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Troubleshooting Tow-Ready T-One Vehicle Wiring Harness # 118449 in 2012 Toyota Highlander  


Hi - I purchased a 4 pole to 7 pole multi-function adapter your part #37185 along with a T-one vehicle wiring harness with 4-pole connector your part #118449 in April and successfully installed it on my 2012 Toyota Highlander. I have used the 4 pole plug on part #37185 successfully, however this week when I used a trailer with a 7-pole plug, the turn signals and brake lights on the trailer did not function. Ive re-checked the installation instructions but it should work. Have you seen this problem before and do you have any suggestions on how I can use these products to get trailer lights and signals using a 7-pole plug on the trailer? Many thanks.


Expert Reply:

Thank you for your recent purchase from etrailer.

There are several possibilities for the trailer lighting issue you are having. First, I suggest you test the trailer that has the 7-way connector, either by connecting it to another vehicle with a 7-way or by directly testing it with a 12V battery. You can apply 12V power directly to the trailer connector's pins to see if the lights function. If they do, then you know the problem is upstream of the trailer; if they do not you need to troubleshoot the trailer's wiring for signs of damaged or corroded wires or connectors. Do note that a short in a trailer's wiring can damage a wiring harness; if you find any issues with the trailer wiring these should be corrected before re-connecting the trailer to your Highlander.

If the trailer functions, then you can focus your efforts on the Highlander's wiring. First, disconnect the # 37185 adapter from the harness's 4-pole and test the 4-pole directly with a circuit tester like # PTW2993 while a helper activates the various lighting signals. If the harness sends out the correct signals on its 4-pole, then the problem is either in the 7-way adapter or its connection. You can apply the tester to the 7-way's pins for the lighting functions (see the linked photo of a 7-way for which pins to test). If it does not send out the correct lighting signals, then apply the tester to the wires leading into the converter box (on the left side). Test the green wire with the right signal activated; the yellow wire with the left turn activated and the red wire with the brakes applied. If the correct signals are entering the converter but not coming out of it then the harness is damaged.

Also check both connector's contacts for signs of even slight greenish or whitish discoloration; this mild corrosion can still interfere with signal transmission.

The linked article will give you additional guidance for troubleshooting.

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