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Left Turn Signal and Brake Light Do Not Work and Running Lights Blink When Right Turn Signal Is On

Question:

I am yet another casualty of issues with trailer lights. I have a 7-pin blade connection between my SUV and my trailer. I have tested the SUV using a test lamp and all tests worked 100. Most light combinations on my trailer work ok as well. However, I have a few combinations that dont. My left turn signal/brake never turns on at all. I have checked te bulb and it is working fine. Also, when i turn on the right turn signal, it works fine unless the running lights are on. If the running lights are on, they all blink very dimly while the right turn signal blinks brightly like it should. As expected, the running lights turned on by themselves work fine, the right turn signal turned on by itself works fine, and the right brake works fine. I have tried seemingly every combination i can think of to get this to work but nothing has fixed the problem yet... Any ideas?

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Expert Reply:

You took the exact first step I would take when trying to solve this issue by testing the trailer connector on your vehicle. That tells us that the issue is definitely on the trailer somewhere and it sounds to me like it is a bad ground.

The next step that I would take is to make sure all of my ground connections are good and clean. There will be a ground connection coming from the trailer connector to somewhere on the trailer tongue and each of your trailer lights should have a ground connection through either a ground wire ran to the trailer frame or grounded through a mounting stud on the light assembly.

If the issue persists after you have checked all of the ground connections, then you will want to check for cut, pinched or damaged wires. You could also use a circuit tester and test the wires at different sections of the trailer to see if you can locate where the signal terminates. I would also recommend checking the inside of the connector for any corrosion.

If there is a plastic license plate bracket, light brackets bolted to the trailer frame or if you have a tilting style trailer, then you will need to check each of these for ground connections as well. If you have a tilting style trailer, then you may need to run a ground strap from the tongue of the trailer to the tilting portion of the trailer to provide a solid ground through the hinge point.

I have attached an article on trailer wiring that may help you locate the different wires on your trailer and see how they are ran to the lights.

expert reply by:
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John H

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