Left Brake and Turn Signal on Trailer Connector Not Working on 2016 Ford F-150

Updated 12/30/2019 | Published 12/28/2019

Question:

Brake and signal lights on my 2016 F150 work fine on truck but no left signal/brake at the trailer tow plug. I’ve replaced the 20A fuse in port 35 in the min fuse box with no success. Any idea what else it could be? Thank you.

asked by: Kalvin

Expert Reply:

It sounds like you've isolated the issue to your 7 way connection on the truck, and the left turn signal/brake light is controlled by the circuit at 9 o'clock when looking at the front of your vehicle's 7 way connection (most likely a yellow or red wire).

I would start with making sure that the ground wire (white) on your 7 way is connected firmly to a rust and paint free spot on the vehicle's frame; it's entirely possible that the ground wire is loose and causing this issue. You'll also want to make sure that the 7 way connector is clean and free from debris/corrosion.

Next, you'll want to turn the left turn signal on and use a circuit tester like the # PTW2993 to check for a signal on the circuit behind the 7 way connection on the vehicle(wire at 9 o'clock when looking at front of plug)- if there is no signal, you'll need to trace the wire back and check for damaged/cut wires along the way. If you do have a signal behind the wire, your 7 way is damaged and it will need to be replaced.

If you've gone through these steps and the issue has not been resolved, let me know and I have a couple of other things you can look at to get this resolved. I've also linked one of our help articles on 7 way wiring that has some diagrams in it of wiring schematics that you may want to refer back to.

expert reply by: Erik B