How do I troubleshoot trailer wiring problems?
Response by Bass Rogue - Dated 11/26/1998

Troubleshooting Approaches
       The wiring between a tow vehicle and a trailer is quite simple. However, when something goes wrong, this simple wiring can become very complex and confusing. Here are some thoughts about trailer wiring that might help when the boat is spending too much time in the garage waiting for you to get a couple of light bulbs working.

       There are two approaches to troubleshooting. The first approach is based on knowing what is wrong. For example, the right rear turn light stops working. You venture a guess, the bulb is burnt out. You change the right rear bulb, and the problem goes away. This can be describe as the "I've seen this problem before" or the "what's the most likely thing that's broken" approach to troubleshooting.

       Finding the most likely failed part is the easiest way to troubleshoot, but it doesn't always work. What would happen if you changed the right rear bulb, and the problem didn't go away? Would you change the light socket, the light fixture, or maybe redo the wiring? It's often this next step that gets us into trouble when the easiest way doesn't work. We start "Easter-egging" or just changing stuff. This can waste a lot of time and cost a little money. It can also be the source of induced failures which really compound the problem. In case you're not familiar with the term, an induced failure is when you break something while trying to fix something else.

       The second approach is based on knowing what is good. For example, the right rear turn light stops working. You check the voltage coming out of the tow vehicle and discover it isn't there. Therefore, you conclude, the trailer is good. You look at the tow vehicle turn lights and discover they work correctly. Therefore, you conclude the tow vehicle's basic wiring is good. You remember the other lights on the trailer were working correctly. Therefore, you conclude, the ground wire is good.

       About this point in time, you conclude there is only one wire left to check and that's probably where the problem lies.

       This second approach to troubleshooting is the best approach when you have a difficult problem.  Systematically go at things and keep shrinking the area where the problem might be - eliminate what is known to be good. Also, don't assume anything, prove the things are good. Too often, it's the stuff we assume is good that gets us.

Troubleshooting Tools
       The most common tool for troubleshooting electrical problems is a voltmeter, a device use to indicate the presence of a voltage. Personally, I think this is a big mistake when chasing difficult electrical problems. The impedance of these devices is so high, you can't load down a faulty connection with them. In other words, you can read 12 volts (really 13.2 volts) on the meter, but you can't pull enough current through a faulty connection to light a bulb. I prefer to use a bulb with two test leads (wires) attached to it. If the bulb comes on when I connect it, I have voltage and current, and current is what makes a bulb glow.

       Besides the test bulb or a device to check voltage, the only other special tools are things needed to make reliable electrical connections. These include a crimping tool, wire stripper, wire cutters, fine sandpaper (emery cloth) and dielectric grease. The thing to remember about electrical connects is they must be clean, mechanical sound and protected. A clean connection means the wires are shiny and free of contaminants before you connect them. A mechanically sound connection is one that is made by crimping or by forcing the conductors (metals) together. Once you make a good connection, protect it. Use dielectric (non-conducting) grease to seal crimps or connections. If you can't find something called dielectric grease, get an automotive silicon grease (clear).

Tow Vehicle Wiring
       When something goes wrong with the lights on a trailer, people automatically assume the problem is in the trailer. However, reality tells us something different. A bunch of the time, the problem is in the tow vehicle. If the "I've seen this problem before" approach doesn't fix the problem, start with the tow vehicle. First of all, make sure all the lights are working correctly on the tow vehicle. Once this is verified, go to the connection at the tow vehicle and test there. Prove to yourself that the tow vehicle is putting out the correct voltages on the correct connector pins. Again, I am not a big fan of using a voltmeter for this task, I prefer to use a bulb with two test leads. If you prefer, you can also purchase a little LED (light emitting diode) tester that plugs into the tow vehicle jack (receptacle). Besides providing a load to draw enough test current, this tester quickly show when too many lights are coming on. For example, the running lights come on when the brakes lights come on.

