Troubleshooting Your Boat’s 12-Volt DC Lights

Let there be light…or not. One of the most annoying and common problems you will encounter as a boat owner are lights that fail to work. At a minimum this is a nuisance; you may not be able to read the book you’re currently addicted to in your bunk. On the more expensive end of things, running at night with an extinguished navigation light could earn you a pricey fine from your local marine police. And for good reason—it’s dangerous.

A bayonet-type incandescent light bulb. Here’s one place to check and see if your incandescent bulb is burned out. If the filament is broken, the bulb is bad. Photo courtesy of Ancor.

Figure 1: A bayonet-type incandescent light bulb. Here’s one place to check and see if your incandescent bulb is burned out. If the filament is broken, the bulb is bad. Photo courtesy of Ancor.

A festoon-type 12-volt light bulb. Photo courtesy of Ancor.

Figure 2: A festoon-type 12-volt light bulb. Photo courtesy of Ancor.

Still, troubleshooting and repairing a malfunctioning light is relatively easy in the grand scheme of things. Before you get started, however, head over to the hardware store and buy yourself a multimeter if you don’t already have one. A basic multimeter is a relatively inexpensive device that every boater should keep in the tool bag.

We’re going start out by covering how to troubleshoot a simple, incandescent bulb and skip LED, halogen, and fluorescent lights for now. In most cases the troubleshooting for those types of lamp will be the same, but there are some differences that we can cover in a later article.

Check the Bulb

Yes, DC bulbs can blow out just like the ones in your AC-powered lamps at home. Make sure that the light is turned on by checking integral on/off switches and any remote bulkhead switches the fixture may be wired to. Also make sure your light is powered on from the control switch or circuit breaker panel. You’d be surprised how many folks forget simply to see if the circuit is turned on first.

Next, go to the offending light fixture, remove the bulb, and examine it. If you’re working on exterior fixtures, such as navigation lights, be very careful not to drop any parts in the water as you disassemble the fixture. Once you’ve retrieved the bulb from its fixture, take a close look at it. Some blown-out bulbs will be obviously defective—they’ll be clouded over inside with grey, white, or black residue. If yours looks like this, head to a marine or automotive store with your bulb in hand as an example and seek out a replacement. Also, grab a couple of extras for your spare kit.

Other bulbs may require closer examination. Examine yours closely and look to see if its filament—a tiny, twisted piece of wire between two prongs—is intact (Figure 1). If it’s broken, the bulb is bad and should be replaced. Even if the bulb looks OK, you may want to consider getting a new one and giving it a try. If it works, you’re set, if not, move on with your troubleshooting.

Check Fuses and Breaker Switches

Next, turn your attention toward circuit protection devices and switches, such as the ones you’ll find at control switch and circuit breaker panels, and make sure the switches are in the on position. If the electrical circuit your light is on is protected by a fuse, check to ensure that it isn’t blown. If it has, the piece of metal that runs through it will be melted and broken. Replace it and then check to see if the light works again.

If you find a circuit breaker toggle for the circuit your light is on has been tripped, switch it back to the on position and see if your light illuminates. If that circuit breaker trips again, there’s a problem somewhere in the circuit that you need to fix, or have a pro check out.

Check Your Battery

You’ll likely know if your battery is dead because nothing else powered by it on the boat will work. But it’s worth checking to see if there’s juice available anyway. If you’ve got a voltmeter in your electrical panel, simply check there. If your electrical system is more simplified than this, use a multimeter at the battery to see if a charge is present. Simply touch the positive probe from the multimeter to the positive terminal on the battery and the negative probe to the negative terminal. A charged battery should read between 12.5 and 14 or so volts, depending on the type of battery.

If the battery is charged, your bulb is not burned out, and your circuit protection devices/switches are working properly (not blown), and other electrical devices on the boat are functioning properly, then it’s time to do some electrical sleuthing, working from the bulb socket backward to the battery.

Check Socket, Connections, and Switches

This is where your multimeter will really come in handy. It’s time to see if there’s electricity reaching your light fixture. There are certainly a number of ways to do this but the simplest method is to diagnose from the fixture backwards, starting at the bulb socket and working back towards the battery.

Generally speaking, there are two types of incandescent light sockets that you might find inside your light fixture: bayonet (Figure 1) and festoon (Figure 2) type. We’ll deal with the bayonet-type first. These sockets have two points that contact the light bulb—one is negative, the other is positive. The negative contact point is generally the socket housing itself, while the positive contact or contacts (sometimes there are two) are located down in the socket (Figure 3).

12-volt bayonet-style light bulb sockets and their negative and positive contact points. Photo courtesy of Perko.

Figure 3: 12-volt bayonet-style light bulb sockets and their negative and positive contact points. Photo courtesy of Perko.

