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Trailer Connector # HM40975 Turn/Stop Light Functions Not Working with 2007 GMC Sierra 2500 HD

Question:

I have a good ground at the plug and the park lights are working. Im testing with tow truck lights. The fuses under the hood for the trailer lights are good. Ive checked the yellow and green at the plug and they are not hot. Where do the yellow and green wires connect to the fuse housing? Ive found the park light at the housing.

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Expert Reply:

You can test the vehicle wiring that leads to U.S. Car connector that plugs into the back of the 7-way # HM40975; this will tell you if the correct turn/stop signals are reaching the 7-way or if there is a problem upstream in the vehicle wiring.

Apply your grounded circuit tester to the yellow and green wires leading to the vehicle's round connector. With the left turn or brake engaged the yellow wire should carry voltage; with the right turn or brake engaged the green wire should carry voltage. If these signals are present on the wires then check the connector's contact surfaces for any signs of discoloration or corrosion. Even a slight greenish or whitish film on the contact surfaces can interfere with signal transfer. Clean the contacts as necessary. If the turn/stop signals are present on the wires leading to the connector but the connector still does not pass these signals after cleaning, then it may require replacement.

The green and yellow wires do not lead directly to the fuse box; they will attach at some point along the vehicle frame to another connector that leads to the front of the vehicle. You can also test at these various points to find where the signal is lost.

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