       A typical tow vehicle jack is an in-line four-pin jack. It has three female pins and one male pin. The female pins carry the voltages for the different lights, the male pin is the ground or return for all the circuits. Starting with the male pin and working toward the other side of the jack, the correct voltages and wire colors are:

  • White - Ground
  • Brown - Running (tail) lights
  • Yellow - Left turn/brake light
  • Green - Right turn/brake light

       To test the voltages at this jack, turn on the running lights and then connect your test bulb between the white wire pin and the brown wire pin. The test light should come on. Connect your test bulb between the white wire pin and the yellow wire pin. The test light should stay off. Connect you test bulb between the white wire pin and the green wire pin. The test light should stay off.

       Turn off the running lights, then repeat this basic test approach for each turn signal and the brake lights. Remember, always connect one test lead to the white wire pin (ground) and see what is on the other pins. The test light should only come on when you are on the correct pin. Also keep in mind, when testing the brakes, the test light should come on at both the yellow and green wire pins.

Common Tow Vehicle problems
       If the problem at the vehicle jack is the test light never comes on, then the problem is probably a broken ground wire or a bad ground connection. Trace the white wire from the jack back to where it connects to the vehicle frame. If it doesn't connect to the vehicle frame, then connect it there. A good connection requires a crimp termination (round lug) on the end of the wire, a metal screw or a bolt and a self-locking nut, and a star washer. The frame connection point should be bare shiny metal. To test this connection point, turn on the vehicle's running lights and connect the test bulb between this point on the frame and the brown wire pin on the vehicle jack. The light should come on.

       If this connection point is good, then check for an open or broken ground wire. If the test light fails to come on at one pin, then you probably have a open wire going to the jack. Retrace the wire back to it's connect point and ensure the wire is not broken. Also check to see if it has a clean, mechanical sound connection that is protected from the weather elements. If in doubt, redo the connection.

       If the test light comes on at too many pins on the vehicle's jack, then the wires are probably shorted together somewhere. Retrace the wires back to their connection points and look for places where two or more wires are pinched together. If the wires are good, make sure the connection points are not shorted. If the connections are physically right next to each other, separate them a couple of inches to avoid the potential for a short circuit.

       If the test light comes on at the wrong pin, then the wires are probably connected to the wrong connection points. Retrace the wires to the connection points and reconnect them correctly.

       One of the things to keep in mind about tow vehicles is they are not all a like. Some vehicles, especially the ones with factory installed tow packages, have a trailer pig-tail wrapped up in the tail light wire bundle. If you find a place where the wire bundle appears to be too big, then you have probably located the pig-tail. Undo the wire bundle and you will probably find a coiled wire bundle that doesn't go anywhere. That wire bundle is your trailer pig-tail. All you need to do is connect a jack to the wires. Just follow the instructions in your vehicle user manual.

       Another thing to keep in mind is some vehicles use five wires instead of four. These vehicles use different bulbs, usually amber, for the turn signals. In other words, the brake and turn signals are different light bulbs. The five wires are:

  • Ground
  • Brakes lights
  • Left turn signal
  • Right turn signal
  • Running lights

       In order to go from these five wires to the four wires discussed about, an adapter is needed. These adapters can be found at any automotive store. Some are universal ones that connect directly to the wires; others are designed for specific vehicles and plug into the wire harness at certain points. Keep in mind, these devices can fail, they have diode logic circuits inside of them.

Trailer Wiring
       The lights on a trailer are just an extension of the tow vehicle lights. The wiring for these lights starts at the tongue of the trailer with an in-line four-pin plug. This plug has three male pins and one female pin. The male pins carry the voltages for the lights, while the female pin provides a ground or return for all circuits. The color code for the wires at this plug are the same as on the tow vehicle.

