Trailer Brake Lights and Turn Signals Not Working When Running Lights Are Turned On  

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Expert Reply:

Since you have just re-wired your trailer with new wiring and lights, the cause for the problem with brake and turn signals not working when your running lights are on is likely to be either a grounding issue or that the total current demand from your new trailer lights is more than can be handled by your wiring harness.

A weak trailer ground connection can often allow enough current for some lights to pass, but not enough for all lights, especially the running lights which remain on continuously once activated, unlike brake and turn signals which are on only some of the time. Check that your trailer connector's ground is tightly secured to a clean bare metal surface on the trailer frame. Also check that each individual light's ground is secure. Some lights use their mounting studs to make the ground connection while others use a separate wire for ground. Either way, make sure that ground connection is solid.

You can also check the current rating of your vehicle wiring harness to make sure that all of your new trailer lights do not exceed its power rating. If this is the case you might replace some of the incandescent lights with LED versions that draw only about 1/10th as much current as incandescent bulbs.

Since I could find no purchase history for you I am not sure of your vehicle or the wiring that you are using. If you'd care to let me know your vehicle year/make/model/version and the wiring kit you're using I will be happy to offer additional recommendations. In the meantime you can try the trailer with a different tow vehicle if possible. The linked troubleshooting article may also be helpful.

expert reply by: Adam R

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