Many boaters own one, or should, but few know how to use it. In fact, some are afraid they'll blow their expensive electronics. We're talking about a digital volt-ohm meter (DVOM), and the truth is you're more likely to damage the meter than your boat. It's time to become the master of your meter. Here's how to perform three useful troubleshooting procedures.
Master Your Meter
Your meter has two leads, one black and one red. The black lead is for the ground, or negative, connection, both to the meter and to the point in the circuit you're checking. Plug it into the COM socket on your meter. The red lead is the positive, or "hot," lead.At the meter end, plug it into the socket labeled V-O, which stands for volts (AC or DC) and the Greek symbol omega, which denotes ohms, for resistance. (This same socket may also have functions like a continuity beeper or a diode test function indicated by a triangle pointing at a T symbol.) Connect the red business end of the lead to points in the circuit to measure voltage. It's important to remember that when you're checking diodes, continuity, or ohms of resistance, the circuit must be turned off. Any current flowing in the circuit could blow the meter's internal fuse. After plugging in, look at the rotary dial on the face of the meter to select which measurement you want to make. AC volts (your shorepower) are indicated by a V with a wavy line above it. DC volts (your battery and alternator output) are indicated by a V with a straight and dotted line above it. To measure ohms, look for the omega sign. Continuity is most often indicated by a series of graduated length lines to indicate the beeper. Diode check will show the diode electrical symbol described above. Some meters can measure AC frequency, indicated by the Hz symbol. Once your leads are set to go, and you've selected the right measurement, try these simple troubleshooting tests.
With your meter set to DC, connect the red lead to the positive terminal on the battery, the black lead to the negative. If the battery's just come off charge from the engine running or the boat's shorepowered charger, crank the engine a few times to remove the surface charge. Measure the open circuit voltage in the battery to get its state of charge, which is: 12.6 volts or more = 100 percent charged; 12.3 volts = 75 percent charged; 12.2 volts = 50 percent charged; 12.0 volts = 25 percent charged; and 11.7 volts = you're in trouble, the battery is dead. Alternator Health
Check the open circuit voltage as described before and write down the reading. Next, with DC electrical loads turned off at your main DC panel, start your engine and rev it up to about 50 percent of normal operating rpm. If the new reading is somewhere between 1 and 2.5 volts more than the open circuit reading, your alternator is working. If it's more than 2.5 volts greater, then your alternator's regulator is faulty and your batteries are getting cooked. It's overhaul time. If there's no increase, then the alternator isn't charging. Next, keeping the engine at the same rpm, turn on all the DC loads at the main panel, and watch your voltmeter. With all the loads on, you should be reading a minimum of 0.5 volts greater than the initial open circuit reading. If not, your alternator is too small for your boat. Time for a high-output one.
Fishfinders are especially susceptible to low voltage and will act up if they're not getting enough juice. To test this, unplug the power supply from the back of the display and identify the DC + and - terminals. Your owner's manual should give this information. With your meter set to DC volts, insert your red lead into the DC + terminal and the black lead into the negative terminal. Turn on the power supply to the fishfinder and take a direct reading. If it's below about 11.7 volts (your owner's manual will give you a voltage operating range in the tech spec section), you have a problem. If the battery's okay, then there's voltage drop between it and the fishfinder, usually from a loose or corroded connection in the wire run. Trace the cable back to the source and fix any bad connections.