How is my boat trailer wired and what does each wire do?

Trailer Wiring Color Code & Functions

Boat trailers typically use either a 4-wire or 5-wire wiring system:

• 4-Wire System (no brakes):
  Used on trailers without surge brakes. Covers ground, running lights, left turn/brake, and right turn/brake.
• 5-Wire System (with surge brakes):
  Adds an extra blue wire to control the reverse lockout solenoid, allowing the trailer to back up without brake engagement.

Standard Color Code

• White Wire – Ground
  Connects to the trailer frame/tongue. Provides the necessary ground for all lighting and electrical systems.
• Brown Wire – Running/Marker Lights
  Carries power to clearance lights, tail lights, and sometimes the 3-bar identification light on the rear crossmember.
  • Brown + Yellow: Routes down the left side of the trailer.
  • Brown + Green: Routes down the right side of the trailer.
• Yellow Wire – Left Turn & Brake Light
  Controls the left stop light and left turn signal.
• Green Wire – Right Turn & Brake Light
  Controls the right stop light and right turn signal.
• Blue Wire – Reverse Lockout (Auxiliary – 5-wire only)
  Connects to the tow vehicle’s reverse lights. Powers the reverse lockout solenoid behind the surge brake actuator, allowing you to back up without brake engagement.
  • Note: Not used on trailers without brakes