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Troubleshooting Trailer Lights With New Harness and New Trailer


New trailer, new light harness, etc but the lights are not working properly. When I press the brake I get brake lights on the trailer, when I use the Rt and Lt turn signal BOTH lights flash on the trailer, when my headlights are on I have tail lights but no flashing turn signals on the trailer. Vehicle has been tested using a Hopkins plug in tester and all lights work properly there. Ground from new harness has been grounded to bare metal and ground has been checked at the coupler to make sure it’s tight and it doesn’t look like any wires are broken, they shouldn’t be as it is a brand new trailer. I haven’t checked the ground on the trailer lights as it looks like it is grounded from the bolts onto the trailer and no ground wire is coming out of the back of those lights like some of the trailer lights I’ve seen. Please help. I will probably call the dealer tomorrow and go see if the other trailers are doing the same thing when I plug them in. Don’t know if they will let me return the trailer or not.


Expert Reply:

Normally when you have bleed over like this it is due to needing to clean connections or fix a ground wire. I recommend using an incandescent tester like # PTW2993 to test each function at the vehicle side connectors and then again at the tail lights of your trailer. If you get proper function at each of those locations, then you know the wiring isn't the issue. If you don't then the ground connections need to be checked.

If you have LED lights, and the power wires and ground connections all check out, then sometimes it has to do with the minimal power the functions draw and adding resistors to the 3 power functions makes everything work correctly. You can use a Load-Resistor Kit # DI34ZR to draw the power of an incandescent light to assist in helping functions at the lights work correctly.

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