Abstract: A tester includes a housing containing a passive test circuit including a plurality of LEDs respectively connected to terminals of a multi-terminal connector jack on the housing for connecting the tester, by means of an associated cable harness, between a keyswitch module and other electronic circuits on an associated vehicle to be tested. Different harnesses may be provided, tailored to different vehicles. The test circuit is powered by a battery on board the vehicle and includes indicators dedicated to checking to see if the battery is connected properly and if an associated battery charging circuit on the vehicle is operating properly. The remaining indicators check different circuits, depending upon the particular vehicle being tested. The test circuit includes override switches associated with certain ones of the indicators and a test switch for simultaneously energizing all of the indicators.