Troubleshooting A 7-Way Trailer Connector With A 4-Way Wiring Harness

Question:

I have a 2015 GMC Denali with a 7 prong outlet to connect trailers. My little cargo pull trailer has a 4 flat wiring scheme...W/B/Y/G I have tried several different connections. the basic one : W-Ground Y-Left Blinker G-Remember got blinker B-Im assuming replaces Brown for brake lights and running lights. No matter what combination I try the rest get blinker always works, when I put the left blinker on all blinkers go on like the hazards and when I put the hazards on it works like it should. I swapped the Y and G but still get the same results.

asked by: Barry

Expert Reply:

In order to help diagnose the problem, I would first need you to check that both trailer connectors are properly wired. You can do this by using a circuit tester like part # PTW2993.

You will then need to have a friend sit in the vehicle and perform each of the trailer connectors functions while testing the corresponding wires for each circuit on the back of the vehicle's 7-way connector both before and after the plug. If you have power to all wires before the connector and not after, the problem is in the trailer connector. If you do not have power running to a wire before the 7-way then you will have to trace the wire from the 7-way back to its corresponding function while checking for breaks or frays in the line. You may also want to check your fuse box for blown fuses.

If you are using a 7-way to 4-way adapter, I also recommend testing this adapter while plugged in to your vehicle to ensure the adapter is working correctly.

Once you have ensured the the vehicle side is wired properly, you will want to test the 4-way connector on the trailer using the Hopkins Tow Doctor Trailer End Test Unit part # HM50928 or by temporarily hardwiring a power circuit to the trailer. You will then want to ensure the wires are attached to the proper functions. The white wire on the four way is responsible for the ground, yellow wire is for the left turn signal and brake, green wire is for the right turn and brake, and the brown wire is for the taillights. Be sure to check that the ground wire is securely attached to a clean bare metal surface on the trailer frame.

If the issue is on the trailer side of things, you will want to trace each wire back from the connector to ensure their are no frays or breaks in the lines.

Once all of the above steps are completed, you can then use a 4-way to 7-way adapter such as the part # PK12716 to mate the two connectors. I have attached our trailer wiring help article for your review.