Just got done doing a mechanical and cosmetic preservation for a great customer of mine Rory Hester out in Idaho. All wrapped up, engine running great, but as soon as I would turn on the lights the engine would die and the fuse would be blown.Ugh! So after an hour or so of tinkering I decided to get out the trusty old multimeter and find the "short to ground". Any time you have a consistent blown fuse it is because Positive voltage from the battery is somehow passing directly to the negative side of the battery with out going through a sufficient load. This can be directly through a hot wire touching a ground wire because the insulation has worn or a hot wire somehow touching the frame or engine case, which in automotive and motorcycle applications, acts like a big wire for the return path to the negative side of the battery. In my case it was the latter. I simply had the hot taillight wire hooked to the ground terminal of the taillight! Since I work on a pretty wide array of vintage motorcycles, sometimes I forget the idiosyncracies of different machines. In the case of this Vespa GS160 MK1, the black wire is a hot and the white wire is ground. How did I figure it out? I used the multimeter to show me the short to ground through a significant voltage drop and then disconnected wires from the ignition switch until the drop went away. I then traced that circuit until I found the issue. In this video I show you how to conduct the test.
This test will work on any battery type dc electrical system. Especially those with battery ignitions. I hope it is informative and feel free to leave a comment