I tried a search and couldnt find anything. If I use a left or right signal, both blink on the trailer, but not on the truck...just the trailer. The truck signals work fine. I took the factory plug a part and cleaned the terminals, etc. It appears I'm getting both signals at the plug testing with a meter.

What would cause both signals to blink on the trailer when using left or right signals?

 

I would check your plug on your truck. the wires coming into the plug have to be touching together before they go in the plug.Just haul the plug apart on the truck and have a look. My .02

 

When you say they're both blinking on the trailer, are they blinking together or alternately?

 

Check your ground first. I had the same thing happen to me. I ran a new wire & connector to the trailer, good as new.

Look carefully at the connection from plug to truck & plug to trailer. 12v dc is not much electrical force.

 

ok thanks guys, I'll try it again. I cleaned the plug connector really good. It needs a new ground for sure.

Both signals flash at the same time on the trailer, but not the truck.

 

The way I understand it is that without a good ground the electricity finds any available ground. For some reason the turn signals have some sort of slight connection. (that is my best explanation, sorry as it is).

I put a 12v battery on the trailer to verify the lights were connected correctly.They were fine with the battery at the time I had problems. You could do the same, get a plug and insert it in the cigarette lighter and run wires to the trailer to check things out. I'll bet a soda that it's the ground ( only bet when I will win)

 

I too think you have a bad ground. For a test just clamp a jumper cable from the trailer frame to the truck. A bad ground can make trailer lights do some strange things.

 

Without a ground, the signal will ground through the tail lamp filaments.

 

Rampage is dead on. The most common problem is the ground is not making good connection on the trailer end.

 

Some trailers use the ball/hitch for a ground. This doesn't always work that great.

If your trailer doesn't use a ground wire in the plug then I would add one.

 

check your trailer bulbs you may have a filament broken and connected to your clearance lights