Troubleshooting Brake and Turn Signals on Trailer Towed by 2000 Toyota Tundra  

Question:

Helpful Expert Reply:

On the 4-pole trailer wiring harnesses we offer for your 2000 Toyota Tundra, Tow Ready # 118367 and Curt # C55367, the connection point is under the vehicle on the driver's side. There will be a connector to mate with the plug shown in the linked photo. The vehicle connector should carry the brake and turn signals as well as running lights (brown wire) and 12V power (black wire) for the harness. The brake signal is on the harnesses' red wire, right turn on the green wire and left turn on the yellow wire. Once these signals reach the module they are combined for the trailer's lights into stop/right turn on the green output wire and stop/left turn on the yellow output wire that go to the 4-pole.

It sounds like you have done a thorough job of checking the fuses in the truck; those that could be affecting the harness would be those for the brake light circuit and for the two turn signals. The lid of the fuse box will indicate the function of each fuse. If these are okay then you can move downstream to the harness. Just the same you may want to try pulling the brake light and signal fuses - even if they are not blown - since there could be a build-up of corrosion inside the fuse holder. Sometimes just moving a fuse can re-activate a circuit that was dead. (This has happened to me.) I checked the online manual for your truck and found nothing regarding towing-related relays.

You can test the wiring on the left-hand input side (the side with the connector as shown in the photo) by applying a circuit tester like # PTW2993 to the red, green and yellow wires while a helper activates each light function. When your helper steps on the brake the red wire should have a steady signal; when the right turn is activated the green wire should have an on-and-off (blinking) signal and the same should occur for left turn signals on the yellow wire. If you detect these signals then you know the truck's wiring is working, signals are reaching the harness and the problem is elsewhere, downstream of the vehicle plug.

At this point you'll repeat the testing process on the output side of the harness. The green wire should carry a steady signal when the brake is pressed and an on-and-off signal when the right turn signal is on. The yellow output wire should do the same for the left side brake and turn. If you find these correct signals at the output side then the issue is further downstream, at the 4-pole connector or on the trailer.

If you do not detect these signals then you will want to inspect the condition of the metal contacts inside both the vehicle plug and the harness plug to ensure there is no dirt, water or corrosion that is interfering with their making a solid electrical connection. If you find any corrosion on the metal contacts (even slight greenish or whitish discoloration) you can try an aerosol electrical contact cleaner like Caig De-Oxit to remove the corrosion. Also check that the pins are not bent or damaged. If signals are going in to the harness but not coming out then the harness may be damaged.

expert reply by: Adam R

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