This handy toolbox essential instantly locates continuity problems, including bad connections, faulty or broken wires and poorly grounded sockets in your 12V system. The light illuminates if the connection is sound. No light will be visible if there is a short circuit or poor connection. Must be used with your 12V battery connected and charged. It includes an alligator ground clip on a 45" lead wire and a sharp 3 metal probe for carefully piercing insulation on wires when necessary.