Trailer Lights Flash When Running Lights and Turn Signals are On

Question:

I have new wiring and new grounds I checked truck on another trailer and found truck OK. truck connector checks OK. I cut off right side light and then left worked OK when lights are reconnected trouble continues. I think it may be defective lights which are also new.

asked by: Steve

Expert Reply:

You could have defective lights or there is a wiring issue on the trailer. This could be related to the type of wiring that is on the trailer. If you have a 4-Way connector on the trailer but it has 5 wires coming out of the back of it, then that is called a wishbone harness. You will have 2 running light wires on a wishbone harness, one for each side of the trailer. A mistake in wiring those up could cause some issues.

The other likely cause is a connection or ground problem. The ground locations you have may not be sufficient and you can try moving them to different locations that are clean and free of corrosion. If added something like a license plate bracket between a light ground and the trailer frame, that can cause this issue. The same goes if the trailer has a tilt bed, folding tongue, or aluminum frame.

The fix for this is to run jumper grounds from the light grounds to the main trailer connector ground.

You can test the lights by applying power directly to them from a separate 12 volt battery. If they work fine then the issue is related to the wiring and/or grounds on the trailer.

expert reply by: Michael H