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The SAE J560 (black or green) plug has been used since 1951 to power the trailer functions: Ground, Side Marker Lights, Left Turn, Brake, ABS Power, Right Turn, and Tail/License per Federal Motor Vehicle Safety Standard (FMVSS108). This Standard includes definition of vehicle lighting, signaling, and reflective markings. The equivalent Canadian Standard (CMVSS) has required daytime running lights since 1990. This is not a US requirement.


SAE J560 dictates the common pin position for use with a green extension cord, but does have alternate wiring options, like those which tie the brake lights to the turn signals on the trailer. These things can be accomplished in the aftermarket.

The ISO 3731 (yellow plug, yellow cover) is an auxiliary seven way that has been in use since 1980. The optional functions are extensive, and include items such as: Ground, ABS Fault indication, Reverse Lamps, Control Ground, Ignition Lock, and Rear Fog Lamps, or any other auxiliary circuits. There is no common pin position.


The pins in each plug are positioned such that J560 and ISO3731 cannot be accidentally switched. That is to say, each cord will only attach to the proper mating plug.


Many newer model chassis today have a Pre-trip Switch which turns power individually to the J560 lights on the trailer to verify that they are operational. This saves the driver, many trips back and forth from the cab to trailer turning on the individual switches during their walk around. The Pre-trip Switch does not include the functions wired to the ISO 3731.


FMVSS121 applies to air brakes. Trailer brakes are pneumatic, so neither plug deals with the brakes, just the signals associated with the brakes (lights and ABS power).

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