Question:
Why do I need the blue wire on a 40 foot semi trailer? What does it control?
asked by: Doug B
Expert Reply:
Sounds like you are referencing the blue wire of the 7-way round connector like what's in the attached picture. If you have no use for an additional 12 volt circuit then it would not be needed. It isn't always needed so it sounds like you may not need it at all.
Pollak Heavy-Duty, 7-Pole, Round Pin, Trailer Wiring Connector - Trailer End
