Troubleshooting Tekonsha 4-Pole Harness 118379 Installation in 2004 Toyota Tacoma

Question

Question:

Several months ago I purchased a 4 pole trailer wiring harness from you for my 2004 Tacoma. I installed it by following the YouTube directions on line and it had worked well until recently it has begun to blow the fuses for my tail and signal lights on the truck. The little black box appears dry and all wiring is in good condition still. The ground seems secure so Im just wondering what else to look for.

asked by: Peter

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Expert Reply:

Thank you for your recent purchase of Tekonsha T-One 4-pole wiring harness # 118379 which is a confirmed fit for your 2004 Toyota Tacoma.

The two most common causes for a wiring harness to blow vehicle fuses are a short in the trailer's wiring or an overload situation where the total power drawn by the trailer's light exceeds the output capacity of the harness. If you have been using the same trailer with the same complement of lighting all along and it suddenly started having this issue the odds are in favor of a short on the trailer wiring. If you recently added more lights or changed any lights on the trailer it could be a case of overload. It is easy to test both the harness and trailer individually, while they are disconnected, to help identify the culprit.

If you have a circuit tester like # PTW2993 you can have a helper activate each light function on the Tacoma, one at a time, while you apply the grounded tester. Please refer to the linked photo that shows you which pins carry which functions. The brake light signal should appear on both the yellow and green wires' pins; headlights turned on should deliver a signal to the pin for the brown running light wire. On-and-off blinking right turn should appear on the green wire and left turn on the yellow. If you get these results, then the harness is okay, and you can move on to the trailer connector.

Use the Tacoma's battery or any 12V spare such as a lawn mower battery to apply power to the trailer one circuit at a time, as you did when testing the truck. Any malfunction of the lights indicates a short on the trailer. In this case trace and inspect all wires, from the trailer plug all the way back to the lights where they connect. Look for pinched wires, wires that rub against the trailer frame and any that may have their insulation worn off or cracked. Any of these could create a short that would blow fuses.

You can also check for corrosion on-in the trailer plug or its contacts. It can happen that water can get into a connector and cause corrosion internally that is hard to see. This condition could result in two circuits becoming bridged together which would blow fuses.

expert reply by: Adam R

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