I was given this tach, and I cannot find any info on who made it and how to hook it up to test it. It has two wires, one wire is for the light, it works when the case is grounded and 12v is applied to the black wire. The only other wire is white with a purple stripe, and I suppose that goes to the negative side of the coil? There are a few identifying marks, on the tach it has AT-5001. Who made this and how did it wire in?