Trailer Lights Flash with Vehicle Flashers and Go Out when Stepping on the Brakes

Question:

I installed a new curbside light. Tail lights and side markers work fine now but when I use 4-way flashers trailer lights flash. When applying brakes, trailer lights go out. Brake lights, turn signals and 4-ways work fine when trailer lights are not on. I replaced truck side wiring harness as it had been dragged to many times and was beat up. Checked with test light and all circuits worked fine. Plugged into trailer harness and turned on flashers, pricked trailer light brown wire just behind trailer plug,test light flashed but test light did not light when green or yellow wire was pricked. All wires run thru trailer frame so not many pinch points. I do have side marker lights not far from coupler.

asked by: Dave

Expert Reply:

So it sounds like the flashers are tied into the running light circuit on the truck side. I am not sure it this is how it is supposed to be or if there is a wiring issue on the truck. The correct 4-Way for a 1998 GMC C/K series is # 118315. It just plugs in line with the vehicle tail light harness, see instructions link.

Applying the brakes may be causing the trailer lights to go out while the flashers are on because it is too much draw on the wiring which is causing the lights not to work.

Check all of the grounds for the trailer lights and make sure they are connected to clean bare metal surfaces. Bad ground connections can cause all sorts of problems.

expert reply by: Michael H