Turn Signals on Trailer Work Opposite of Truck With Brake Applied

Updated 09/17/2025 | Published 04/12/2020

Question:

i have a 2012 chevy colorado with a utility traler when i turn on turn signals both truck and trailer are in sync but when i apply brake pedal with turn signals on truck and trailer turn signals are not in sync meaning traler turn signals flash opposite my truck btw my truck has seperate brake and turn signal bulbs could that be the problem

asked by: Thomas L

Helpful Expert Reply:

What you describe is a classic symptom of a insufficient ground connection. Then there is minimal amperage running through the system, everything works fine but with the brake signal thrown in, more power is running through which is when the problem will rear its head.

Make sure that the trailer connector's ground point which is usually on the trailer tongue is secure and not corroded, and is making direct contact to clean metal on the trailer tongue or frame. Check each lamp to make sure it's grounded either by its ground wire to the trailer frame or by it by its direct connection to the trailer frame if the fixture only has one wire.

It sounds very counter intuitive, but a bad ground connection will make trailer wiring do all sorts of crazy things. I've linked to some help articles about trailer wiring that should provide some assistance.

expert reply by: Mike L

Bradley

9/2/2025

2004 gmc sierra 1500 sle. Brand new trailer same thing happens. When no brake is applied light work as normal. When brake is applied the flash opposite. What i went through is. 1. Grounds on trailer and truck side. 2.7 way plug is free of corrosion on truck and trailer side 3. Fuses are good 4. 7 way electrical is confirmed working and proper wiring 5. Replace brake sensor in truck 6. Even a brand new brake controller No fix to the problem Also my trailer brakes when I apply them it will come up with how much I set them too. But as fast as I apply them them reset to zero