I have a 70hp Nissan, and the fuel gage doesn't work (most annoying). How do i tell if the guage is bad or the sending unit ? When I first got boat, guage would work sometimes, now nothing. The sending unit has 2 wires, a red and a black, but the gage has 3 wires ? I bought boat used, and lot of 'rigging' has been done to the wiring, how should it be wired, and what do I do to find which part is bad ???????????? 