Troubleshooting Tail Light Issue on a Trailer  

Question:

Expert Reply:

There are a few different things that could cause taillight failure. Issues that you will want to look for are a weak ground connection, a short on the taillight wire or a possible issue on the vehicle side.

To start, I like to test the trailer connector on the tow vehicle to make sure power is reaching the trailer. I recommend using a circuit tester with an incandescent bulb, like part # PTW2992. You will want to test the pin on the trailer connector for the taillight function. If you have a 4-way or 5-way flat trailer connector, then it will be the pin next to the exposed ground pin (see photo). If you have a 7-way trailer connector, then it will be the pin in the 11 o'clock position.

If you have power on that pin with the vehicle running lights on, then we know the problem is on the trailer. Start at the trailer side connector and look for any corrosion or debris and repair or replace as necessary. Next, check the grounds for the trailer connector, the marker lights and the taillights. Make sure they are grounded to a clean bare surface on the trailer frame. If the grounds look good, follow the taillight wire from the trailer connector to each light. Look for any sections that may be pinched, frayed or damaged. This could cause a short and cause the lights to not work.

If you do not have power for the taillights at the vehicle side connector, then the issue is likely on the vehicle. If your vehicle has a factory tow package, then this can sometimes be caused by a blown fuse or a bad relay. Your owner's manual will have information on fuse and relay locations. Check for the trailer taillight fuse or relay and replace if necessary.

If you are using an aftermarket trailer wiring harness, then you will want to inspect the wiring harness. Check for any loose connections, damaged wires or poor grounds. If it has a converter box, you can test the taillight wire going into the input side of the converter box by probing the wire with the circuit tester. If there is power going into the converter box, but not coming out, then the issue is likely a blown converter box and it would need to be replaced.

I have attached a trailer wiring help article that you can use for reference when troubleshooting your wiring issue. If you are not able to find a solution or the problem, let me know what you come up with after following these steps and we can do further troubleshooting.

expert reply by: John H

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