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Troubleshooting Trailer Turn Signals That Do Not Operate When Headlights Are Turned On

Question:

Trailer turn signal lights do not work once the headlights are put on of the vehicle thats pulling it

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Expert Reply:

The problem you describe sounds like a classic case of a bad ground connection on either your vehicle's trailer wiring harness or the one on the trailer.

When you turn on your vehicle headlights you also activate the trailer's running lights. Since these lights remain on continuously they draw more current through the trailer wiring. If the ground wire connections are loose or corroded the wiring will not be able to pass all of the required 12-volt power the lights need and this can cause one or more light functions to stop working. The fix is easy enough.

Remove the white ground wire's ring terminal and sand off that grounding point to remove any rust, grease, dirt, paint or undercoating. The ring terminal needs to make contact with bare metal in order to establish a good ground connection. You can do this on either the vehicle or trailer harness to see if that corrects the issue. If not then repeat this process on the other.

Once you get a good ground and have everything working properly I suggest applying protective dielectric grease # 11755 to keep water and dirt from degrading those connections.

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