Trailer Turn Signals and Brake Lights Not Working on a Boat Trailer  

Question:

Expert Reply:

If you are getting the correct signals from the truck side trailer connectors, then the problem is on the trailer. There could be a number of issues causing the problem. The easiest to check would be the trailer side connector. With the trailer connected, test the wires right behind the trailer side connector using a circuit tester like # PTW2993.

If no signals are coming through, then you know the connector is bad and it will need to be replaced. While you are at the trailer side connector, check for any corrosion or dirt in the connector and clean it out thoroughly. If you are still having problems, make sure the main ground wire from the trailer side connector is attached to a clean and corrosion free bare metal surface on the trailer frame.

You can then go on to test the wiring working your way back to the tail lights. If there is a break in the wiring anywhere, that can be causing the problem. Next you can check the tail lights. Trailer tail lights usually ground through the mounting studs if no obvious ground wire is present. In both cases, the ground or studs need to be attached to a clean and corrosion free bare metal surface. If a plastic license plate is between the light and the trailer, that, too, could be causing problems.

Most likely you are experiencing a bad or missed ground, a dirty connector, of the trailer side connector is wired differently than your truck side trailer connector; or it could be a combination of these. The good news is that even though troubleshooting the problem takes some time, the fix for each of these issues is pretty easy.

Another possible issue is depending on the year, make, and model of the truck, you may need relays and/or fuses to make the towing package fully functional. You can check your owners manual to determine if fuses and/or relays are used and where to check for them in the power distribution box under the hood.

expert reply by: Michael H

0

Jerry

7/27/2020

I had an issue with my trailer lights were only the running lights would work but turn signals and break lights wouldn’t. I replaced the entire harness and upgraded to LED lights. Every light is connected properly with their respective ground. When I tested it with the truck it does the same thing the old lights did, only running lights but no signals or break lights. Then I replaced the truck harness for a new one, same issue, I checked the fuse panel on the truck and all fuses are good. Can’t think in any other thing.

Jon G.

8/4/2020

It sounds like you're using the same method to test both the truck and trailer wiring; hooking the trailer to your tow vehicle. What you'll want to do it test them separately by using a circuit tester for the truck and applying 12V power to the pins on the trailer (or hooking it up to a different vehicle) so that you can isolate where the problem is. Since you replaced literally everything on your trailer I would bet that the problem is with the OEM wiring on your truck. If the wiring you replaced on the truck was just a smaller harness at the end then you might need to do an overhaul and replace all of the wiring for your trailer connector. You'll need the # ETCB7 kit and an active 4-Way connector as well. You can use our fitguide to see what 4-Way wiring will fit.

Jerry

8/4/2020

@JonG thank you. The OEM harness from the truck was replaced as well. Tester indicated there was no signal sent to the break lights or turn signals. I checked the fuselage and bam, found a blown fuse. Problem solved with $3. Thanks for your comment.

Chris R.

8/14/2020

@Jerry That's great news!