Abstract: A portable diagnostic device for checking electrical signalling systems of for example tractors and their trailers. The device has several receptacles for connection to a corresponding receptacle or pin connector of a trailer. One electrical circuit includes switches for energizing individual signalling or other circuits of the trailer individually, and indicating lamps and/or horns for annunciating circuit operability. Another electrical circuit includes switches for energizing individual signalling or illumination circuits individually, and indicating lamps and/or horns for annunciating circuit operability. The device, which is contained on a wheeled stand, has its own power supply and circuit overcurrent protective devices, and is thus independent.