Originally Posted by ej1 wilcox

If there's just a pos, neg, and a "signal wire", then it's a mechanical gauge, and the pos and neg are for the back light.

If it were electrical it would have a pos and neg for the light, plus an ignition, signal, and neg for the sensor as well. There would be 5 connections if it were electrical. At least that's how my autometer electronic oil press gauge is.