The Standard SAE J1239 was established in 1978 by the American standards developing organization SAE International (formerly the Society of Automotive Engineers). It was revised in 1995 and 2003, whereas the current edition dates back to 2010. SAE J1239 was developed to create a standard connector system featuring plug, socket and wiring for the connection of light trailers and towing vehicles. The purpose of the system is to ensure the power supply of the trailer lighting and the transfer of lighting signals from towing vehicle to trailer.

Scope of SAE J1239

The standard SAE J1239 covers the wiring and rectangularly shaped connector standards for all types of trailers whose gross weight does not exceed 4,540 kg (10,000 lb). These trailers are grouped in the standard SAE J684 with running light circuit loads not to exceed 7.5 A per circuit.

The standard defines a mechanically and electrically secure connection including a well-insulated strain relief between the receptacle and the towing vehicle wiring harness connections. This avoids strain on the vehicle harness in case of an unexpectedly strong pull on the receptacle. All wire and insulation conforms to the requirements of SAE J1128 for low voltage primary cables in electrical systems of surface vehicles.

SAE J1239 also includes requirements concerning 5- and 8-way connectors, however, these are rarely used in practice.