How to Trouble Shoot Brake and Turn Signals Not Working On a Trailer  

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This can be caused by a few things. First and foremost, you will need to check a couple fuses to make sure they are still in good condition. Under the hood of your 2018 Toyota Tundra, you will want to check the fuses in positions 6 and 37. Then you will need to check the cluster under the instrument panel for fuses 20 and 22. These can be confirmed in your owner's manual, but these are responsible for the towing lights. If one or more have blown, it will definitely cause the issue you are having.

If these are all good, you will need to move to the wiring on the end of the vehicle. Using a tester, like the Circuit Tester # PTW2993. You will need to test the pins at the 3 o'clock and 9 o'clock positions if you have a 7 Way connection. The 3 o'clock position will be the right turn signal and right brake signal. The 9 o'clock position will be the left turn signal and right brake. If one or both of these are not working, you will need to replace the 7 Way with something like the Hopkins # HM40975. This would be a great replacement as long as the vehicle came pre-wired with the 7 Way plug. If it did not, you will need to race the wires back from the 7 Way connection as there is most likely either a break in one or more of the wires or the ground has become corroded or not secure. If this is the case, you can simply repair the wire and get back to towing, or you could rewire everything and use the Hopkins # HM40975-11998 at the end of the vehicle to give you a clean look.

If you have a 4 Flat connection, you will want to check the green wire for the right turn signal, the yellow for the left turn signal, and the brown wire for the brake signal. If one or more of these isn't working, you will need to replace the harness. This will again depend on if the vehicle was wired for a 7 way from the manufacturer. If it was pre-wired, you will need the Tekonsha # 118384. If it was not pre-wired for a 7 Way, you will need the Hopkins # HM11141835.

That covers the vehicle side. If all that checks out, it is something on the trailer that is causing the issue. You will want to check all your ground connections to make sure they have a good clean connection to bare metal, making sure it is not aluminum. If they are LED lights, it is best to have each light grounded.

Once all the grounds are checked and or re-grounded, test the lights by plugging securely into the vehicle. If they still aren't working, you probably have a break in one or more wires and will need to follow them to find said break. That being said, you should always start this directly behind the plug that connects to the vehicle. These do go bad over time and are easily replaced. For a replacement on a trailer with a 4 Flat, I suggest going with the Hopkins # HM38134. This features their Easy-Pull design, making it quite a bit easier to disconnect from the vehicle. For a trailer with a 7 Way, the Hopkins # HM20086 has built in test lights to make things easier if this ever happens again.

expert reply by: Lazer S

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