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Troubleshooting No Trailer Brake Light or Turn Signal on Left Side  


I have a 2012 Ford f 250 and my left turn signal and brake light will not work on trailers using 7 way or 4way plugs I know the trailer lights work because the lights work on other trucks with these trailers. Here’s my dilemma, when I put a multimeter to the plugs on truck the voltage is pulsing on as the light blinks. Approximately the same voltage for right and left lights at the plug However when you plug the trailer to the truck the left side won’t work. What am I missing?


Expert Reply:

Since you have tested your 2012 Ford F-250 with a multimeter like # PT89ZR, you most likely have an issue with your trailer ground connection. Some vehicles using the same trailer may not have issue as the voltage sent may be ever so slightly different.

The place to start now is checking that left side tail light assembly. You will have a ground connection whether using a separate wire or in the mounting hardware itself which is often used for a ground. Make sure that ground is bare metal to metal connection free of corrosion, debris, even paint. You can test your connection using a circuit tester like # PTW2993.

If that doesn't solve your problem try running a jump wire from that ground to your main trailer ground to see if that solves the issue. This can still be the issue even if your running/parking lights work on the same assembly.

If you need further help, let me know.

expert reply by:
Jason S

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