The oil pressure wire is tan or tan with a black stripe (kinda foggy there). The choke wire is constant 12v I believe where the tan wire is just for the gauge and isn't supposed to be hooked up to anything else. Think of the sending unit as a potentiometer and the gauge as an ohm meter, because that's basically what that setup is (along with fuel level and water temp.). There is a specific resistance range the gauge and sender have to operate in for the gauge to be accurate.