Welcome to iboats.
I'm not sure that we got all of your post. So I'm assuming that it's connected like the diagram but doesn't work. You should have a 12V, a good ground and a signal wire. (Plus maybe something for the light) The 12V and ground should be self explanatory. The signal wire should flash an old fashion incandescent test light with the engine idling. (maybe more of a flicker) If all this is good and the gauge still doesn't work, let us know what the markings are on the terminals that each is connected to.
