Troubleshooting Electrical Problems An Introduction to the Digital Multimeter

At the Cruisers’ University of the Annapolis Boat Show, I was asked to make a presentation on the use of a digital multi-meter to diagnose electrical issues on boats. During the class, it became clear that many were unfamiliar with the various ways a meter can be utilized in basic troubleshooting so I’m offering an introduction here. A later article will focus on more detailed and advanced troubleshooting.

A digital multimeter, or DMM for short, is a very versatile tool that can be used to measure electrical voltage, current, frequency, resistance, capacitance, connectivity, and more. On a typical cruising boat

We’ll start with an introduction to the basic principles of electricity and then apply them to problems
that tend to creep on most boats.

A Quick Review of Basic Electrical Theory

Two basic laws explain the relationship between voltage or electromotive force (‘e’), current in amps (from Ampere’s courant intensité, ‘i’), power (watts, ‘w’), and resistance, (‘r’). Ohm’s Law tells us:
voltage (e) = current (i) X resistance (r)

This can also be written as one of the following:

e=i*r
i=e/r
r=e/i

If we know a voltage and either the resistance or the current, we can calculate the other value. This is especially important when calculating the permissible voltage drop in a circuit. It’s a chance to use something you might have learned in high school physics class and, if you’re like many people, promptly forgot!

The Power Formula states:

power (w) = voltage (e) X current (i).

Again, if we know that an appliance, like a space heater for example, is rated at 1500 watts and is plugged into a 120-volt outlet that it would draw 12.5 amps of current. When we combine these two formulas with a meter that can calculate the actual values. we are well on our way to diagnosing a device that isn’t operating according to its manufacturer’s specifications.

Measuring Volts, Amps, Ohms, Farads, and More:

to fully charged, so having a good multi-meter that can accurately measure small voltage differences is critical to effective battery management and troubleshooting. In the DC voltage test image below the voltage we are seeing is the resting, no load voltage. That is, there is no load (such as a light bulb or radio) and no charging source (such as a solar panel or alternator). When voltage is used to determine state of charge of a battery, it is important to remember: charging and loads off; we want to measure the battery, not the power going into or out of it as well.

We measure AC voltages in parallel but be careful not to touch the any part of the circuit with your hands, as typical AC voltages on boats can be lethal.

While we measure voltage in PARALLEL, current is measured in SERIES. Low levels of current flow that are less than 10 amps, such as lights and VHF radios, can be measured directly with the meter probes, while larger loads, such as engine starters or air conditioners, require a clamp on style of meter. All DMM’s that can measure the flow of amps (current) in series have a fuse that protects them from over- current. The DMM manual will tell you which fuse to keep as a spare; we’ve all blown one now and then! For measuring heavy currents, you will need either a “clamp on†meter or a probe that connects

We measure resistance and capacitance (ohms and farads) in parallel, just as we do voltage. For example, a capacitor can be tested by setting the meter to the Capacitance Symbol ( -|(- ). The meter reading should match the value printed on the capacitor.

Let’s look at some common troubleshooting

Problem 1: Starter won’t crank. You turn the key or push the start button and you hear nothing. Not a click, not a slow crank, nothing.

• First, use your voltmeter to check the voltage at the starting battery. You should have a built-in meter for that purpose but double check.
• Now check the voltage at the starter motor itself.
• Finally, check the voltage as the engine starter is engaged.
• If you have 12V or so at the battery and not the starter, look for a loose wire, probably the engine ground.
• If you have 10V or less you have a discharged battery, check your charging circuits (and look for a blown fuse in the shore side battery charger).
• If you have 12v at the starter and it drops below 9 when starting, your battery is probably dead.
• If you have a clamp on ammeter you can confirm this by verifying the current going (or, in this case, not going) to the starter motor by clamping onto the wire from the battery to the starter

Problem 2: VHF Radio won’t work but all other electronics do

• Check to confirm that there is no voltage getting to the radio by measuring at the back of the radio.
• Remove the fuse at the radio and, using the continuity function of the meter, confirm that the fuse is blown and replace it. Don’t laugh, it happens to everyone. And for some reason the blown fuse is the last place some people look. Remember, some fuses look good but aren’t; that’s what the meter is for.

Problem 3: Air Conditioner won’t Start

If you are lucky enough to have an AC on your boat (many cruisers find them necessary during summer months on the Chesapeake Bay), you know they can be finicky. While many repairs require a certified AC tech, a common failure is the loss of a start or run capacitor. They are easy to test using your multimeter. Refer to the previous image. Label and remove the wires and test the capacitor; it will either meet the labeled value or not. They can be purchased at hardware and plumbing stores that sell AC parts as well as at any AC supplier.

And many more. The digital multimeter is one of the most versatile tools you will have aboard and can be used to diagnose a huge range of electrical problems. This article is meant to introduce the electrical novice to the meter and its uses. Future articles will focus on advanced troubleshooting and other applications valuable to the cruising boater.

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