Troubleshooting trailer turn signal light problems might seem like a daunting task for those unfamiliar with automotive and trailer wiring, but, relax. It's not difficult. Learning just a few basics about these lights will enable you to find the problem quickly and fix it yourself. The only tools you'll need are a 12-volt test light, a wire cutter and stripper and some electrical tape.
Check the Tow Vehicle
Trailer turn signals get their power from the tow vehicle, so start with verifying the proper operation of the tow vehicle's turn signals. With the ignition key on, enable the left turn signal and look at the rear of the vehicle and trailer to see whether the light is flashing. Do the same for the other side. If the tow vehicle lights flash, is only one side of the trailer out, or both?
Check the Trailer
If only one light is out, shut off the vehicle turn signals and have a helper depress the brake pedal while you check the trailer lights. If the same light is out when the brake is applied, the next step in troubleshooting the turn signal problem is to replace the bulb in the affected trailer light. Bad bulbs are the most common culprit when a single light fails to work, so a new bulb might fix the problem.
Check the Fuses
When the problem is not eliminated by replacement of the bulb, there are a few other areas to check. One often overlooked area when the tow vehicle is a late-model truck is the fuse box. Check your owner's manual to see whether your vehicle has fuses for the trailer circuits. The fuses, if so equipped, can be located in a separate fuse box, which is generally under the hood. Check the indicated fuses with a test light after turning the ignition key and turn signal for the defective side on.
Check the Connector
Move to the trailer light connector if all fuses are good, or the vehicle is not equipped with them. Disconnect the plug from the connector and probe the vehicle side of the connector with a test light. Make certain your test light has a good ground connection, even if that requires running a ground wire all the way from the battery's negative terminal. With the test light, probe each terminal, allowing enough time for the light to flash several times. You should find two terminals that flash the test light, one for the left side, and one for the right. If only one terminal makes the test light flash, the wiring problem is in the tow vehicle. But if two terminals make the light flash, it's time to plug the connector back in and go back to the trailer.
Check the Wiring
Perform a visual check of the wiring to the trailer side of the trailer connector and the offending light itself. If all looks well, turn the vehicle running lights back on. Verify the operation of the trailer tail lights. If the tail light is out in the same lamp as the bad turn signal, the lamp might have lost its ground connection because of corrosion. Remove the screw holding the ground terminal to the trailer frame and sand off all rust and corrosion from the terminal and trailer frame with coarse sandpaper. Reattach the terminal to the trailer frame with a new screw and retest the lights.
Should the turn signal still fail to work, a problem with the wire from the trailer connector to the lamp is indicated. Replace the wire to cure the problem.
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