Troubleshooting 4-Pole Trailer Wiring with No Stop/Turn Signals on Honda Pilot

Question:

Running lights work Brake and turn signal lights do not work , but do on a different vehicle Appears problem with Pilot at the trailer resepticle In Canada concerned about return home

asked by: Mike B

Expert Reply:

In both 4-pole and 7-way trailer wiring the brake and turn signals are carried on the same wires. On a 4-pole connector like the one # 18004 that you referenced the left turn and brake signals are carried on the yellow wire and the right turn and brake signals are on the green wire. On a 7-way vehicle socket, see the linked photo please, the stop/turn signals are carried on the pins at the 3- and 9-o-clock positions.

There are several possible causes for the stop/turn lights on your trailer to not work when connected to your Honda Pilot.

First, you can check the vehicle 4-pole connector for any signs of discoloration or corrosion on the actual metal contacts. Even slight greenish or whitish discoloration indicates corrosion that could interfere with signal transmission. Use a wire brush and electrical contact cleaner to remove any corrosion. Once corrected, those clean contacts can be protected using dielectric grease # 11755.

If you happen to have a circuit tester like # PTW2993 you can disconnect the trailer and directly test the vehicle wiring. Ground the tester to the exposed ground pin and apply the probe to the pin for the yellow wire while a helper steps on the brake and then engages the left turn signal. You should detect a steady signal when the brake is pressed and an on-and-off blinking signal when the turn lights are activated.

If you do not find these signals, you can check the main ground wire for the harness to ensure it is securely attached to a clean bare metal surface and you can also check the harness connections at your tail lights where it plugs in. Check for any corroded, bent or broken metal contacts within those connectors; clean any corrosion and apply dielectric grease as needed.

You can also check the vehicle power distribution box for any blown fuses related to the lighting functions, but if the car's own stop/turn lights work then you know the fuse is okay.

Please let me know the model year of your Honda Pilot so I can recommend a replacement harness in the event yours is damaged. Do please note that if a trailer with a short somewhere in its wiring were to be connected to the vehicle harness this could cause failure of the entire unit and require that it be replaced.

A helpful article on troubleshooting 4-pole wiring is linked for you.