When the boat is plugged in to the truck the running and backup lights work, but no turn signal or brake lights on the trailer. All lights on the truck are working correctly.

Have had the plugged checked on the back of the truck and everything is working there. Fuses are good.

Suggestions on what to check or recommendations on repair person appreciated.

I bet you’re pins in the plug use a screw driver and bend them out slightly and re-plug not making a good connection

How many wires are on the plug to the trailer?

Just picked up a new boat at cabela's last week and this is what they did to get mine working.

5

5 wires on trailer

Will try adjusting the pins.

Thank you.

Issue on the turn signal/ brake light wires. Usually the yellow and the green. Yellow should be left side and green the right side. Brown is usually the running lights.

If the yellow / green and the brown are reversed you won’t see the turn or brake while the running light are on. This is because the brown is illuminating the brighter element and turn / brake is illuminating the weaker element.

Get one of those plug ins for the auto parts with lights that slow that’s the plug for the vehicle are working correctly. If that checks out good then the issue is 100% on the trailer.

White should be to the trailer frame. That has the trailer and truck sharing the same ground and not relying on the ground thru the hitch. Also make sure each light has its white wire to the trailer frame. Essentially they are using the trailer as the ground wire.

You could have the brighter turn / brake elements out on the lights. That’s probably the least likely as both sides would need to be out.

check the fuse in your tow vehicle. some have 2

Mine was the ground. I was grounding to the trailer from the plug and then from the trailer to the light. So basically, the trailer became the ground. It worked on one side but not the other. So, I did away with that idea and ran a new ground wire to both lights from the truck plug. Took me a couple of days to figure all that out checking continuity, fuses and voltage and getting it, but not enough. Replaced the plug and rewired everything. Still no dice. Hit and miss. Then started working with the ground. That was the culprit. The ground was the problem. Works fine now.

Using the trailer for the ground is the way most are wired. You have to make sure both the plug and each light have a good connection to the trailer. They do this to keep from running a ground wire to every light on the trailer including the small marker lights.

Ground was my first thought based on having the same experience.
Make sure your trailer ball isn’t so rusted or in a state that you’re not getting good contact between the ball and the trailer.

99% of the time it is the ground.

Since the running lights are working on both sides I suspect the ground is not your issue. Best thing you can do is go get you one of the test lights that look like an ice pick on the end that you can push through the wire. They are not expensive. You said you tested the plug on the truck. If everything worked at the plug on the truck you can rule the fuses out. Now turn on your emergency flashers and check your wires coming out of the plug on the trailer. Check the yellow and green wires coming out of the plug on the trailer first. Most modern trailers use green as the right blinker, yellow as the left blinker. If you do not get a flashing light on the tester you have an issue either inside the plug or the trailer plug is not making good connection with the truck. If you get power coming out of the plug, repeat that process all the way back until you locate the problem. If it is a round Rv plug my guess is you have a connection issue inside the plug on the trailer. Plugs go bad pretty often. Some you can take apart and check the wiring inside and some are sealed. If it is a sealed plug you can go to any auto parts store and buy a replacement plug. Just cut that one off and re wire it. I had to replace the plug on my ez loader trailer about 2 weeks ago. Right blinker quit working. I guess the solder inside the sealed plug came loose.

he has running and backup lights, he has ground but might want to clean it anyway. ground should be plug to plug too, white wire, using the ball is sketchy a lot of the time. just attach truck side to frame or body and trailer frame on other end.

have you turned on the flashers to see if they work? if not, go to the plug on trailer side and as said test with the ice pick tester, watch for fingers cuz it will hurt if you slip.

is you wiring exposed or in the frame? exposed just go check for connections that may have come loose or wires corroded bad etc. if in the frame just check for current at both ends where you can as close to where it goes in and comes out, could be a corroded wire.

sounds to me like you have a broken wire up around the tongue or a bad connection in the wiring.

one other thing, if you can, hook it to someone elses truck and see what happens.