Trailer lights problems are common, and would in most cases happen when your utility trailer is starting to show its age. For safety reasons, you need your trailer lights functioning properly and should therefore be checking your lights every time you use your trailer. While some problems with trailer lights can be a pain to diagnose and fix, others such as a bad ground wire or burned-out bulb are easy to troubleshoot and fix. Follow the steps on this guide to troubleshoot and fix some common trailer lights issues easily on your own.
What Would Cause Trailer Lights Not to Work?
Trailer light problems are in most cases to do with the tow vehicle’s electrical system or trailer wiring. Visual inspection and testing are thus the first things to do. Because the most cause for trailer lights not working properly is bad or missing ground, check the ground connection of the trailer and truck first.
When some lighting functions are working and when lights go out when you push the brake pedal, poor ground is in most cases the culprit. This means that using several functions at the same time creates a load that a weak ground cannot handle.
Also, if you have a faulty ground connection, there is a likelihood that trailer lights won’t work at all.
How Do You Test Trailer Lights?
Having your trailer lights in proper working condition is very important as it allows other drivers to see your signal and brake. If your trailer lights seem to be malfunctioning, there are several ways you can test them to diagnose and fix the problem yourself. First, you will want to perform a simple test with an assistant to verify that the lights are working. If they are not working, you can use a tow-vehicle light tester and multi-meter among other tools to check for broken wires and contacts within your trailer’s circuitry.
Diagnosing the problem
To diagnose the problem:
- Start up the tow vehicle with the trailer attached to it. Ensure that the wire to the trailer is plugged into the connector on the back of your vehicle.
- Press the brake, hazard lights, and the blinker lights while an assistant stands behind the trailer to see if the lights are working properly. The trailer lights should correspond with the lights on the back of the tow vehicle. Take note of the lights that are malfunctioning (don’t turn on or are dim).
- If only one light is not working, it is likely the bulb has blown out. Unscrew the blown bulb and replace it with another one of the same voltage. Test the lights again by pressing on the brakes in your tow vehicle. If the light still does not come on, you know there is a problem with the wiring.
- Disconnect the tow vehicle from the trailer. Unplug the black trailer cord from the tow vehicle so that you can test all the connections individually.
- Plug a tow-vehicle tester into your vehicle’s connector. If the tester turns red or yellow, you know there is a problem with your vehicle’s connector and not the trailer lights. In this case, check that all of your tow vehicle lights are working to rule out a blown fuse. Use a contact cleaner to wipe down the contacts on the connector to resolve potential issues with the plug.
- Look for broken wires connecting to your trailer lights. Check for visible damage to the wires, or take the trailer to a professional for repairs if you suspect the wires inside the frame might be broken or frayed. The white wire is the trailer’s ground wire, the green wire is for your right turn, the brown wire is for your tail lights, and the yellow wire operates your left turn signal and left brake light.
Testing for continuity using a multi-meter
- Set the multimeter to continuity mode and clip it to the green contact on the trailer cord. Make sure that the wires are long enough and you can still be able to reach the back of the trailer.
- Unscrew the cap to the light that does not work to access the wire contacts in the light. Set the cap aside so that you can re-attach it later.
- Touch the green contact under the light with the multi-meter. The continuity on your trailer wire should be close to .6 or .7 ohms. If you do not get a reading, you know the specific wire running to that contact is malfunctioning or broken.
- Unclip the multi-meter and repeat the process on the other wires.
- If all the wires seem to be working, then you may have to clean or fix the contacts on the plug, or there could be a problem with your tow vehicle circuitry.
Why Does Only One Side of my Trailer Lights Work?
If only the lights on one side of your trailer work, you may have a break in the wire. To test for a broken wire, check the color of the wire going to the socket and then find that wire on the connector in the front.
- Clip one end of a long jumper wire to the connector pin and then clip the other end to the continuity tester.
- Probe inside the socket with the tester. You will see the color of the wire that connects to the socket inside.
- If the lights on one side fail to go on, trace the wire and check for bare spots or breaks. If you find a break, cut the wire at the brake, solder a new connection and repair the insulation with heat-shrink tubing (available at hardware stores).
