Trailer Running Lights Do Not Work With 1999 4Runner Headlights On Using T-One Harness # 118341

Question:

I purchased this wiring harness for my 99 4-runner. Everything hooked up fine... no problem installing. The turn signals and brakes work fine until I turn on the headlights. After I turn on the headlights nothing else works except the headlights on the trailer. I have one of the fourflat testing lights and it does the same thing on that. The trailer lights work fine on my buddies truck. Do you have any suggestions?

asked by: Michael T

Expert Reply:

If your trailer turn signals and brake lights work but stop working when your running lights are on the most common cause is a bad ground connection. An insufficient ground connection may allow enough current to pass through to operate the brake and turn lights, which are on intermittently, but not enough for the running lights which draw power continuously.

The particular harness you are using on your 1999 Toyota 4Runner, Tow Ready # 118341, is rated for stop/turn lights that draw up to 2.1 amps per circuit, and tail lights that draw up to 4 amps per circuit. You might check the current draw from your trailer running lights to confirm it does not exceed the 4 amp capacity of the harness.

The white wire with ring terminal is the ground for the harness. This needs to be firmly attached to a clean bare metal surface (or to an existing ground point) in the rear cargo area of the vehicle where the harness plugs in. The ground wire ring terminal must make solid metal-to-metal contact with a clean unpainted metal surface in order to pass the full current capacity of the harness. If needed, you can try using fine grit sandpaper to prepare the surface for the ring terminal.

I have linked a troubleshooting article that will give you additional guidance.

Since your trailer works fine with another tow vehicle chances are that there is no issue within the trailer, but it is also a good idea to periodically check your trailer wiring. Inspect the wiring for signs of cracked or worn insulation and tape or replace such damaged wires; also confirm that the ground wire from the trailer connector is firmly attached to the trailer frame.

In some circumstances the mechanical connection between the tow vehicle hitch ball and the trailer coupler may make a partial/intermittent ground connection. This may mask a grounding issue with the wiring. You can uncouple the trailer (while leaving the 4-pole connected) to check.

expert reply by: Adam R