Troubleshooting Trailer Wiring That Stops Working When Headlights are On

Updated 11/17/2021 | Published 03/06/2020

Question:

When I hook up my trailer, when the head lights are turned off the trailer lights function normal, brake lights and R/L turn signals and 4 way hazards. But when I turn the headlights on the 4 way hazards stop and go out, if I turn the headlights back off, the 4 way hazards will start blinking again. Everything use to work normally 2019 a couple of times. With a 3 light tester plugged into the factory 4 way plug, same thing happens. It has been at the dealer for a month. New battery New Flasher New Multi function switch I even tried one of the kits that plug into the back brake/tail lights like it didnt have a factory trailer harness, same thing....

asked by: Gardian

Expert Reply:

Your main problem is an insufficient ground for the trailer wiring. Try improving the connection of the main ground wire of the trailer wiring harness. Make sure it is attached to a clean metal surface. Or you can run all of the trailer wires back to the main ground. This is actually the ideal solution. But you can also check the current grounds for the lights to make sure they are making a solid connection to a clean metal surface.

What you described is the classic insufficient ground scenario so finding a way to improve the grounds will fix the problem. The issue is that when your headlights are on the maximum amount of amperage draw is put on the setup which if there's an insufficient ground this would be when this would show up.

expert reply by: Jameson C

Gardian

7/4/2020

Still not working. They re-grounded the harness back at the rear bumper. Dealer said they attached it to a trailer simulator and it worked fine. But it is still doing the same thing. I ran a 8 gauge wire from the negative on the battery to the connector at the rear bumper, still will not work. Battery replaced Flasher replaced Turn signal/hazards assemble replaced. Rear connector replaced. Re-grounded the frame to the body. At two different places. Still, same thing. Hazards on working till parking/headlights are turned on with motor running, then hazard lights go out and it shows onlt the running lights work. I don't know where to turn next???

Chris R.

7/8/2020

Can you tell me what model vehicle you have along with the type of trailer harness that was installed?

Gardian

7/9/2020

1997 E150 has the factory trailer wiring back to the rear axle. Then I purchased a 4/7 dual plug connector. But, I have a tester with 3 little red lights that I plug into the factory 4 wire plug at the axle, which takes the connector out of the loop. Parking light works. Brake lights work. R and L turn signals work. Hazards work, till you pull the parking lights and or headlights on and then they stop. So we have narrowed it down to Hazards stop when the parking lights are activated. Headlights on doesn't matter. Is it possible it is a bad headlight switch? This setup worked twice (two road trips) now this problem. Thx

Chris R.

7/14/2020

It certainly could be a bad headlight switch. That's probably the next place you'll want to look. You've honestly ruled just about everything out on the trailer wiring itself.

Gardian

11/17/2021

Well, update. The Ford Dealer could find nothing wrong. So, I took it to our local Midas, they had it for 3 weeks: Findings: Low voltage at the Trailer wiring Harness 9 volts. They went completely through the wiring, even pulled the dash and was not able to fix it. So I ordered the "Code: C59496" "Curt Upgraded Circuit Protected Powered Tail Light Converter w Hardwire Kit and Tester - 4-Pole Flat" and it works fine now. The low voltage wires are enough of a signal to make the lights work.... Something is still wrong, but this was a work around as it is powered from the battery.