US 4-Pin Flat Connector
Towing & Hitches · 7 specs
Direct Answer
The US 4-Pin Flat Connector (standardized under SAE J1239) is the ubiquitous North American electrical plug for light-duty utility trailers, small boat trailers, and snowmobile haulers without electric brakes. Utilizing an inline rubber molded connector with one exposed male pin and three shrouded female sockets.
Source: SAE International Standard J1239 (Four-Conductor Electrical Connector for Light Duty Trailers) · Last reviewed Aug 28, 2026
Specs
| Standardization | SAE J1239 / 4-Pole Flat Connector |
| Pin 1 (White–Exposed Male Pin) | Ground / Earth Return (Exposed male pin ensures ground connects first upon insertion) |
| Pin 2 (Brown–Shrouded Female) | Tail Lights, License Plate Lamp, and Side Marker Running Lights |
| Pin 3 (Yellow–Shrouded Female) | Left Turn Signal and Left Stop/Brake Light |
| Pin 4 (Green–Shrouded Female) | Right Turn Signal and Right Stop/Brake Light |
| Combined Stop/Turn Architecture | Brake and turn signals share the same wire and bulb filament (flashing one filament while braking) |
| 5-Pin Flat Extension | Adds a 5th Blue wire for Reverse Lights / Surge Brake Hydraulic Lockout Solenoid (boat trailers) |
Why the Ground Pin is Exposed Male
On the vehicle side of a 4-pin flat harness, the three powered circuits (Tail, Left Turn, Right Turn) are deeply recessed female sockets to prevent live 12V pins from short-circuiting against the steel bumper or hitch receiver if dangling. The Ground wire is the single exposed male bullet pin, ensuring safe, spark-free chassis earthing.