Tutorials about electrical systems and multimeters often involve theoretical analogies to flowing water. In these primers, the authors test well-designed, functioning circuits, and everything behaves exactly as anticipated. But when we break out a multimeter in the real world, it’s usually because something is wrong and broken. We are detectives, using multimeters to delve into the invisible world of electricity.

This time I’d like to skip the theory and jump right into four common electrical problems, ranging from easy to advanced. But first a warning: even a small amount of current passing through your heart can kill you, and battery acid can cause severe burns or blindness. When testing electrical circuits, be careful, and know what you are testing.

MYSTERY 1

Is my battery charger working?

First disconnect the shorepower battery charger (and all other charging sources) and let the battery sit for 20 minutes or so. Turn on some lights to drain the battery a bit. With our meter set to volts, we then go straight to the battery. With our red lead on the positive pole and the black lead on the negative pole, we take a reading. Now we plug in our charger and take a second reading. The voltage jumped up when we plugged in the charger, and this is how we know our charger is working.

Rested 12-volt batteries will show voltages somewhere below 12.6 volts, whether fully charged or in various states of discharge. Anything up in the 13- to 14-volt range means they’re getting a charge—or have recently received a charge—from somewhere. This methodology is the same whether you’re checking charging voltage from solar panels, a wind generator, an alternator or any other charging source.

MYSTERY 2

This light doesn’t work

First we want to see if the bulb is bad. We take it out and with our meter set to ohms test the resistance across the two contacts at the bottom. If there is infinite resistance the bulb is shot; if there is little or no resistance the bulb is good. Here we don’t really care about accurate measurements, just if there is continuity. With bulbs, fuses and many things aboard, resistance is mostly an all or nothing proposition. Many multimeters beep when they find continuity.

While we’ve got the bulb out, let’s test for voltage in the socket. With our meter set back to volts, we carefully touch the leads to the contacts in the socket. We forgot which way is on for the light switch, so we try it both ways. We measure no voltage, which means the light is not receiving power.

Perhaps the fuse or circuit breaker feeding the light has blown. Our light is fed by a glass fuse, which can be tested just like a light bulb. Set the meter to ohms and check for continuity across the fuse. The fuse is good, too.

Now we need to test for voltage or continuity from our light socket all the way back to the battery, if necessary. By touching our meter’s positive lead to a known source of positive juice on the boat’s distribution panel, we can check for negative voltage or continuity at the socket. We’ve got it, so our problem is somewhere on the positive side.

With our black lead touching the negative bus on the panel, we can work our way back toward the battery from the positive side of the light socket. We get no juice at the socket again. We get no juice on either side of the light switch. We track the wire and the next stop is our fuse. We know our fuse is good because we checked it, but now we read voltage on one side of the fuse holder and not on the other.

The fuse holder looks fine, but may be invisibly corroded. We remove the fuse, clean the holder with a wire brush, replace the fuse, replace the bulb, and our light goes back on. Our multimeter has pinpointed a break in a circuit that looked fine to the naked eye. In fact, most electrical problems on boats result from bad contacts or some sort of corrosion. Cleaning and checking contacts is often the most efficient way to go about solving any electrical mystery.

MYSTERY 3

My engine’s temperature gauge doesn’t work

First we check that the temperature sensor on the engine is making contact with hot cooling water. Often there is a lack of cooling water, not a temperature gauge problem. But in this case there appears to be plenty of water.

We start with the gauge, which has three poles on the back labeled S, I and GND (Sender, Ignition and Ground). To make things confusing, there may also be an instrument light, with two more positive and negative connections. Ignore the light. GND must be connected, directly or indirectly, to the negative pole of the battery. I is the positive lead, and usually comes from the ignition key. These two are easy to check out. With the key turned on and our meter set to volts, we check for voltage across the GND and I poles. We find the gauge does have power.

If we bridge the GND and S posts, the needle on the gauge should also jump to hot. In this case it does jump, but if it didn’t we’d know we had a bad gauge.

We remove the wire from the sending unit on the engine. With the ignition key still on, we touch the wire to a clean metal part on the engine. Our multimeter needle again jumps to hot. We already knew the gauge was good, but this confirms it, and now we also know the wire from the sending unit to the gauge is good.

This points to a bad sending unit, but we can futz with the sending unit (and of course clean the connections) to be sure. The sending unit is a variable resistor that allows more negative juice to flow from the engine block (which is usually grounded to the negative side of the battery) to the temperature gauge when the engine is hot and less when it is cold. This is what moves the needle on the gauge.

We don’t know what the resistance through our sending unit should be, but we do know it should vary. With the engine cold and our meter set to ohms, we test the resistance between the top of the sending unit (where the wire attaches) and the engine block. We do the same with the engine hot to see if there is a significant difference (usually it’s around 200 ohms).

In our case this is moot, because when we do our first resistance check we get infinite ohms, telling us the sending unit is shot. It’s broken inside, and there is no connection, at any resistance, between the place where the wire attaches and the engine block. We buy a new sending unit, and the gauge works fine when we warm up the engine.

This same methodology works for most electric instruments: water temperature, oil pressure, transmission pressure, and fuel and water tank gauges. All of them have S, I and GND poles on the back, with I and GND being switched 12-volt power from the battery, and S being negative juice, controlled by a variable resistor in a sending unit.

MYSTERY 4

My electric windlass is dead

Troubleshooting the windlass isn’t too different from troubleshooting the light. But with our windlass we’ll work the other way, from the batteries up to the bow. Windlasses draw a lot of power and are fed by big cables. In our case the positive cable leads off the battery switch, and the negative cable runs to the main negative bus. Windlasses also need big breakers for circuit protection, usually something around 100 amps, so we’ll check this first. With the meter’s black lead grounded to the main negative bus, we check both sides of the breaker. Yes, there’s power leaving the engine room, running forward through our big cables.

Upside down with our head in the chain locker, we’ll begin troubleshooting right at the windlass. We test the voltage at the ends of our big cables and find there is juice, but we see there’s a foot switch and a windlass control solenoid up here, either of which may have a bad connection.

We’re worried most about the windlass being broken, so let’s jump ahead and test the windlass motor by bridging the positive lead. We make sure the windlass brake is off and use a big cable (an automotive jumper cable or a wrench should work fine) to bridge the positive feed across the contacts on the solenoid. The windlass jumps to life, and our wallet breaths a sigh of relief.

We turn to the solenoid and foot switch. With our meter set to volts we touch the negative lead to the negative wire to the windlass, so we can test for voltage on both sides of the foot switch. A helper on deck steps on the switch, and we read over 12 volts, telling us the switch is good. Also, we hear the solenoid click each time our helper steps on the switch.

A solenoid is a big magnetic switch. Because our windlass draws so much power, we use the foot switch to actuate the control solenoid, which does the heavy switching. With our negative lead still touching the negative wire, we test for positive juice coming from the solenoid when our helper steps on the foot switch. We’ve got good voltage going into the solenoid, and it clicks, but there is no juice coming out. There must be a bad contact on or in it. We disassemble the solenoid and clean the carbonized contacts inside, which fixes the problem—until next time.

The same methodology can be used to troubleshoot an engine that won’t turn over. We should be especially suspicious if we hear the starter motor’s solenoid clicking, but the starter won’t engage.