Troubleshooting Trailer Running Light Not Working  

Question:

Expert Reply:

If by main light you refer to the tail light (also known as a running light) on your trailer that should be on whenever your vehicle headlights are on, this function not working on the left side of your trailer could be a bad light bulb that needs replacement; it could be a result of loose wiring at the light; it could be from corrosion (either within the light itself or in the vehicle/trailer connectors); or it could be a result of a weak ground connection at the trailer or vehicle wiring.

It often happens that when the main ground wire for the vehicle or trailer wiring harness gets loose, develops corrosion or simply gets packed with dirt and road grit, the running light function will be the first one to develop trouble.

Lights that are only on briefly, such as brake lights and turn signals, can still work when the running lights do not because running lights stay on continuously and draw much more current. A weak ground can allow lower-current light functions to work okay when the running lights do not.

The easiest troubleshooting you can do is to disconnect the trailer from the vehicle and test the trailer on its own using a spare battery or the one in the vehicle. Use a length of wire to connect the battery negative terminal to the ground pin on the trailer connector. Then connect a wire to the positive terminal and apply it to each trailer contact, one at a time, to see if all the functions work. If they do, then this tells you the issue may be on the vehicle's harness.

To test the vehicle wiring use a tester like # PTW2993 to check for the correct signals on the individual contacts while a helper activates each function.

expert reply by: Adam R

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