Troubleshooting Trailer 7-Way Wiring With Constant Power on Lighting Circuit

Question:

A while back Adam R. responded to a question asked by Denis. He asked: For some reasons there is constant power going to the right turn pin #6. I causes my trailer light on the right side to stay on all the time. Even when i use the right signal light in my vehicle, the trailer light stays on. It even stays on when my vehicle is off. Any suggestions on what the problem is? I have the exact same question, except that I have confirmed that the power to the 12v Power pin is not active as there is no fuse allowing current to flow through that wire. The right side brake light wire does have power coming through it most of the time. It is somewhat intermittent, but most of the time it is live. My question is, where do I look next? The turn signal lever or ??? Looking for where to look next.

asked by: Stu B

Expert Reply:

When power appears on a vehicle's trailer wiring when it should not the reason is usually a short or corrosion. The short-circuit or corrosion can let power from one circuit bleed over to another. Sometimes you can see the problem at the trailer connector but other times you may not find it until you disconnect things.

I understand that you have checked the fuse box and found that there is not a fuse in the position for the trailer 12V power circuit. Please note that the 2006 Suburban has three separate fuse panels, according to the online manual that I reviewed. One is under the hood and the other two are within the dash. There is one at the left side of the instrument panel and one in the center. These are shown in the manual on pages 5-110 through 5-117.

Just to be absolutely sure that the 12V circuit is not causing the problem I suggest you apply a circuit tester like # PTW2992 to all of the contacts on your Suburban's 7-way trailer socket, one by one. If you do this with the vehicle off you'll be able to narrow down the cause for the always-active right turn signal circuit. Ground the tester and then apply the probe to each circuit, one at a time.

I also suggest you unplug the 7-way socket (one such as items # 30955 and # HM40975) from the round U.S. Car connector that plugs in at the back of it. This lets you also test the contacts on that U.S. Car connector. If there is a corrosion problem there in that connection it might not appear on the socket.

If the wiring is the original OEM 7-way setup then based on the vehicle's age I would definitely consider this a likely possibility. You can repeat this process as you move upstream on the wiring.

Since you mention that the Suburban turn lights function correctly (even when the trailer turn lights do not) I would not expect this to be related to the turn signal switch itself. If that were the case then the Suburban lights would probably not function.