Troubleshooting Trailer Wiring

Question:

Hello, I was wiring up my trailer and it seems everything is working great. Until I have my headlights turned on, and if I were to signal left or have my hazards on.. my left side tail light on my trailer stays constantly lit up and doesn’t blink. My ride side works well throughout all light variations. But for some reason my left side is having issues with blinking when the lights are on in the truck. I also have a video for reference if needed. Thank you.

asked by: Eddie

Expert Reply:

In order to determine the specific problem you will need to purchase a circuit tester if you do not have one - the very handy Cordless Circuit Tester # PTW2979 would be my strong recommendation due to how easy it is to use - and test the connector on your vehicle after disconnecting it from your trailer. Check the wiring going into your harness from your tail lights to see if those wires are carrying the correct signals, if not then the issue is with the vehicle wiring.

If the signals going to your harness are okay, you'll need to test the signals coming out of the harness, and if they are correct then your issue is likely going to be with the trailer side wiring. You will also want to check your connectors - of one of them is older - for corrosion, which can be green or white in color. A build-up of corrosion can interfere with the signals getting through the connectors so if it is present you are going to need to replace the trailer-side 4-way plug with # 18004.

It is also vitally important to check your grounds as these can be the cause of many different issues. You'll need to make sure your grounds are attached to bare metal surfaces and free of any paint, dirt or corrosion.

I have added links to a pair of help articles on trailer wiring, including a troubleshooting guide which I highly recommend taking a look at.

Additionally, I also highly recommend using Dielectric Grease for Electrical Connectors # 11755 to help prevent any type of corrosion in your connector; using it will increase your 4-way's durability and extend its lifespan.