I've got one, to be honest, not sure if I know how to use it! I mean I can check batteries and alternators and stuff, but what is an acceptable drop? At what point should I worry about my switches and wiring? If I test on either side of the switch, how much voltage drop is OK? Weird thing is, my headlights are not dim. I had an ignition switch burn out in a truck and a headlight switch actually catch fire in another car and never really got either of those fixed "right", so sorry if I sound a little paniced!