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Truck and Trailer Left Turn Signal Will Not Work when Truck Headlights are Turned On

Question:

All trailer and truck lights signal everything. When headlights are turned on the left truck and trailer turn signal will not function?

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Expert Reply:

It sounds like there is a bad ground connection. Bad grounds can cause all sorts of strange things to happen. The first thing you are going to want to do is disconnect the trailer from the truck. Then use a circuit tester, # PTW2993 if you need one, to test the connector on the truck. Have someone sit in the truck and run through the lighting functions (with and without the headlights on) while you test the pins on the connector.

If the signals are not correct on the vehicle, then make sure that the connector is clean and free of corrosion. Trace the wiring as far as you can and check for breaks in the wiring or places it may be shorting out.

If the truck checks out fine, then move on to the trailer. Start with the trailer connector and make sure it is clean and free of corrosion. Make sure that the main trailer connector ground and the grounds for all the lights are attached to clean and corrosion free bare metal surfaces.

If the trailer has a lot of lights on it, it may be overloading the amperage rating of the tow package wiring on the truck. It is also possible that there is a loose connection or frayed wire somewhere on the trailer that you will need to track down and fix.

If you recently added a plastic license plate frame between a light and its ground connection that can cause some problems. It the trailer has an aluminum frame, tilt bed, or folding tongue then running jumper grounds from the lights to the main trailer connector ground can help. I have included a link to an FAQ article that should help.

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