Troubleshooting Running Light Circuit on 2011 Toyota FJ Cruiser 4-Pole Trailer Wiring

Question:

I have the factory wired 4 flat and I am having trailer lights problem.. The turn signals work and brake lights work fine .. unhooked trailer and tested ground and green wire and yellow wire have power ... but nothing out of brown wire.. I see in the fuse panel under the hood there is a tow tailight relay .? please help

asked by: Tim P

Helpful Expert Reply:

When trailer stop and turn lights function but the running/tail lights do not, the most common cause is a weak ground connection, either on the vehicle's or on the trailer's wiring. Stop and turn signals are not on continuously the way running lights are, and this is why those running lights may need a better ground connection in order to be able to draw enough power to operate.

You can check this easily enough by disconnecting the trailer and applying 12V power directly to the trailer's 4-pole pins for ground and the running light circuit. These will be the pins for the white and brown wires as shown in the linked photo. If the running lights still do not function when you apply direct 12V power from your battery then check the trailer's ground wire where it attaches to the frame.

Disconnect the ground wire ring terminal and sand off that grounding point to expose clean bare metal. Apply dielectric grease # 11755 to protect it from corrosion and re-attach the ground ring terminal. Re-test the running light circuit. If it still does not function then inspect all the wiring for that circuit, from the 4-pole connector all the way to the light assemblies; check for nicks or worn spots in the insulation, especially where that wire may come into contact with the frame or other trailer structures.

On the FJ side, first make sure that all required fuses/relays are present in the FJ's power distribution box for all trailer-related functions (it looks like fuse position 10 requires a 30-amp fuse). Then apply a circuit tester like # PTW2992. Attach the tester's clip to the truck 4-pole's ground pin and apply the probe to the pin for the running light circuit. Then have a helper activate the headlights on the vehicle while you test. If you get a steady signal then you know the issue is on the trailer side. If you do not, then check the main ground wire for the truck's 4-pole in the same way you did on the trailer.

expert reply by: Adam R