PURPOSE: To provide switch means changing over a plurality of signal lines between a truck tractor and a trailer, to selectively execute connection and interruption and to use high speed data communication wiring for control/ diagnosis.

CONSTITUTION: A tractor electronic controller 8 sets a state in accordance with whether an adaptation device is contained in a trailer or not and it controls electric switch devices 10A, 10B, 12A, 12B, 13A, 13B and 16. The tractor electronic controller 8 and a standard trailer electric package 20 are connected to a conventional trailer electric package 19 and a standard trailer electric device 28 through tractor/trailer connector packages 22 and 24. Information sufficient for detecting a problem is contained in electronic controller diagnosis lines 30A and 30B and high speed data link lines 32A and 32B are connected to different devices and information is exchanged. Thus, the linkage/detachment of electric connection to an electric connector between the tractor and the trailer and high speed data communication wiring can be used for control and diagnosis.