Utility Trailer Lights Go Out When Vehicle Brakes Are Applied

Updated 05/18/2015 | Published 05/15/2015

Products Featured in This Question

T-One Vehicle Wiring Harness with 4-Pole Flat Trailer Connector

(201)

$34.15

Hook Probe Circuit Tester

(62)

$13.94

Question:

I have a 5x10 utility trailer being pulled by a Chevy Blazer. The trailer lights are on Ok with the headlights but when I apply the brake the trailer lights go out until the brake pedal is released. Also, the turn signals work but when one turn signal is on the other will blink also but dimmer. Any idea what might be wrong?

asked by: Ron

Helpful Expert Reply:

It sounds like you have a bad ground connection. A weak ground can allow some lights to function but may not pass enough current for additional functions like your brake lights.

First, check the condition of the contacts on both the Blazer and the trailer connectors. Clean off any visible greenish or whitish discoloration with a spray-on contact cleaner or brush.

You can also check the main ground wire on the vehicle wiring harness where it attaches to the frame and on the trailer side as well. Make sure these ground connections are made to clean bare metal surfaces that are free of grease and dirt. You can clean the ground point and re-attach the ground wires or sand off any paint/primer on a new spot and affix the ground wire there.

If you have a circuit tester like # PTW2993 you can test the vehicle by disconnecting the trailer and applying the tester one at a time to each pin on the harness while a helper activates each light function. Please refer to the linked video for a demonstration of using the tester.

You can also test the trailer lights by applying 12V power from a battery directly to the trailer connector's pins.

expert reply by: Adam R