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Old 04-16-2018, 06:16 AM   #7
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Join Date: Mar 2011
Location: Gaylord
Posts: 14,363
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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