To test your socket, first make sure that your multimeter is set to measure 12 volts. (We’re assuming here that your boat, like most, has a 12-volt DC system, but some have 24-volt systems, so set up your meter accordingly.) Next, make sure the light fixture is switched on, including at your control switch or circuit breaker panels. Now touch the red probe to a positive contact inside the socket and the black probe to the socket housing. If you get any reading above 12 volts, you know that the socket is getting power. You may want to check for corrosion on the contacts or socket housing, or trying a couple of new bulbs. If you don’t get a voltage reading here, we’ll need to dig back farther.

With a festoon bulb socket, keep in mind that they generally have two contacts—one is positive and the other negative. You may not know which terminal is which, so testing the socket may require touching the positive and negative probes from the multimeter to each side of the festoon socket and then reversing them until you get a result. If you have power to the socket, you’ll get a reading of 12 volts or more; if you don’t, it’s time to move down the circuit toward the battery.

Components in the Mix

There may be different components between the battery and the light bulb itself, including items like switches, fuses, buss bars, and circuit breaker panels. Again, the goal of troubleshooting is to work our way back from the light bulb toward the battery until you find a faulty leg of the circuit. Once you find a section of the light circuit that has power, you can generally isolate the faulty component in front of it, replace or fix it, and be on your way.

Bulb Sockets

If your light fixture does not have an integral switch—most navigation lights are wired this way—and your bulb socket failed the voltage test, the next step is to test the negative and positive leads connected or leading to the bulb socket. Sometimes—especially if the leads are soldered to the light socket—you can simply touch the positive and negative probes to the solder connections on the bulb socket. Other bulb sockets may be connected to their supply wires by connectors such as spade- or socket-type connectors. If this is the case, you can simply remove them (make sure you do not touch the ends together) and then connect the positive and negative leads to the probes on the multimeter to see if you get a reading.

If you get a voltage reading from these leads, the likelihood is your socket is bad and should be replaced. You can usually find them at your local marine supply shop or automotive store. Bring your defective socket along as an example. Oh, and pick up a spare, too.

If you weren’t able to read any voltage at the leads that connect to the bulb socket, you may have a bad switch in your circuit, or other problems farther down the line toward the battery.

Switches

Let’s assume you’ve got an on/off switch either installed integrally as part of the light fixture, or somewhere back farther in the circuit. Switches, by the way, are sort of like on/off valves installed in the positive supply wire to an electrical appliance. Turn it off and it closes the valve to the energy heading toward your light. Turn it on and the juice flows and the light lights up. A simple diagram of a fused and switched DC circuit can be seen in Figure 4.

Figure 4: A simple, fused and switched DC light circuit.

Figure 4: A simple, fused and switched DC light circuit.

Now, take a closer look at that illustration. Since we’ve already confirmed that we’re not getting power to the back of the light bulb socket we’ve moved back to the switch, which is wired inline on the positive side of the circuit. That switch might be in a control panel in a dash or bulkhead, or on the light fixture itself. What we need to confirm is whether power is or is not leaving the switch.

To test the switch, you need to touch the positive probe from the multimeter to the terminal leading out of the switch toward the light and the negative probe to a “test” wire. You can make one from a length of 14-gauge black marine electrical wire long enough to reach your battery. Strip the wire from one end and install a male spade connector and do the same on the other end but use a female spade. Connect the male spade to the negative post on the battery and insert the negative probe from the multimeter to the female spade.

With the on/off switch in the “on” position, touch the positive probe from your multimeter to the connection point on the switch that supplies power to light, then attach the negative probe from the multimeter to the test wire (Figure 5). If you get a reading on the multimeter, this means the switch is functioning and there’s a break, corrosion, or other defect in one of the wires leading to the light, most likely the positive lead.

Figure 5: Multimeter test points for diagnosing a possible defective on/off switch in a 12-volt light circuit.

Figure 5: Multimeter test points for diagnosing a possible defective on/off switch in a 12-volt light circuit.

If you get no reading, it means the switch is likely defective, but you’ll want to test the incoming positive lead that connects to the switch to be sure. If you get a voltage reading here, the switch is definitely the source of your problems and needs to be replaced. If you don’t, you will have to continue to work backwards toward the battery until you find out where the fault is. The next logical place to check is the fuse holder, testing each side just like you did with the switch to diagnose it. (You did check the fuse and tried installing a new one, right?) If you’ve gotten to this point and you still can’t find the source of your problem, work your way along the circuit from the battery to the light, checking and troubleshooting using the steps we’ve described.

Truth be told, 99.9 percent of the problems with a light circuit are at the light itself, at a terminal (where corrosion from damp air, especially damp salt air, is usually the culprit), at a fuse or fuse-holder, or in a control switch. While this may all seem as if it would take hours to track down and diagnose, it honestly shouldn’t take more than an hour or so to be back up and running again. Also, if you get to a point where your comfort with working with DC electricity becomes less than your desire to fix the problem yourself, get a pro involved to save you the headaches and avoid any potential safety issues.

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