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Trailer Lights Do Not Work when Vehicle Lights are On, 2004 Honda Odyssey

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Hi, I have a similar problem that I saw in Ask The Experts. The hookup of the 118336 Wiring Harness to my 2004 Honda Odyssey was very easy. When I connected my trailer lights, they did not work properly. With the headlights on, the trailer lights do not work. With the headlights off, both rear lights on the trailer blink when I use either the left or right blinker in the Odyssey. When hitting the brakes, both rear trailer lights work as expected. In all cases, the side lights on the trailer do not work. I want to start trouble shooting, but I need the electrical schematic for the 118336 harness. The 118336 harness appears to be color coded to match the trailer wiring. There is a Green, Yellow, Brown and White wire on both the 118336 harness and the trailer connector. How should these wires test when I turn on blinkers, emergency lights, headlights, brake lights, etc? I want to make sure the 118336 harness is working properly before starting to look at the trailer lighting.

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Expert Reply:

This sounds like a classic ground problem. A bad or weak ground will show up when there is more load on the system. That is why when you turn the vehicle lights on, the constant draw of the trailer running lights creates more of a load and the bad ground shows up.

There are some common places to check for a bad ground. First make sure all of the trailer lights and the main trailer connector are attached to clean and corrosion free bare metal surfaces. You mentioned having side lights on the trailer. If the running lights draw more than 4.2 amps combined then will overload the system and cause the trailer lights not to work.

To test the converter you will need to use a circuit tester like # 40376 unless you already have one. First, disconnect the trailer then test the wires that go into the converter for the proper signals. Green should be right turn, yellow is left turn, brown is running lights and red is brake lights.

If the signals are not right going into the converter that indicates a problem with the vehicle wiring. If they are correct test the wires coming out of the converter. White is the ground, brown is running lights, green is right turn/brake lights, and yellow is left turn/brake lights.

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