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I've just acquired a 2010 Honda Ridgeline. The truck was equipped with a 7 pins trailer plug. I have very little knowledge on the history of the truck, so no idea how it was installed, if it was used or even if it ever worked. Now I want to connect my basic trailer that has 4 pins. I've bought a 7 pins to 4 pins adapter and connected everything, however neither the lights, the flashers or the break lights turn on on my trailer. My trailer is find as I have tested it yesterday with my Honda CRV that has a 4 pins plug and everything worked flawlessly.

I wanted to test if the power was getting to the plug, so I used my voltmeter to calculate the voltage coming out of the plug. That is where it gets weird. If I fire up the car, turn on the light and the hazard flasher, I get a normal (I guess) ~13V out of the light pin, but only ~0.30V out of both left and right flasher/breaks lights. The same thing happen if I just hit the breaks instead of the hazard lights. It's weird because with those voltage, I would have expected to lights to works, but not the flasher/break lights. However, neither worked.

My 7pins to 4 pins adapter is not likely the problem has I'm getting the same voltage reading with or without it.

The plug's wires attached to my hitch run into my frame and disappear (meaning I can't follow it) around the back wheel drive side. They are not connected to my truck tail lights.

There is also a tiny white wire that is on my trailer plug that isn't connected to anything. I though it may be a ground and when I touch it to my trailer frame, it make some sparks. However, connecting it to the frame did not solve the issue. Here is a picture to clarify this point (it's the white wire): enter image description here

Am I missing something basic about connecting a 4 pins trailer to a 7 pins plug? It's my first time trying it. If not, any ideas what to look for, where to check? I couldn't find fuses or any controller for my trailer lights because I don't know what to look for.

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You should be getting +12V on each line when activated. I think you need to trace back wherever this was installed and I'm guessing its not OEM. I'd put money on the fact that its wired underneath the drivers side panel. Possibly also clean the connector and check that bending the cord while measuring doesn't give you a higher or lower reading showing a faulty connection.

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So it seem I had 2 problems. The ground not connected to the trailer prevented the lights to turn on (normal night lights). Connecting it fix the lights, however it didn't fix the break lights. To fix that, I found the installation instruction for the original hitch at: https://www.hondapartsnow.com/genuine/honda~trailer~hitch~harnes~08l91-sjc-100c.html

On this site, I founc a pdf where I could see all the fuse and other parts of the trailer hitch. I check all of the fuses and found 1 that was missing and another that was blown. I replaced them and it fixed my problem.

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