       Generally speaking, the ground wire or white wire from the trailer plug connects to the trailer frame within three feet of the plug. Often this ground connection is a poor mechanical connection which leads to frequent problems. Sometimes, the length of the wire bundle at the plug is such that it puts strain on this ground connection. If in doubt, redo this connections to ensure a clean, mechanical sound connection that is protected from the elements. To relieve any strain on the ground wire, splice in a short length of additional wire and use a cable clamp to secure the whole wire bundle to the trailer tongue.

       The wiring color code on most trailers is only good for the first three feet, the length of the trailer plug pig-tail. After that, the wiring can be any color. Therefore, it is often necessary to physically follow the wire instead of following a wire color. On the other hand, there are really only three wires to troubleshoot and fix.

Trailer Troubleshooting Basics
       The most important part of troubleshooting a trailer problem is a good visual inspection. Check all ground connections and make sure they are clean, mechanical sound connections that are protected from the elements. Examine all bulbs and light fixtures up close. Look for water trapped inside the light fixtures. Look for discolored bulbs. When in doubt, fix the connection or replace the bulb or fixture.

       One of the most common problems with trailer wiring is a broken ground connection. If you are troubleshooting a trailer, do not connect the trailer hitch to the tow vehicle ball and do not connect the safety chains or cables. These can provide false grounds to the trailer. The only connection between the two vehicles should be the four-pin jack and plug connection. The way I prefer to troubleshoot trailer wiring problems is with an inexpensive 4-amp battery charger with a current meter on it. Do not use one of the more expensive chargers. Some of them have protect circuits that won't allow any current to flow unless they detect a battery on the end of the charger cables.

       The first test should be one of the turn/brake lights circuits. These are the simplest circuits, usually only one bulb filament is involved, sometimes two. If you are using a battery charger, connect the negative cable to the white wire pin (ground) on the trailer plug and connect the positive battery charger lead to the yellow wire pin. If you are using a tow vehicle, turn the left turn signal on. The left turn light should come on. If it doesn't, first check the bulb, then the wiring to it. If one of the lights on the other side comes on, see the Strange Trailer Problems discussion below. After checking the left turn light, check the right one. Leave the battery charger on the white wire pin and move the other lead to the green wire pin; or turn the tow vehicle's right turn signal on. The right turn light should come on. When you test the turn signals, you also check the brake lights. They are the same circuits.

       The last check should be the running lights. Leave the battery charger on the white wire pin and move the other lead to the brown wire pin; or turn the tow vehicle's right turn signal off and turn on the parking lights. All the trailer running lights should be on. If not, check the bulbs, then the ground connections, then the wiring.

Strange Trailer Problems
       It's very difficult to get people to check the ground connections, even though most trailer problems are probably caused by bad grounds. Some of the strange symptoms of bad ground connections are things like: the lights on the wrong side of the trailer come on; the lights on one side are brighter; the lights are on, but they're dim. Here's what's happening when these type of symptoms show up. Some of the bulbs on the trailer have two filaments in them.

       One filament is for the running lights, they other is for the turn/brake lights. Each filament has a wire going to it. Both filaments use the same return, the base case which is connected to the vehicle's frame or ground. Under normal situations, current (hole current) flows from the positive terminal of the battery, through the wire to the filament, through the filament, through the base, through the ground, through the vehicle frame back to the battery negative terminal.

       Okay, what happens if the ground connection is bad. Well, current (hole current) flows from the positive terminal of the battery, through the wire to the filament, through the filament, through the base. Okay, it can't go out the ground because the ground is bad. So, what happens? Well, it goes back from the base through the other filament, through the wire to the bulb on the other side of the trailer, through that filament, through that base, through that ground, through the vehicle frame back to the battery negative terminal. When all this happens, the filaments won't have the full voltage across them, so they will be dimmer, but on one side two filaments could be lit and that should appear brighter unless the other side filament is the brake filament. Okay, you get the picture. When things start appearing to be real strange, start looking at those ground connections.

       In summary, trailer wiring problems can appear to be complicated. However, a systematic approach based on eliminating things that are proven good can help you focus in on the real problem. Also, when things start really appearing strange, start checking ground connections.

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