AS you know this can be in either location being the truck or the trailer. I always have one of these 7-way adapters (AMAZON-ETRAILER) in my tool to plug-in the truck side and make sure those signals are working correctly. Then I know where to start looking for the problems...
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7-way BLADE TESTER
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Wire colors may be different depending on the manufaturer...
Etrailer.com is a great source for wiring info for the RV trailers...
My F150 Truck side 7-way connector has a pin-out decal on the side of the blade connections which helps big time when using a multimeter to check the blade readings...
I usually squirt WD-40 into the 7-way connector before plugging in the trailer. This keeps the blade connections clean for me... Some folks like to use a compound from the auto parts places that probably does a better job then WD-40 for electrical connections. WD-40 works OK for me as I always carry a PLAN B fresh can of it in my door side pocket. I just do a couple of squirts of it and move the the plug around a few times and never have a dirty connection problem it seems... There is also a roll of duct tape in my side pocket as well haha...
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Might be hank of bailing wire there as well...
Roy Ken