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Troubleshooting Trailer Lights Not Functioning When Tow Vehicle Headlights Are Turned On  


when I hook up my trailer and then head lights are turned off the trailer lights function normal brake light turn signals hazard. but when I turn the headlights on they do not work


Expert Reply:

Since I do not know what tow vehicle or wiring harness you have I will try to give you the best general guidance I can. Usually the best way to troubleshoot your wiring issue is to disconnect the trailer from the truck's 4-pole and test each one individually.

However, based on your description of the problem - all trailer lights working until you turn on the vehicle headlights - there is one most-likely cause that should be easy to correct. This type of issue is typically related to a bad ground wire connection.

If the ground connection for the wiring harness on your truck is not completely solid, or if it is connected at a point that now has rust or grease, it may not allow enough current to pass through. Some lighting functions like turn and stop lights that do not remain on all the time will need to draw less power and they may operate properly, even when there is a ground problem. But when always-on lights, your headlights, are turned on the total current draw may be more than can be passed by the harness due to the weak ground connection.

The easiest potential fix is to make sure the harness ground wire is tightly affixed to a clean bare-metal surface that is free of dirt, grease and corrosion. You can try cleaning the surface of the existing ground point or you can make a new one on a clean spot on the vehicle frame.

Another potential issue is that your trailer's total lighting system is overloading the harness. Trailer wiring harnesses carry a specific maximum current capacity. For example, # 118578 has a maximum rated current capacity of 4.2-amps for the stop/turn circuits and 7-amps for the taillight circuit. If you happened to have recently added extra lights to your trailer it is possible that the harness just can't handle the total power demand.

If you have a circuit tester like # PTW2992 you can test the vehicle's 4-pole to see if the correct signals are present (with the trailer disconnected). Please refer to the linked video.

You will need a helper to activate the brake, turn and head lights one at a time while you apply the grounded tester. On the 4-pole pin for the yellow wire you should detect a steady signal when the brake is pressed and an on-and-off signal when the left turn signal is on. On the pin for the green wire, you should detect a steady signal when the brake is pressed and an on-and-off signal when the right turn signal is on. On the pin for the brown running light wire you should detect a signal only when the vehicle headlights are on. If this is what you find when you test the vehicle, but the problem still persists when you connect the trailer, then you do most likely have a ground issue.

You may want to refer to the linked article for additional help.

expert reply by:
Adam R

Kurt H.


Just hooked up new LED lights on my trailer. When I hit brakes in my truck, trailer lights illuminate but they are just as bright as my running lights. When I turn on my head lights on my truck, then my trailer lights show the brightness of my brake lights. Please help.

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