Trailer Running Lights Come on Even When Vehicle Lights Are Off After Re-Wiring Trailer Connector

Updated 04/28/2015 | Published 04/26/2015

Products Featured in This Question

Ford Replacement OEM Tow Package Wiring Harness, 7-Way (Super Duty)

(423)

$68.90

Hook Probe Circuit Tester

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$12.41

Question:

I have a 2005 ford f250 with the factory tow package,recently the 7 pin plug from my trailer was damaged,when replacing the plug I got the hot and the running light wires reversed and when I connected the trailer to the truck I blew a fuse. I corrected the wiring problem on the trailer plug and replaced the blown fuse but now as soon as I connect the trailer to the truck the running lights are on without the headlights or parking lights being turned on,What did I fowl up?

asked by: Carl R

Expert Reply:

It sounds like you may still have a mis-wired trailer connector or that there could be a corrosion problem in the vehicle's trailer socket or on the trailer's plug.

If your trailer's running lights are coming on as soon as you plug in the trailer connector (even when the truck lights are off) this tells us that power from another circuit is reaching the wiring for the running light circuit on the trailer.

If you look at the photo I linked for you you'll see that the adjacent socket contacts at the 11- and 1-o-clock positions carry running light and 12V power signals, respectively. If there is corrosion on or around these two contacts (or on the back side of the socket) these two circuits can become bridged together. You can inspect and clean both sides of the socket as needed. The linked article shows this and the trailer-side connector and the usual wiring configurations.

A circuit tester like # PTW2993 will let you test the output at the truck's socket (while a helper activates each function in the truck). If testing the truck socket shows the correct signals are carried on each pin, then you know the issue is on the trailer.

You can also test the trailer by itself when disconnected from the tow vehicle. Simply tap into the truck's battery or use another 12V power source to apply power one at a time to each circuit on the trailer plug. This will let you know what function each wire on the trailer is carrying. This test will also tell you if there is a problem in the connector or if it simply has two circuits reversed.

expert reply by: Adam R