Trailer Lights All Come On w/Running Lights Turned on and Trailer Connected to 2010 Volkswagen Golf  

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Adding trailer wiring to a Volkswagen can be tricky due to the complicated electrical system in them. I know that on the 2010 Golf TDI wagon models the tail light wire is gray with a black stripe, brake lights are black with a red stripe, ground is a brown wire, left turn signal is a black wire with white stripe, and right turn signal is black with a green stripe. You can test those wires to see if they are in fact carrying those signals or if different wires are used on your Golf.

The way to test the converter box is to use a circuit tester such as # PTW2993 or a meter if you have one. Have someone sit in the vehicle to operate the lights. Since the issue occurs when the running lights are on go ahead and have them on when performing the test. Have the person operating the lights work the brakes and turn signals while you test the input wires going into the converter from the vehicle.

While testing the running light wire should have power all the time since it is on during this test. If any of the other wires have power then something is not connected properly. If a turn signal is activated then that turn signal/brake wire, along with the running light wire, should have power. Any other wires with power would indicate an input error. The same would go for the right turn signal/brake light.

If everything checks out fine on the input side the next part of the test is to test the output wires (4-Way) in the same manner. If everything is good going in but not coming out then it is a problem inside the converter box and the whole harness will need to be replaced.

If everything is good going in and coming out then the issue is on the trailer; likely a bad ground or a short. This sounds like a ground issue. Ground problems normally show up when there is more draw on a system such as when you have the running lights on and then use another function like a turn signal or brake light but can show up any time.

Make sure that the vehicle side and trailer side connectors are clean and free of corrosion inside and out. The main connector grounds should be attached to clean and corrosion free bare metal surfaces.

The light grounds should also be attached to clean and corrosion free bare metal surfaces. If there is anything between a light ground and the metal trailer frame or if the trailer has an aluminum frame, tilt bed, or folding tongue, any of these things can interrupt the ground. Running jumper wires from the light grounds to the main trailer connector ground can help.

expert reply by: Michael H

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