Trailer Lights Not Working with Wiring Harness Installed on a 1995 Ford Ranger

Expert Reply:

To determine definitively if the converter is bad use a circuit tester like # PTW2979 to test the wires going into the converter box. If the signals are correct going in, test the 4-Way end. If they are not correct coming out of the box then the converter is bad and it will need to be replaced.

You will not be able to just eliminate the box. The issue is that the vehicle has what is called a separate 3-wire system meaning that all the functions are on separate circuits (brake lights, turn signals, and tail lights). The trailer on the other hand uses a 2-wire system with the brake lights and turn signals combined. This is what the converter box does, it takes the separate signals from the vehicle, combines them, and delivers it to the trailer.

When you test the converter, if the wires going into the converter are not correct then the issue is on the vehicle wiring somewhere. If the signals are correct going in to the converter AND coming out then the issue is on the trailer. One easy way to check is, if you have another trailer, plug it in to see if it works. If it does then the issue is with the trailer. And if you have another vehicle with a trailer connector, plug in the trailer to see if it works with a different vehicle. If it does then you know the problem is with the vehicle.