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Troubleshooting 2007 Subaru Outback Wagon OEM 4-Pole Trailer Connector and Boat Trailer

Question:

I have a 2007 outback with a dealer installed hitch and 4-pin harness. My boat trailer has a 4-pin as well. However, there are noticeably 5 wires leading into the trailer end 4 pin connecter. When connected, trailer lights go on, but there are no brake/turn signals. Do I need to update my vehicle side wiring with a converter? or but a new harness such as #118461? Thanks.

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Expert Reply:

There are several potential causes for some trailer lighting functions to not work while others do. It could be something as simple as a bit of corrosion on the connector pins (which can appear as a slight whitish or greenish film), a blown fuse or missing relay in the tow vehicle, a short in the wiring somewhere on the vehicle or trailer, or an insufficient ground. You can do a visual inspection of the wiring to check for worn spots or cracks in the insulation that could cause a short. Also check that the white ground wire on both the vehicle harness and trailer harness is firmly affixed to a clean unpainted spot on the frame. A bad ground may allow enough current for some light functions to pass through but not allow enough current for all light functions. I have linked an article on troubleshooting that may help you.

The first step in troubleshooting your vehicle and trailer wiring is to disconnect the trailer and test the 4-pole on the vehicle with a circuit tester like the Draw-Tite # PTW2993. This will let you know if the issue is in the vehicle wiring or the trailer wiring. If the vehicle 4-pole displays the correct signals when the various functions are activated, then you know to move on to your trailer for further troubleshooting. The linked video shows how to do this 4-pole testing on your vehicle.

If your Outback does not pass the correct signals then its wiring may be damaged and require replacement. The Tow Ready quick-install harness you referenced, part # 118461, will work as long as your Outback is not the Sport model. I have linked an installation video showing use in your year/make/model vehicle as well as written instructions. You can use part # 119147KIT if you have the Outback Sport. This is a hard-wire kit that will require you to splice a few wires. It is possible that a fault in trailer wiring, such as a dead short, can damage a powered converter; you should inspect the trailer connector and wiring for any signs of damage before installing a new harness just to ensure the new unit is not damaged by the same fault.

If the vehicle 4-pole wiring functions correctly you can next test the trailer wiring and lighting. You can use the tow vehicle or another 12V battery to test the trailer light circuits one at a time. Apply the ground lead from the testing battery to the ground pin on the trailer 4-pole, and one at a time, apply the hot 12V power lead to the three other pins. With 12V power going directly into each trailer light circuit you will be able to see if the trailer lighting functions correctly independent of the vehicle 4-pole harness.

It is possible that the 5th wire on your trailer harness is a duplicate running light wire, part of what is called a wishbone harness. Some trailers use dual running light wires to avoid running a jumper wire from one side of the trailer to the other. Since running light signals are the same for both sides of the trailer, either method works.


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