Troubleshooting 12 Volt Electric, Fatty-Style!

A friend of mine recently purchased a vessel to sail around the world, went down below, flipped a switch on the panel marked running lights—and nothing happened.

He was dumbfounded. He did not know what to do next. So he called me up and asked, “WTF?”

It dawned on me that an increasing number of relatively intelligent owners, with very complex vessels, do not know the first thing about their electronics and electrical system.

Let’s see if we can remediate that—in a very practical, down-to-earth way. This will be a win-win, because the newbies will learn something and those who already know that Ohm’s Law and chanting ‘Om’ are different—will get a chuckle from me attempting to describe the complex as simplistically as possible.

First off, everything I am about to tell you concerns 12 volts only. Most vessels have a 12 volt system for use away from the dock, and a 110 or 220 volt system to use while tied up or when the gen/set is running.

These are different. 110 volts (and higher) can and will kill you—12 volts is much more benign. (You can think of voltage like the pressure in a hose—the higher the pressure, the more likely to cause damage.)

The simplest thing to do, if you are tied up, is to unplug your power cord and then put it somewhere where nobody ‘helpful’ can plug it back in. (Many a 110 volt electrician has died this way.)

Okay. We’re talking about 12 volts only (some bigger boats have 24 volts but we’re not going to get into that) and your shore cord is unplugged.

Let’s assume that most things are working on your boat—thus, you know that your electrical system is mostly okay and functioning as designed.

You just flipped on your running lights—and nothing happened. What to do?

First, rejoice! This is a simple problem, and we’re going to fix it within a couple of minutes … an hour, max.

If your circuit breaker is different than your switch, make sure both are on and conducting electricity. Ditto, if there’s a fuse involved. The fuse or circuit breaker is often the cause—and you can save a lot of time by checking them first.

The cause of your problem is most likely saltwater corrosion. Something is preventing the electrons from flowing through the wire, the same as if you parked your car on the garden hose or it broke or became blocked.

I repeat—the electrical stuff you want to work ain’t working because it ain’t getting any juice—just like your car won’t run without gas.

Once you resupply your electric gizmo with electricity, it will work—presto!

First, some simple theory—fear not, we’re not going to dwell on it.

Everything electrical on your boat is connected to one side of your battery and the other. The electrons flow from the negative terminal on your battery, through the device (in this case, a running light or three) and then on to the positive terminal on the battery.

If the device isn’t working, there are only two reasons:

  1. The device itself is defective, AND
  2. The wiring isn’t getting 12 volts to the device. Number Two is almost always it.

Trust me.

When it comes to solving electrical problems, this is $25 well spent. Photo: OceanMedia
When it comes to solving electrical problems, this is $25 well spent. Photo: OceanMedia

But, just to be sure, let’s start at the device. Take out the incandescent bulb—if it has one—and see if it has ‘continuity’.

We use a volt/ohm meter to do this, widely available for ten to 20 bucks from Amazon, etc. Set the meter so it beeps when you touch the red lead to the black lead. Once you hear the beep, put one lead on one contact on the bulb and then the other lead on the other contact—and listen. If it beeps, the bulb is okay. If it does not, the bulb is burnt out and is not completing the circuit. Replace bulb.


But if three running lights are suppose to turn on and none do—chances are it ain’t three burnt bulbs, chances are almost 100% it is the wiring.

I repeat: the commonest electrical job aboard is corroded wiring either making no contact or a poor contact.

If you learn how to troubleshoot and fix this problem, 99% of your electrical woes will be a piece of cake. I actually look forward to fixing electrical stuff, because it is so easy, quick, and satisfying.

Okay! We now know the bulb works. Yippee! We are making real progress.

There are two terminals in the bulb socket. One is connected through your switch panel/circuit breaker to the positive side of the battery, and the other is (usually) connected directly to the negative side of your battery.

Let’s go to the battery—and test your test light. This is a tiny little thingy which looks like small screwdriver or ice pick … with a wire coming out the top of it. If it senses 12 volts, it lights up.

Put the alligator clip attached to the wire onto the negative post of your battery, and touch the positive post with the pointy part. The bulb will light. Excellent!

