Abstract: The invention is a device for testing the electrical circuitry on all trailers used in the cargo industry. The device plugs into the electrical connector of a trailer and allows a single mechanic to test and make brake adjustments without the assistance of another individual, to check the operation of all lighting and signal lights of the trailer, and to test the integrity of each electrical circuit and locate shorts in the circuits. An electrical short in a circuit is located by applying a low amperage current, at 12 volts, to the inoperative circuit and locating the warm point in the wiring where the short exists. The brake system adjustments provided by the device include checking sticky or frozen cams, moving the cams while lubricating them, and testing the various valve functions in the air brakes system of the cargo trailer.