TO UNDERSTAND WHAT’S HAPPENING inside your boat’s electrical systems, the single best tool is the digital multimeter, sometimes called a DVOM (digital volt-ohm meter). Available for as little as $20, a DVOM reveals three main things about an electrical circuit:
the electrical potential (voltage), the electrical current (amperage),
and the electrical resistance (ohms). Voltage is like water pressure in
a hose. Current is the rate of flow — like, say, water passing through
a nozzle. And resistance is like a crimp in the hose, or even the hose
itself: the force that restricts flow. What’s a circuit? It’s the chain of six components
that powers a device: a power source (battery or distribution panel), a conductor
(wire), a switch, circuit protection (a fuse or circuit breaker), the device, and the
return path to ground (another wire). If the VHF works only intermittently or not
at all, chances are one of those six components is letting you down. The DVOM
will let you sort out what’s at fault. Here’s how to go about it.
OPEN-CIRCUIT VOLTAGE TEST
The first step is to determine if the boat’s power source is good, and exactly
how much voltage it can supply, using the open-circuit voltage test to determine the battery’s state of charge. A reading of 12. 6 volts or more indicates a
full charge; 12. 3 volts, a 75-percent charge; 12. 2 volts, a 50-percent charge;
12.0 volts, a 25-percent charge. Readings below 11. 7 volts indicate a discharged battery. Follow the steps below.
1. Turn the DVOM selector switch to DC volts (scale for less than 20 volts).
2. Turn all circuits on the boat OFF (battery selector switch to OFF).
3. Engine and charging system OFF.
4. Touch and hold the DVOM black lead (COM port on the DVOM) to the
battery’s negative terminal, and the red lead (DC volts port on DVOM) to the
You should get a voltage reading that indicates your battery’s state of charge.
Write it down. If it’s above 12 volts, continue to the next test. If not, charge
THE VOLTAGE-DROP TEST
Voltage drop is the inevitable loss of electrical potential through a circuit. The
American Boat and Yacht Council (ABYC) allows for a three-percent voltage
drop in critical circuits (nav lights, VHF radios), and a 10-percent voltage drop
in all noncritical circuits. A higher voltage drop indicates a problem — faulty
connections, corroded terminals, undersized wiring — that must be corrected.
You should’ve noted the battery voltage in the first test. Now you need to
FIVE WAYS TO USE A DIGITAL MULTIMETER
What do you do when your VHF or live-well pump is dead?
Start sleuthing, with the help of a multimeter
PRACTICAL DO IT YOURSELF
BOATER ENGINE TROUBLESHOOTING BY TIM MURPHY
A DVOM uses two
leads: a black lead
and a red lead. Plug
your black lead into
the device’s “COM”
port, or common terminal. Similar to a negative terminal, this is
the reference lead. Plug
the red lead into the
port that corresponds
with the setting you’ve
chosen on the DVOM
selector switch. On
some DVOMs, these are
combined in a single
port; other devices provide separate ports for
amps, volts, and ohms.