Troubleshooting 7-Way Wiring Harness Lighting Issues

Question:

I have a trail king double axle trailer the lights used to work. The 7-way connector was broke. Replaced connector. I have brake lights. When it try the turn signals they just stay on solid and i have no trailer lights at all. Any thoughts

asked by: Alex

Expert Reply:

I suggest you test the trailer lights directly, without connecting the trailer to any vehicle, by applying power from a 12V battery. You can use a well-charged spare 12V battery if you have one around or you can use a set of jumper cables to draw power from your vehicle battery and then use short wire leads from the cable clamps to the trailer plug. What you want to do is secure the battery negative to the trailer plug's ground pin. Then one by one you can test each lighting circuit on the trailer by applying the +12V lead from the battery positive terminal to each pin.

Please refer to the linked photo that shows a 7-way trailer plug and the typical circuit assignments for each of the contacts. Apply the battery ground to the pin at the 5-o-clock position and then apply the +12V wire one at a time to each of the circuits. This will let you easily see which trailer wiring circuits work and which do not, without the vehicle's wiring being involved.

You can also check the vehicle socket for any signs of corrosion. Corrosion can build up inside a connector too and this can cause two different circuits to becomes bridged together. This corrosion can cause all sorts of issues with light functions. If the +12V auxiliary trailer power circuit became bridged to one of the lighting circuits, then that light might stay on continuously.

expert reply by: Adam R