Troubleshooting Wiring Harness That Stops Working Properly When Running Lights Are On

Updated 06/11/2019 | Published 06/09/2019

Products Featured in This Question

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

(54)

$91.73

Dielectric Grease for Electrical Connectors, 2 oz.

(1489)

$10.79

Digital Multimeter with 42" Long Test Leads - LED Display

(3)

$21.03

Question:

Harness install as per directions: Without running lights on: Brake lights work Signals work With running lights on: No brake lights Signals work when brake pedal pressed

asked by: Paul M

Expert Reply:

The issue with your running lights causing the brake and turn signal to not work sounds like a common grounding problem that occurs often. Since the running lights remain on continuously they will draw more power than blinking turn signals or brake lights that are typically on for only a moment. This higher power draw is why those other functions can fail when you put on the vehicle headlights.

The solution is usually very simple and easy. What you want to do on both the vehicle and on the trailer is locate the main ground wire for the harnesses. Disconnect the ring terminal, sand off that grounding point to expose clean bare metal, apply protective dielectric grease # 11755 and then securely re-attach the ground. This remedy fixes this specific issue in most cases.

The Curt wiring harness # C56270 that you referenced can support up to 7.5-amps current for the running light circuit so if you still have an issue after addressing the vehicle and trailer grounds you might confirm how much power your trailer's running lights are drawing. You can do this by applying a meter like # PT89ZR to the running light circuit while applying direct battery power to the running light contact on the trailer connector.

expert reply by: Adam R