Troubleshooting Trailer Lights Not Working
Bench tested green light goes on take tester out green light goes out all trailer lights work but no brake lights brake lights work on truck
asked by: Willis
There are a number of troubleshooting issues you will need to take a look at if the brake lights on your trailer are not working.
The first thing to do is take a look at your owner's manual to locate any trailer fuses or relays and ensure they are all there and in working order. If they are, 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 both your vehicle and trailer connectors 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 will need to either replace the plug or clean the corrosion. You'll also need to make sure your grounds are attached to bare metal surfaces and free of any paint, dirt or corrosion.
Finally, check your wiring thoroughly to ensure there are none that are crimped, broken or otherwise damaged. 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.
- Q&A: Trailer Brakes Lock Up When Connected to Truck
- Q&A: Testing for Proper Voltage on the Brake Output Circuit
- Article: Trailer Wiring Diagrams
- Article: Wiring Trailer Lights with a 7-Way Plug (It's Easier Than You Think)
- Article: Testing Trailer Brake Magnets for Proper Function
- Q&A: Trailer Lights Work But Brakes are Not Working
- Article: Brake Controller 7- and 4-Way Installation Kit (ETBC7)
- Q&A: How Much Voltage Should Be Applied To Brakes on Trailer w/ Electric Brake Controller
- Article: Troubleshooting Brake Controller Installations
- Q&A: Troubleshooting Voltage Drop to Trailer Brake Assemblies
- Q&A: How to Test Brake Controller without Trailer Connected
- Article: Wiring Trailer Lights with a 4-Way Plug (It's Easier Than You Think)
- Q&A: How to Tell if 2019 Toyota Highlander Has Factory Tow Package
- Article: Brake Controller Installation: Starting from Scratch