I edited the link above. It should work now. Thanks to the both of you for responding even though my link was bad. I was looking at the circuit board this morning and there are 3 actual inputs BUT only two are fuctional. If you see the picture there are three prongs I guess you can call them but two prongs actually are connected to the circuit. I don't think the tachometer is being grounded by the case like some older electronics are. So I don't think this tach uses the normal 12v/GNd/Sig. I spoke to the guy who asked me to look at this and he told me it only uses signal and ground. *shrug* I've never seen anything like that before. As far the mauf. of the tach it say: Sun Electric Corp. Chicago, ILL. I hope someone can give me some insight I'm completely stumped.
Thanks again!