Blinking marker lights

[Indy Steve]

I have a mystery...noticed the past couple nights our side-rear running lights have a very small blinking light on both sides. It's only the red ones closest to the rear bumper, each side. I figure if it was a short, all of the running lights would be doing it. When I look very closely, it's almost like there's a dedicated led in there for this. Anyone know why this is happening? 2018 Cougar fifth wheel.

[jsmith948]

Some additional info would help:
1. are the running lights on or off?
2. Is the trailer cord connected to the truck?
3. Are the four-ways flashing on the truck?
4. Is it just the rear side marker lights or are the tail lights flashing?

My first thought would be that you have a short. Also, if you have a bad ground somewhere, the marker lights will dim when the turn signals are activated making it appear that they are blinking.

[Indy Steve]

Truck off but plugged in and just the rear side markers flashing. To be clear though, it's not a bright flash, almost not noticeable, just one little led on each side...when truck is unplugged, nothing. Just very strange that it's only two running lights, and not all of them. Truck is a 2018 Ford.

[flybouy]

As you list a 2018 model, have asked your dealer about it? I would suggest starting there.

[jsmith948]

Truck off but plugged in and just the rear side markers flashing. To be clear though, it's not a bright flash, almost not noticeable, just one little led on each side...when truck is unplugged, nothing. Just very strange that it's only two running lights, and not all of them. Truck is a 2018 Ford.

This seems to point to the problem being in the truck's wiring

[Canonman]

This seems to point to the problem being in the truck's wiring

ON our Cougar, the marker lights are only active if the 5er is connected to the TV. OP notices that only one LED on each light seems to be blinking. I'm of the opinion that the marker light has a defective LED module. Not a truck problem. I'd suggest checking the part number stamped on the light itself. Likely they are the same fixture. Might be a defective batch? Wouldn't be big job to take one of the other marker lights that is working correctly and swap it with the problem light and see if it starts blinking. My guess it's the light:popcorn:

[JRTJH]

It's a simple test to remove the truck from the circuit. Just unplug it. Then, the clearance/running lights on the trailer will be "completely off" (as stated by the OP). Next, jumper between the 12 VDC and the running light pin on the trailer umbilical connector. Jumper between pins 3 and 4 on the trailer umbilical. They are the two pins closest to the "keyway index". The trailer running lights will light. Check to see if the "problem LED's" illuminate with this power source. If they do, then it's not the truck, but a problem with the trailer wiring or those two LED assemblies. Chances are there's a "feedback through ground" or a "screw through a wire" somewhere in the circuit.

Here's a diagram of the umbilical wiring. Just use a paperclip, a short piece of copper wire or something conductive, bent into a "U" shape to jumper between the contacts.

DO NOT JUMPER BETWEEN THE CONTACTS ON THE TRUCK UMBILICAL !!!!! You will short out the contacts and possibly damage the truck wiring. Only jumper the trailer end !!!

Note: If you want to be "completely safe" and not risk damage to your trailer wiring, you can use a 10 amp fuse, bend the "prongs" at an angle and use that fuse to jumper the contacts at position 3 and 4, but that is "going way beyond" necessary for troubleshooting this kind of problem.

[craftguy]

Just another thought to check for. We have had our new Laredo 5th wheel just a month or so and on the return from last trip my wife did a check of the lights. We had tail lights but no turn signal/flashers/brake lights. After spending an afternoon trying to figure it out I found a blown fuse in my truck, under the trail wiring position. But why, so I went through all the hook up wiring on my plug in in the truck, no change. I then checked under the fifth wheel hook up, where the pig tail hooks into the wiring. Some dummy, and I'm being nice, at the factory wired the bare wires to the junction box very tight. Tight enough that I thought this was probably an issue and could rub the plastic off the wires, so I loosened the connection and there it was, a wire had rubbed the plastic coating off and bare wire was touching the junction box. I taped it up and now it works. This may or may not be something to look at but throwing out what I experienced. That was just pure lazyness in the wiring.

[KevinH268]

Did you ever figure this out , I have the same issue with a 2019 f150