Trailer lights only work if truck lights are off

Went and got the boat out of storage the other day. The brake and blinker lights all worked on the trailer. Then I turned the lights on for the truck and tested the trailer lights again. Nothing... Checked the trailer lights with the truck lights off and they all worked. Went back and forth a couple more times with the results staying the same.

Truck has a 4 pin hookup same as the trailer. I'm not an electrical person so I'm not sure if it would be a problem with the trailer wiring or the truck. Just wondering if anyone has any ideas. I'm going to take a closer look tomorrow.

 

Your problem sounds like a ground wire fault to your trailer. Most trailers ground through the trailer ball and if the ball or the trailer receiver are rusty, you may not be making a complete circuit. If you have some spare wire, try turning on your lights and touch the wire with the insulation stripped on each end from the rear bumper to a bare metal spot on the trailer frame and see if the lights work then.
I hope this helps.

 

Thanks, I'll go out and give it a try.

ETA: Okay went out and tried that. It worked. I also took a picture of the plug on the trailer because I noticed that there is a spot on the plug not being used. Now my only questin is whats the fix? I could scab a wire from the trailer tongue to the frame of the truck, but i want to do it right.

Got this thing last year from a friend. He had redone the wiring and now I'm thinking about doing it again to try and get it right.

 

I am glad that your trailer works now. You could fix this two ways. One is the easiest and the cheapest, you buy a seperate ground wire that has a male and female plug and hook it up seperately of your old wiring harness .
The other solution would be to purchase a new complete wiring harness and rewire the trailer. I say the connector picture and I would bet the missing wire is the black ground wire. I would get a nice water proof connector style of wiring harness and really fix it correctly. Either way will work, it is your choice according to how good of a job the rest of Wiring harness is.
Congradulations and good luck.

 

Thank you for the help. Think I'm going to take this opportunity to rewire the trailer. Good learning opportunity. The lights on the trailer are new, but I'm not sure the guy that put them on knew too much more than I do.