ALL trailer lights flash when 4-ways or turn signals are turned on: How to fix?

Here's an easier fix. Pop out the pig tail, get yourself a flathead screwdriver, and GENTLY spread the plug ends on the trailer side a bit to get better contact. Also check and make sure both the pig tail and the trailer box are clean and free of verdegris on the copper. If that doesn't work, your gonna need a shop to check the wiring on both ends before you start sinking money into it.

 

I had similar problem except that when you applied the brakes all lights came on including clearance marker lights.

Problem was one of the LED lights had a bad connection and shorted causing all lights to come on.

What I would do is start unpluging each bulb until your lights work normal. If the problem persists then it could has to be something else.

 

That's a good solution.

I don't see how the symptoms could be the result of a ground issue.

 

A bad ground will cause it to try to pull ground through the filaments in the unlit bulbs. They'll glow dimmer than usual and the lights that should be lit will be dimmer also. With the flasher on they'll oscillate like cop lights.

 

How is the current going to get to the unlit bulb filaments unless there is a short between that circuit and the circuit carrying power to the ones that are lit?

12 volts isn't very good at jumping through air.

 

I had that happen but it was only when the trailer lights were turned off, when I used the right turn signal almost every light would blink, even backfed into one side on the tractor... turned out to be a bad LED side turn signal bulb.

 

Exactly what I'm talking about. I didn't know led's could do it too.

 

It happened to me after years of running a teflon 5th wheel plate. After I took it off the trailer was to rusty to make a good contact with the 5th wheel. Blinker would blink ALL lights off and on. If I hit the brakes they would all go out. If I hit the brakes with the lights off, my interior lights in the trailer and turn signals would come on dim without power to them. If I turned the blinker on to change lanes it would kick off my cruise, I assume by sending power back through the brake lights. It can happen.

My solution was to run a ground wire from the truck along the pigtail and hooked it to the trailer with a quick disconnect. Problem solved.

 

I'll try to explain it without drawing a schematic...

Both filaments in a dual filament bulb usually have a common ground. In the case of an 1157, it is the shell. In many turn/park lamp circuits, each side will have its own ground with a short lead coming off the bulb socket attached to chassis ground.

When one side has a poor ground, the current will flow back though the parking lamp filament to the side that has a good ground.

Make any sense? I can draw a schematic if it would help you understand better.

 

Ok... I fired up GIMP and made these schematics. Hopefully this makes it a lot easier to understand why bad grounds cause funky things to happen with your lights. Too bad I couldn't get paid for this kinda stuff, lol.

In this first example, the left turn signal is on and the grounds are connected properly. Think of electricity as water flowing through pipes. The red arrows indicate the direction of flow.



Now in the second example, the ground wire is broken on the left side.



Electricity follows the path of least resistance--in this case, through the parking lamp filaments to the right side chassis ground.

This concludes today's lesson. Remember your term papers are due on Friday... class dismissed.