can be confusing if the multimeter reads millivolts (a thousandth of a volt). In this case, the display will show an mV symbol instead of a V symbol. Millivolt readings are useful if you are looking for parasitic draws on your DC system, but you want volts if you’re testing for faulty circuits.

For any DC voltage test, touch one probe to the metal positive terminal or wire end, and touch the other probe to the negative, which should be black or yellow. You can perform this test at any point in the circuit between positive and negative, and it will show how much of the battery voltage is reaching the device.

Some multimeters use symbols to differentiate between DC and AC voltage. The DC symbol is an equal sign with the bottom bar broken into three dashes, symbolizing the direct flow of current. The AC symbol uses a bar with a squiggle under it, showing the sine curve of alternating current.

Resistance

Resistance in the circuit causes a voltage drop. An example is when you start out with 13.3 volts at your battery, but you end up with 11.6 volts at the freshwater pump. Devices run poorly when they don’t have enough voltage. The ABYC recommends that most devices experience less than a 10 percent drop, and that safety-related devices such as bilge pumps and navigation lights have no more than a 3 percent drop.

Voltage drop is usually caused by resistance from wiring that is too small, too long or too cruddy, or from poor connections. Use your multimeter to find the voltage at the battery, and then test for it again at the device. If you have too large of a voltage drop, then check the voltage at any switches or connections in the circuit. If they aren’t the culprit, you may need to run new, larger or clean wiring.

If there is no voltage at your device, but there is voltage at the battery, then the circuit is likely broken. This is a situation where a continuity check can be useful via the resistance setting (Ω).

Resistance should be measured when the circuit is depowered. The meter will send a small amount of current from one probe to the other, and then measure it. Switch your multimeter to the resistance setting, and then touch the two leads together. If your multimeter has a tone for this check, you’ll hear a beep (when performing the test on a wire, it is referred to as “ringing out the wire”).

When the two leads are connected, resistance is very low, the multimeter reads “0,” and the tone sounds. If you pull out the fuse in the circuit and touch the leads to both sides of the fuse, the multimeter should ring again. If the fuse is blown, the multimeter will read “OL,” which means infinite resistance.

You can perform the same test by checking each end of the wire to see if it is broken. You may need a coil of wire with alligator clips installed on each end, called a jumper wire. The jumper wire will extend the probe wires so they can reach both ends of the boat’s wire. This test can be used to find the other end of wire back at the source. You may need to disconnect each wire individually for the test, but it is a reliable way to identify the correct wire.

You can also use the resistance test to check switches and connections. A properly working switch will have very low or 0 resistance. A higher reading means the faulty switch or connection is blocking some of the current flow.

DC Amperage

Most multimeters, using the DC amperage setting, will let you check to see how much amperage a device is pulling.

Move the red test lead into the amperage socket on the multimeter, and be sure that the device you’re checking will not exceed the multimeter’s amperage rating (which is usually 10 amps). Then, disconnect one of the wires powering the device, or pull the fuse. Place the multimeter in series by touching both sides of the fuse holder with each probe, or by touching the black probe to one end of the disconnected wire and the red probe to the other.

Next, turn on the device and watch for the amperage reading. If the device exceeds 10 amps, it will blow the fuse in your multimeter. To read larger loads, such as checking to see how much amperage an alternator is producing, a clamp ammeter lets you place the clamp around one of wires in the circuit without disconnecting it. Before clamping the wire, zero out the amp clamp for a more accurate reading (there is usually a button for this).

AC Voltage and Millivoltage

You might want to know why the coffeemaker isn’t working. Again, take more caution when troubleshooting AC gremlins because of the danger of electrocution. A low-risk method of determining if there is AC voltage at the outlet is to set the meter to AC voltage, and then place your probes into the two slots of the AC outlet. You should read about 120 volts.

If not, then the shore-power breaker may have tripped, or the inverter may not be on or outputting. The problem may be as simple as a disconnected wire from the back of the outlet.

If your troubleshooting takes you behind the electrical panel, make sure the AC is turned off. Even then, check for voltage with your multimeter before touching anything metal.

In the United States, AC wiring usually follows this color code: black and red are hot; white is neutral; and green is ground. European boats will use brown, blue and green/yellow, respectively.

With a properly working system, you should see voltage on your multimeter between hot and neutral, or between hot and ground, but not between neutral and ground. If the boat is 240 volt, then you should see 240 by touching the probes to the red and black hot conductors.

Better multimeters will utilize true rms (root mean square). This more accurately reads inverter output. Multimeters without true rms will often read a little more than half voltage from inverter output.

AC Amperage

Even though some multimeters can read AC amperage in series, it is usually limited to less than 10 amps. I wouldn’t recommend the test without utilizing insulated alligator clip test leads—and exercising extreme caution. An AC clamp amperage meter is much safer for this test.

AC Frequency

It can be useful while troubleshooting to know what AC cycle (Hz) your generator or inverter is outputting. A generator that is running too slow or too fast may produce the wrong cycle.

In the United States, this reading should be 60 Hz. Some multimeters utilize a primary and secondary function for each switch, and there may be a button (similar to the shift key on a keyboard) that enables the Hz function. This reading can be taken by placing your probes on the AC hot and neutral wire terminations.

Diagnosing and Repairing

It is logical to be cautious around things we don’t understand and can’t see. We’ve all tried to fix something and ended up causing more damage by mistake. Having the ability to figure out when you need to seek help is one of the most important traits in a good boat tech or troubleshooter.

However, we never really learn until we try. It can be extremely satisfying to repair or install something. Just remember that while parts can be replaced, you cannot. A multimeter can help you become a better and safer electrician.

This article was originally published in the May 2024 issue.