Troubleshooting Trailer Lights After Installation of 7-Way Socket on 2004 Ford Expedition

Question:

I replaced the 4 pole to make it a 7 pole with 4pole adapter on the expy. I used the screw termination 7 pole adapter since I had all the wires except my constant 12V for AUX. I have running lights, left turn. But when I test right turn the vehicle turn works and no right turn at the plug using a plugin tester. Plugged to the trailer I get the left turn signal flash on the trailer on lh signal and rh signal Left flashes on both. When I brake, the trailer left goes out and the right comes on. hazards the left only flashes. It seems there is something missing, I have checked the fuses, added the 30 amp fuse for the Ebrake, which is also showing No Connection. The last thing I have not checked is the turn signal fuse, but it should work since the left works and rh not. Is it possible there is a RH side ground lug for the tail lights? I have never had this much trouble hooking up trailer lights. Simple things made difficult by stupid engineering. BTW I am an design engineer and it irks me when I see stupid being propagated on large scales. I guess I am one of the few common sense approach Design Engineers left.

asked by: William W

Helpful Expert Reply:

It sounds you have installed a screw-terminal type 7-way socket like # HM48470 on your vehicle to convert it from the 4-pole wiring format. Similar items like # 37185 allow this same format change but use a 4-pole connector that joins your original 4-pole, thereby requiring that you make only three spliced connections for the other circuits.

The screw terminal connections on part # HM48470 are intended primarily for convenience when wiring in a replacement for a damaged 7-way socket, as with the mating trailer side plug part # HM48500.

The best way to troubleshoot is to test the wires themselves, with a circuit tester like # PTW2993, before connection to the screw terminals on the replacement socket, since this eliminates that variable. If you test the pigtail leads on the vehicle and find your various trailer signals are present then you know the issue is in the adapter or its connections, or elsewhere further downstream toward the trailer. If signals are not present on the leads then missing fuses, worn or cracked wiring insulation or a grounding issue at the light assembly itself could be responsible.

You can also test the trailer on its own when it is NOT connected to the vehicle by applying 12V power from the vehicle battery (or a spare) to the contacts on the 7-way trailer plug. You can refer to the linked photo that shows the wiring configurations common for a 7-way. This shows you where to apply power for each lighting function. If you find a circuit does not respond to direct 12V power input you can troubleshooting that wiring, checking for damaged wire, a loose power connection or a bad ground. If the light happens to use the mounting studs for ground you might need to sand the mounting point to expose clean bare metal for a good ground.