7 Pin Trailer Running Lights Not Working
The best way to fix your trailer lights wiring issue is to eliminate possible problems one-by-one until you have pinpointed the root cause of the malfunction.
- It is possible that one of the bulbs on your trailer burned out. So, check the lights and replace the bulb if burned out.
- If the bulbs are not the problem, disconnect the trailer’s wiring system from the tow vehicle. Test all the lights, from the backup lights to brake lights and turn signals to ensure the problem is not the tow vehicle itself.
- If the vehicle’s lighting system functions properly, use a trailer wiring tester to check the tow vehicle socket. Plug in the tester to the socket and check the indicator(s) to determine if there is an electric current. If the tester responds to each individual system, the trailer wiring system is likely the cause of the problem. If one or several test results don’t provide a response from the tester, the problem might be the tow vehicle itself.
- If the tester illuminates when it should not, it may indicate there is a short in the tow vehicle wiring. Check the tow vehicle wiring to determine if several wires contact one another.
- Several wires could also be connected to improper connection points. If this is the case, use wire cutters, a crimper and a wire stripper to perform a swap.
How to Ground Trailer Lights
You need to ground each lamp either through its mounting studs being in direct contact with the trailer frame or by having a jumper wire ran from the lamp to the trailer frame. Ground the trailer connector at the front of the trailer, which will use the trailer frame as a ground wire.
Common LED Trailer Lights Wiring Problems & Their Fixes
LED trailer lights are not working
- Check that the ground wires for the lights are securely attached to a clean metal surface on the trailer frame.
- Make sure the wiring on the trailer is wire correctly and wire by function, not just by color.
LED trailer lights flickering
- Install a load resistor at each affected LED light. Load resistors mimic the electrical signature of incandescent lights to let the trailer wiring work as intended. This can cause hyperblinking to your trailer’s indicator lights.
LED trailer lights appear dim or have no lights
- Check the trailer light harness at the tow vehicle.
- Check the operation of all your vehicle lights to make sure you don’t have a blown fuse, a bad flasher or a burned-out light.
- Clean the connector and check again.
- Check for broken wires near the connector.
- If the trailer lights still appear dim or aren’t lighting at all, clean the connector plug.
One light out or blinker/stop lights
- Replace the bulb
Troubleshooting Trailer Brake Lights
On a trailer, the brake lights and turn signals operate on the same wire/bulb. When the turn signals work but the brake lights don’t, this could be because of the following possible causes:
- There could be a problem on the vehicle side. To troubleshoot:
- Use a circuit tester to test the vehicle side trailer connector while someone steps on the brakes in the tow vehicle.
- If there is no power on any of the pins for this test, then there is likely a blown or missing tow package, stop light, or CHMSL fuse on the vehicle.
- If there is power, then the issue is on the trailer. A bad grounding can be the culprit. Ground issues normally show up when there is more draw on a system like when you have the running lights on and then use another function like a brake light or turn signal but can show up any time.
- Make sure that the vehicle side and trailer side connectors are clean and free of corrosion inside and out. The main connector grounds should be attached to clean and corrosion-free bare metal surfaces.
- Light grounds should also be attached to clean and corrosion-free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these can interrupt the ground. It helps jumper wires from the light grounds to main trailer connector.
- If the brake lights for some reason draw more power that the tow package wiring can handle, then the lights will either not work or it will blow a fuse. If this is the case then switching to LED lights will solve the problem because they draw much less power as compared to incandescent lights.
Troubleshooting Trailer Lights Turn Signal
Both Trailer Lights Turn Signals Flashing Troubleshooting
When both trailer lights turn signals flash, this is normally caused by one of three issues:
- There could be some corrosion in or on the connectors on the tow vehicle or trailer side or both.
- There could be bad ground.
- Wiring mistakes if the trailer connector is a wishbone type harness (4-way flat but with 5 wires coming out, 2 for running lights)
To troubleshoot:
- First, disconnect the trailer and use a circuit tester to test the tow vehicle side connector for proper functions.
- On the trailer side, make sure the connector is clean and that the ground is connected to a clean and corrosion-free bare metal surface. Repeat the same for each of the trailer lights.
- If the trailer has an aluminum frame, tilt bed, or folding tongue, it will help to run jumper grounds from the lights to the main trailer connector ground.