Troubleshooting Running Lights on Trailer Not Working

Question:

running light on trailer does not work but brake and turn signals do - is it a fuse or relay and where is these fuses and relays located

asked by: James L

Expert Reply:

There are a number of troubleshooting steps you can take in order to determine the specific problem, but if these were working before it likely isn't a fuse or relay, it is almost certainly the wiring on your trailer or tow vehicle. To start you will need to use a circuit tester such as # PTW2979 to test the connector on your tow vehicle after disconnecting it from your trailer. Check the wiring going into your harness from your tail lights to see if those wires are carrying the correct signals, if not then the issue is with the vehicle wiring.

If the signals going to your harness are okay, you'll need to test the signals coming out of the harness, and if they are correct then your issue is likely going to be with the trailer side wiring. You will also want to check both your vehicle and trailer connectors for corrosion, which can be green or white in color. A build-up of corrosion can interfere with the signals getting through the connectors so if it is present you will need to either replace the plug or clean the corrosion and be sure to use Dielectric Grease # LT37534 to help stop any future corrosion.

Another important issue you will want to take a look at are the ground connections, as they can be the cause of numerous lighting problems. You'll want to make sure your grounds are securely attached to bare metal surfaces free of any paint, dirt or corrosion of any kind. Also check your wiring thoroughly to ensure there are none that are crimped, broken or otherwise damaged. If your running lights are incandescent and not LED it may also be an overload so check your harness to determine what its max amperage is to be sure your lights are not drawing more power than the harness is made for. You can also check the lights on your trailer directly by using an external 12V battery.

I have added links to a pair of help articles on trailer wiring, including a troubleshooting guide which I highly recommend taking a look at.

expert reply by: Kyle S