trailer marker light trouble
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Senior User
Joined: Feb 2005
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trailer marker light trouble
i have an 02 250sd and recently i started having trouble blowing the 20a fuse for the trailer marker light circuit. it doesnt blow while i have a trailer connected. it only started doing this recently. i can have marker lights when i drop the trailer, but the fuse will be smoked when i go to hook back up to either of my trailers. i have heard there is some kind of relay that causes this? any help guys?
"Mifflin Clay"
Joined: Jul 2005
Posts: 3,177
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From: Mifflin, PA
At a guess sounds like something loose or chafed wiring in the truck harness - something that can short when you unhook. I'd check the wiring, including inside the connectors. Does it do it with both trailers? If so it's something with the truck. If only one trailer, it's the trailer.
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yes it does it with any trailer that i hook up. it blows the fuse whether there is a load on the circuit or not. i can replace the fuse as soon as i notice it has popped, and it will stay intact for a random amount of time. sometimes for an hour, sometimes a day. i keep lots of spares in the glovebox, so i keep a good fuse in it, and make sure it is working when i drop the trailer. it has been popped by the time i hook back up, or test that spade on the connector with a tester. the wiring at the back of the connector seems to be in good shape, and not damaged. any ideas of a likely place it may have a stress point?
Posting Guru
Joined: Jan 2003
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From: Back in the TX Coast
I just started having the same problem...replaced fuse 38, hooked trailer up everything works fine move truck and trailer...blow fuse 
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Joe Kucinski
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Joined: Apr 2004
Posts: 127
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From: Palm Bay, FL
Encountered similar difficulty with my truck. Talked to a tech at Bloomington Ford yesterday and K356 is built into the back side of the cjb and not serviceable, requiring replacement of the cjb if it fails. He also stated that, in his experience, he has not seen a K356 failure....it's always been a trailer wiring issue and 9 out of 10 times a grounding issue. Says Ford circuitry is ground-finicky.
I know my car trailer has some iffy wiring, so I'm spending the day going back through it as well as replacing the truck connector. I'm somewhat skeptical as I found a 25A (instead of the correct 20A) fuse in position #38 and I had no such problems with my last towrig pulling this trailer, nor have the people had a problem who've borrowed the trailer.
If the above efforts don't cure the problem, I'm going to add a stand-alone wire and switch for the trailer running lights rather than screw with the cjb.
Good luck with yours.
I know my car trailer has some iffy wiring, so I'm spending the day going back through it as well as replacing the truck connector. I'm somewhat skeptical as I found a 25A (instead of the correct 20A) fuse in position #38 and I had no such problems with my last towrig pulling this trailer, nor have the people had a problem who've borrowed the trailer.
If the above efforts don't cure the problem, I'm going to add a stand-alone wire and switch for the trailer running lights rather than screw with the cjb.
Good luck with yours.
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one of the trailers i have the issue with is almost brand new, and the problem didnt surface until after i had pulled it many times. i have gone over both of the trailers thoroughly, so it sounds like i need to just run a stand alone wire to that circuit. probably will run a relay from the truck marker circuit for a trigger and back from there. kinda irritated about it. with it getting dark early and not knowing if im gonna have my lights. if i wanted wiring gremlins, i would have bought a dodge
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Joined: Apr 2004
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From: Palm Bay, FL
In my case, it's obvious that my trailer wiring was the culprit. I don't know for certain what the exact problem was as I started from the back forward; cleaning sealed beam terminals/grounds, replacing crimps with silicone-sealed wire nuts, and I replaced the complete breakaway system, trailer connector and harness, and the factory truck connector (Pollak #11-893).
Even though my trailer is barely 5 years old, I was surprised at all the corrosion and brittle insulation I found; regardless that all wiring is boxframe/pvc enclosed. In the Spring when I'll again be eager to attack projects, I'll replace all the wiring, solder and shrinkwrap all connections, and extend all ground wires back to a bussbar on the tongue for direct connection to truck ground. Slithering around on my back most of a day on gravel at 12*F doesn't deliver the intense level of joy it used to.
Good luck.
Even though my trailer is barely 5 years old, I was surprised at all the corrosion and brittle insulation I found; regardless that all wiring is boxframe/pvc enclosed. In the Spring when I'll again be eager to attack projects, I'll replace all the wiring, solder and shrinkwrap all connections, and extend all ground wires back to a bussbar on the tongue for direct connection to truck ground. Slithering around on my back most of a day on gravel at 12*F doesn't deliver the intense level of joy it used to.
Good luck.
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