Now set your volt/ohm meter for voltage, 12 volts, and put the black wire on the negative terminal of your battery and the red one on the positive—and it should read 12+ volts.

Great. Now you have determined that your 12 volt test bulb is working, that your voltage meter is working, and that your battery has juice in it at a pressure of (usually) around between 12.5 and 12.7 volts. (Unless the battery is running something, then it will be lower—or higher if solar cells, etc, are hooked up.

Now take a long (preferably fused) test wire and test it for continuity—so you are absolutely sure this wire conducts electricity. Next, attach this test wire to the negative side of the battery and carefully (so it doesn’t touch anything metal and make a short circuit which will blow the fuse and/or make a spark) bring the other end to your bulb socket. Place the alligator clip of the circuit tester (the ice pick one with the tiny bulb) on the end of this wire, and then touch the ice pick end of the tester to the metal contacts in the light socket. If the switch is on, it should light.

If one of the terminals makes the test light turn on—your positive side is okay, and lacks a ground to your negative terminal. Mark the nub which lights up as +positive. We know this because it completes the circuit with the negative. Confirm this by switching your volt ohm meter (set to continuity/beeping)… and touching your extension wire to the light socket nub which DID NOT light up, i.e., the negative side of the socket. It should not beep. This confirms you have no current flow through negative, and the positive side is fine.

Wow! You are almost done.

Replace the bulb, and (if possible; sometimes easy, sometimes not) touch your negative ground wire to the black wire negative ground going into the negative nub on the socket—and the light should light.

GREAT. You have now FIXED the problem—sloppy and temporarily, true—but the light didn’t work and now it does … you are a freak’n electrical genius!

Pat yourself on the back. (Not time for drinks yet.)

Now physically follow the wire (usually black for negative) back to the negative post of the battery. Chances are you will find an area of greenish discoloration along the wire (water has leaked into it and corroded the wire) or discover it is loose by a terminal box, etc. Repair or replace using the same components (if its big wire, use the same size, etc).

Keep testing it as you go towards the battery—and at some point you will discover it is ‘live’ with electricity. Narrow it down to exactly where there is 12 volts and no 12 volts, and there’s your problem.

You have now trouble-shot and fixed your first major electrical problem.

Let’s say, however, it wasn’t the negative side which was faulty, but the positive. You would have known this because—when you first touched the test light from the negative side of the battery to the nubs of the light socket—neither of the nubs would have lit up the bulb—telling you that either both wires weren’t connected to the battery (unlikely but possible) or it was the positive side which wasn’t making contact.

To confirm, you’d take your long extension wire and now (being careful again) attach it to the positive (+) terminal on the battery—and use your bulb tester again.

When it lights up (the circuit is complete) that’s the negative side of the socket.

You can confirm this by testing continuity on the neg side. It should have it.

On the positive side, it should not.

Trace the positive (usually red) back to the battery. This is trickier as it will go through a switch and circuit breaker or fuse box—in addition to a terminal or two.

Is there more to it? Sure! A bulb has no polarity and can be wired up with either one of the other terminal wires leading to either battery terminal—it doesn’t matter. (Incandescent light bulbs are just small toasters; but optimized for light, not heat.)  However, a radio, a motor, or some LED lights have polarity—meaning that the electricity must flow in a certain direction. Thus, the red wire must eventually go to the positive battery terminal and the black one to the negative.

Occasionally, a wire will make enough contact to allow a little bit of juice through it—but not the high load demanded.  This is common with windlasses, etc.

This can be spotted with the volt ohm meter by measuring the wire’s resistance to current flow—but that’s another article.

But for the most part, all you need is a 12 volt bulb tester and Volt Ohm meter (both under $25) to get 99% percent of your stuff working together.

If you need to trace which particular wire is where at your switches, just shut off your main battery switch so there is now 12 volts in the circuit—and then put your continuity meter on one end of the wire (say, at the running lights, for instance)—and touch the other end of it to the individual wires at the terminal block leading to the switches. Bingo! The beeping one is it!

Remember—unplug your 110/220 volt before you begin if you’re not sure which wire is AC (shorepower) and DC (12 volt).

Good luck!

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