Troubleshooting Running Light Circuit on Wiring for 2011 Mazda CX 9 Trailer Harness  

Question:

Helpful Expert Reply:

The most common cause for some trailer light functions to operate while others do not is a bad ground connection at either the vehicle wiring harness or the trailer-side wiring. This can happen because a weak ground will still allow some current to pass through - say enough for brake lights and turn signals that are not on all the time - but not for other functions like the running lights and license plate light that ARE on continuously.

Check that the white ground wire's ring terminal on the CX-9's harness is tightly affixed to a clean bare metal surface that is free of paint, primer, grease or corrosion. You can use a touch of sand paper if needed to clean up the wire's attachment point. Also check that the trailer's ground wire is similarly attached to a clean bare metal surface on the trailer.

If that does not correct the problem check both vehicle and trailer connectors for any signs of trapped dirt, water or even slight greenish or whitish corrosion - any of which could interfere with signal transmission.

You can also test the trailer lights directly by applying 12V power to the pin for the running light circuit, which is most often carried on a brown wire. If the lights work when you give them a direct power input then you know the issue is on the vehicle side or within the connector.

Dielectric grease like # 11755 is a great way to protect those ground connections once everything is working.

expert reply by: Adam R

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