My fuel gauge registers as I would expect it to according to the ammount of fuel in the boat when I turn the key on. After just a couple of seconds it falls to empty. I was told to check the sending unit by touching the hot wire to the ground. This should make the gauge spike to full and stay indicating that the gauge and wiring are fine. I did this and the gauge did spike to full but after a couple of seconds it fell to empty again. I ran jumper wires on the hot and ground side anyway and the wires seem to be fine. The gauge does the same thing. I took the gauge out and hooked it up to a battery charger and was able to get it to spike to full and stay. At his point have I not tested everything with positive results?  I did notice that there a lot of wires on the fuel gauge. At the left bolt (S) there is just one wire leading to the sending unit. The upper bolt (G) has two grounding wires. One leading into the wiring harness and one leading to what looks like a large relay near the gauges. The last bolt on the right (I) has 3 wires. One goes to the low oil alarm, one goes to the speedo gauge next to it and the last one goes to the same relay I mentioned. There are 2 other wires coming out of the relay that go into the main harness. Is it a bad relay? Too much on the gauge? Wired wrong?
The way to test the sending unit is with an ohm meter....maybe the manufacturer can tell ya what ohms it is suposed to read full and empty....I think thay are pretty standard...but I ouldnt tell ya what it is.
Thank you for reading this far and thank you for any help you may be able to offer.