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Troubleshooting 4-Way Flat Trailer Wiring That Blows Running Light Fuse in 1996 Toyota 4Runner

Question:

My tail light fuse keeps popping on my 96 4Runner as soon as I turn on the running or headlights when my boat trailer is hooked up to my truck. If I dont turn on the headlights/running lights when the trailer is hooked up, my brake and turn signals work fine on truck and trailer. When I use the 4 pin tester on the truck all work except the tail light. Does that mean that the trailer wiring adapter on the 4Runner might be shot? When I used a test lamp on the truck pigtail to test the tail light wire, it worked however it was grounded to the bumper. So Im thinking that maybe the truck ground or the wiring adapter might be dead. Am I on the right track? Thanks.

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Helpful Expert Reply:

The problem with your running light fuse blowing as soon as you turn on the 4Runner headlights could be caused by any of several problems.

One possibility is that the wiring harness you have on the 4Runner does not have the required current capacity for all of the lights connected to the running light circuit. We offer two 4-pole harnesses for your vehicle; part # 118341 and the higher-current-output Curt harness # C55341. If you recently added extra lights to that circuit, then that could explain the issue. In this case you might need to replace some of those lights with higher-efficiency LED type lights that need only about 1/10th the power of typical incandescent lights.

You may also just have a damaged wire in your running light circuit. Check for cracks or worn spots in the insulation, especially where the wire has to make sharp bends and where it comes close to making contact with the truck frame.

Another possibility is corrosion/damage to the vehicle connector, your adapter or trailer connector. It can happen that corrosion will build up on a connector's contacts, even inside the connector, and cause two different circuits to become bridged to together. This would also explain the blown fuses.

If your testing shows the 4-pole works fine without the adapter, then you know the adapter is the likely cause for the problem and it should either be cleaned or replaced.

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