Troubleshooting 12V Circuit in 7-Way Trailer Wiring

Question:

I have a 7 pole tester and have determined the 12v pin isnt working. How do I diagnose the issue?

asked by: Andrew W

Expert Reply:

I can give you some general troubleshooting help for your 7-way wiring setup that does not have power on its 12V auxiliary circuit.

First, you can check your vehicle's power distribution (fuse) panels for a missing or blown fuse or a bad relay. Some vehicles can have power boxes in both the engine compartment and the passenger compartment so it is best to check your vehicle owner's manual to find all fuse locations related to trailers/towing. There should be one 30- or 40-amp fuse or a relay specifically for auxiliary 12V power, which should appear on the 7-way contact at the 1:00 position (see photo, please). This is often called the "trailer battery charge line".

Please note that some vehicles' 7-way will not carry 12V power unless the key is turned on. Others will have a circuit that remains inactive until you plug in the trailer and shift the vehicle into gear.

You may have power making it part of the way from the power box; if there are connectors under the vehicle between the power box and the 7-way then check these too, for signs of corrosion, damage or simply a loose connection.

If you used a circuit tester like # PTW2992 to test your vehicle socket and it has power, but this power is not reaching the trailer, then you'll need to inspect your trailer's 7-way setup all the way from the plug back to your junction box like # HS381-9000 or the battery or other devices connected to that circuit. You could have an issue with corrosion inside the plug, a pinched wire that is shorted, or a wire that has worn insulation. You could even have simply a loose connection somewhere. Tracing the trailer wiring from end to end is really the only way to isolate the issue.

Please feel free to let us know the result of your testing an inspection and if necessary we can provide additional recommendations. In that case it would also be helpful to know the specifics about your vehicle year, make, model.