Have this tool on your boat. It will save you a lot of frustration and time. Trust me!!!!!!

Electrical Troubleshooting: Using A Multimeter

Do you really need a multimeter? Yes! Once you understand the

basics, it can save you time, money, and hassle when trying to locate

the source of a power problem.

WARNING

Many boats have AC power, which can kill. Ensure all AC power is turned off to the boat before starting work. Disconnect or isolate shore-power connections, inverters, and generators. Be certain of your capabilities, and if in any doubt, consult a qualified marine electrician.

Your brand-new bilge pump blows a fuse every other trip, and your VHF goes dim whenever you try to transmit. An easy way to find the source of your problem is to get out your multimeter and do some electrical investigation. But first, read the instructions and be sure you understand them. If not, you could hurt yourself and your equipment. Also, you must have a basic understanding of wiring and what is dangerous. This includes the differences between AC and DC electricity and wires on your boat.

Got it? Great! Now, let's review the basics of using a multimeter to measure DC current, resistance, and voltage.

Current is the rate of flow of electricity running through the wires in your boat. Think of it like a garden hose, where current is the amount of water coming out. It is measured in amps. When a current is interrupted by something like a bad connection or improperly sized wiring causing a device to stop working, or at least stop working as it should, this is known as resistance — like a kink in the hose. And voltage is, to put it very simply, somewhat similar to the amount of water pressure in the hose. The more pressure, in theory, the more power the current can produce.

A multimeter can be used to measure all of these things and more. Some units are more complex than others, performing more functions. Some have different features to better measure the same functions (such as auto-ranging vs. manual). Some are digital and some are analog (with a needle). Most of us have simple needs, such as determining basic current, resistance, and voltage, and don't need to spend the extra money for complicated features. However, it's worth spending money for quality, regardless of usage, because electricity can be very dangerous.

Voltage

When you have a problem with onboard electronics, the first thing to check is that you have an adequate source of power available. On a boat, 12-volt DC power is supplied by a battery or batteries, so that's your starting point.

The multimeter I'm using has several capabilities, so because I'm measuring 12 volts, I turn the scale on the meter to read DC volts, and specifically the 20-volt range, which is closest to the voltage that I need to read. (Meters with auto-range may make this adjustment automatically.)

To get an accurate reading, first turn off your battery charger. Then check that the black probe is inserted into the black "common" receptacle on the meter and the red probe is plugged in to the red "voltage" receptacle. Now touch the red probe to the positive (red) battery terminal and the black probe to the negative (black) terminal, and make a note of the voltage. A reading of 12.65 volts or more indicates a full charge, 12.3 volts is a 75 percent charge, 12.2 a 50 percent charge, and 12 volts a 25 percent charge. If the battery reads below 12 volts, it is effectively dead and will need to be recharged or replaced.

Measuring Current

If the problem you're having isn't getting devices to turn on but, rather, keeping them on because of a circuit breaker tripping or a fuse blowing, you might be drawing too much current through a given circuit. Refitting older boats with new, power-hungry devices can exceed the capacity of the original circuits.

If you don't have good documentation on how many amps a given device draws, here's how to use your multimeter to find out. This test uses the meter itself to complete the circuit. A good place to insert the meter leads is at the circuit protection (fuse block). If there's a fuse, remove it — the meter will be measuring between both sides of the fuse block.

Switch OFF the circuit you're testing, then set your meter's selector switch to DC amps. You'll probably need to move the meter's red probe to the "amp" plug. (Some meters only read a very small amount of amps. See that yours is adequate.)

Disconnect a wire or fuse where you'll measure, and hold the black lead on one side of the fuse holder or wire and the red lead on the other side. Again, take care not to touch the leads together. Finally, switch on the circuit, being careful not to let your body parts complete the circuit. The meter will give you a reading in amps to record. If the total amp draw on a circuit exceeds its protection rating, the breaker will trip. Measure the load of all devices on a given circuit and add together to find out if it's overloaded. If so, you'll need to upgrade the wiring and overcurrent protection.

