Abstract: A testing unit for testing the lighting of truck tractors and trailers wherein the tester can be alternately connected to the truck or the tractor and when connected to the tractor a battery in the tester provides power to energize the various circuits in the trailer such as left turn signal, right turn signal, stop lights, tail and marker lights, auxiliary feed and ground. When the tester is connected to the trailer connector of the tractor, various accessories on the tractor are turned on such as the tail lights, turn signals, stop lights, etc. and the indicator lights on the tester will light and correspond to the accessories turned on in the tractor. The tester can also check itself by attaching the negative and positive leads to a 12 volt battery and attaching the male trailer connector of the tester harness to the female connector of the tester by flipping the toggle switches to the ON position. Indicator lights of the tester will light up corresponding to the toggle switches.