i just purchased all aftermarket gauges for a progect boat i have. I have a question on wiring. they are beede gauges and my question is this. the back of the gauges have letter to label the posts. I know what that the g is ground, but what is I, S, And O. The gauge fastener jumps g to o. Does that me o isnt anything? I think i is ignition, and s would be sender, correct?