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Old 04-15-2018, 06:09 AM   #1
Indy Steve
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Blinking marker lights

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.
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Old 04-15-2018, 09:14 AM   #2
jsmith948
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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.
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Old 04-15-2018, 09:05 PM   #3
Indy Steve
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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.
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Old 04-16-2018, 05:10 AM   #4
flybouy
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As you list a 2018 model, have asked your dealer about it? I would suggest starting there.
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Old 04-16-2018, 05:38 AM   #5
jsmith948
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Quote:
Originally Posted by Indy Steve View Post
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
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Old 04-16-2018, 06:03 AM   #6
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Quote:
Originally Posted by jsmith948 View Post
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
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Old 04-16-2018, 06:16 AM   #7
JRTJH
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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.
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