Pigtail problem

Likely either the receptacle on the truck or trailer is a bit loose and not connecting, or just a cheap junk 15 footers. Many are imported now, that's why they're so cheap now.

The coiled cord helps put a constant pull on both ends.

 

Both ends of the pigtail..check for wires that came loose. Just need a small screwdriver to remove the one screw and then slide back the metal covering.
I’ve had brand new ones with wires not connected properly. I’ve had better luck with the ones that are crimped vs screw tightened.

It’s really easy to check if a wire got disconnected and a good place to start looking

 

Get a thin screwdriver or pocket knife into the slotted pin on the male receptacles, and spread them puppies a little.

Little rolled up like a toothpick piece of sandpaper or emery cloth to clean the female ends.

Dab of dilectric grease .

 

Wedge an old seal or a splinter of wood in the socket with the pigtail, sounds like it’s not getting a good connection.
It happens on our newer trailers quite a bit.

 

Oh, be careful when doing that. I pulled a lot of rail wagons and those pins break off pretty easy. Rather than contact, it's usually corrosion.

 

I hope this helps. This is my very first time being on a forum. I recently decided to see what they're about.

Verify that the male connector doesn't have vertical play. The problem could likely be in the bulkhead connector of the trailer. You should only have 2 points of contact: which gives 4 testing locations; 2 male, 2 female. Whenver I do electrical diagnostics I follow these few steps: 1) check common failures, physical damage, and ground, 2) isolate the circuit, 3) check resistance/diode tester check, 4) load test circuit for voltage drop. If you can't find the issue within those four steps. You're most likely in the wrong area of the problem. Whether you're a driver or a mechanic I recommend owning a
7-Way Round Pin Tractor Trailer Circuit Chaser™