Troubleshooting 4-Pole Trailer Wiring Harness After 18-Months of Use  

Question:

Expert Reply:

The Tow Ready T-One wiring harness you referenced, part # 118566, is indeed very similar to the 4-pole wiring harness you purchased from etrailer in October 2013, which is a part from Curt. Thank you for that purchase and also for your hitch purchase.

A 4-pole wiring harness such as what you have carries power only for the trailer's stop, turn and taillights. The problem you are having with all trailer lights staying on continuously once the trailer is plugged in to the Escape suggests that the harness may be at fault, but we can help you troubleshoot to find out for sure. The first step is to check the white ground wire on both the vehicle and trailer to make sure both are solidly attached to a clean bare metal surface. A weak ground can cause all sorts of problems. Since this is the easiest possible fix, do this first. If this does not correct the problem, then you'll need to test.

The best way to test is to unplug the trailer from the vehicle and test the 4-pole on the Escape. You will need a circuit tester like # PTW2993. Use of this type of tester is shown in the linked video. Have a helper activate the light functions on the vehicle, one at a time, while you apply the tester to the various pins on the 4-pole. Please refer to the linked photo of a 4-pole that shows where to ground the tester and each of the pins' functions. First test the pin for the brown wire while the car's headlights are on. Then test the pin for the yellow wire while the brake is applied and then while the left turn signal is activated. Repeat for the green wire with the brake and then the right turn signal.

The correct results of these tests are: a continuous signal on the taillight pin when the lights are on, and on both of the other pins when the brakes are applied. The Yellow and green wires should also have an on-and-off "blinking" signal when the turn signals are activated. If this is what you see when you test, then you know the wiring harness is still working properly. If not, then the harness may have gone bad and needs to be replaced.

If you have access to either a different tow vehicle or trailer you can try substituting one to see if the problem goes away or remains. If another trailer works fine, then your trailer is likely to be the problem. If another trailer gives the same results, then your vehicle wiring may be at fault.

If the harness is good, then you need to move on to the trailer wiring. Sometimes bad corrosion within a trailer's plug, or a short from damaged trailer wiring that touches the metal frame, can cause more than one circuit to be bridged together. Under some circumstances this could cause all the lights to stay on. Carefully inspect the trailer plug for signs of corrosion and also check each wire on the trailer all the way from the plug to the light fixtures. Look for cracks in the insulation, especially at points where the wire has to make sharp bends, or where it may touch the trailer frame. Friction and vibration can over time wear into the wires' insulation and could cause a short.
You can try applying 12V power directly to each of the circuits on the trailer plug, again one at a time, using the car's battery or a spare 12V battery if you have one. This will let you see what the trailer lights do independent of the wiring harness.

Damaged spots in the wiring should be repaired, or the wiring can be replaced. If you want to replace the trailer wiring you can use 35-foot wishbone harness # A35W42B. This has dual brown running light wires to make connection to your lights easier. I have linked an installation video for you to look at. I also linked an article on troubleshooting that may be helpful.

expert reply by: Adam R

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