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...http://www.google.fr/patents/US4547722?utm_source=gb-gplus-shareBrevet US4547722 - Test panel for tractor and trailer lights