Continuity Check

For electrical equipment to work, there has to be a complete circuit from the power source to the thing that is being powered and back to the power source. Sometimes, even when everything looks OK, that bulb, fan, or other gizmo just won't work. This could be due to a continuity problem — a break in the circuit. Using your multimeter, you can determine if a bulb has blown or a fuse is bad, or ensure that any other number of components has good electrical connectivity.

To test the circuit, make sure your power is turned off. Start by setting the multimeter to the ohms scale, usually represented by the Greek letter Ω (omega). An ohm is a measurement of resistance. When there is a break in the circuit, your meter will show "OL," which means "overload" or "open loop," i.e., a break in the circuit. When the circuit is complete, the meter will show a very small reading, indicating that all is well. If it registers more than a few ohms, it could indicate a poor connection or a bad wire in the circuit causing unwanted resistance. Address this by carefully checking all circuit components.

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.

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.

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.

• Turn the DVOM selector switch to DC volts (scale for less than 20 volts).

• Turn all circuits on the boat OFF (battery selector switch to OFF).

• Engine and charging system OFF.

• 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 positive terminal.

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 battery.

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 check the voltage at the VHF.

• Battery and circuit you're testing ON (flip the appropriate breaker on the DC panel).

• All other circuits OFF.

• Turn the DVOM selector to DC volts.

• At the VHF, touch your red lead to the positive terminal, black lead to the negative.

Compare the result to your voltage at the power source. A drop greater than 0.4 V (the ABYC deems a VHF radio a critical circuit) or 1.2 V in any circuit should send you scurrying to clean terminals and check for ample wire gauges. If the voltage reads zero, you've got continuity issues.

Checking Continuity

A typical electrical fault is "an unwanted open" — an undesigned break in the circuit — that interrupts the flow of electricity. That can come from something as simple as a blown fuse or something more complicated like a broken or extremely corroded conductor, or a loose or separated connection. Continuity describes a circuit that is closed, as it's designed to be. To find out if you have a break in the circuit:

• Turn the DVOM selector switch to ohms (Ω).

• Check the DVOM battery. With the leads separated, you should see "OL" for "overloaded" or no continuity. Now touch the leads together, and you should hear a beep for continuity or a value near "0" on the meter. If you don't get these, replace the DVOM battery.

• Turn off the circuit to be tested.

• Disconnect the two ends of the circuit component you suspect is bad (say, the wire leading to VHF from the distribution panel). If your DVOM leads can't reach, add a length of properly sized wire to complete the circuit.

• Connect your probes to each end of the conductor.

A beep or a meter reading near zero indicates continuity. "OL" indicates overload: no continuity. No continuity means there's a break in the wire. You can also check for blown fuses and tripped circuit breakers with this test by touching each end with your probes (breaker must be on and disconnected from power). It's also possible the device itself has burnt up, breaking the circuit. If the continuity is high, but not OL, go back and look for a loose or corroded connection, or jiggle the breaker or switch if they're part of the portion of the circuit you're testing. When these go bad, they sometimes work when pressure is applied, but not when it's released.

Measuring Current Using Leads

If the problem you're having isn't getting devices to turn on but rather keeping them on, you might be drawing too much current (amps) through a given circuit. Fitting older boats with new, power-hungry devices can exceed the capacity of the original circuits. If you don't have good documentation on how many amps a given device draws, here's how to find out. Use a pair of meter leads that terminate in insulated alligator clips. You'll need to disconnect the power lead somewhere in the circuit; the meter itself will complete the circuit. A good place to insert the meter leads is at the circuit protection (fuse block). If there's a fuse, remove it.

• Switch OFF the circuit you're testing.

• Set your DVOM's selector switch to DC amps, at the proper scale.

• Disconnect a wire where you'll measure; alternatively, remove a fuse from its holder.

• Clip the black lead to the terminal closest to the battery, the red lead to the other.

• Switch on the circuit, being careful not to let your body parts complete the circuit.

The meter will give you a reading in amps to record. If the total amp draw on a circuit exceeds its protection rating, the breaker will pop. Measure all the devices on a given circuit to find out if it's overloaded.