How to access gas tank sending unit?

Take the back seat out.

To expand on Tim's answer,

The sending unit is the round doo-hicky, with wires coming off of it, mounted in the top center of the tank. There are three or four screw that hold it in place. When you remove the screws, the sending unit and float can be pulled up and out of the tank. In the case of my 89 SN, I was able to remove the sending unit without removing my gas tank. If you don't have much clearance above the tank, you may have to pull the entire tank. If you decide to try and pull it out with the tank in place, clean everything around the area so that you don't knock a bunch of gunk into your tank as you maneuver the sending unit out.

Sending units are prone to wear and will develop a dead spot. The behavior is that the gauge will read full when the tank is full. Then, as the fuel level drops it will hit the dead spot. At this point, the gauge will bounce around eratically as fuel sloshes around in the tank. Once the fuel level drops sufficiently below the dead spot, the gauge will read empty, even though there is still fuel in the tank.

Before you go to the trouble of removing the sending unit, you can do a simple test to verify that the gauge is working correctly. Remove the wire from the center post of the sending unit. The gas gauge should read empty. Then briefly touch the wire to the ground wire. You should see your gas gauge jump to full. Again, only do this briefy.

Finally, if you do need to replace the sending unit, you should order it from a CC dealer. Some CC boats have VDO gauges which operate over a slightly different resistance range than the typically replacement sending unit that skidim and others carry.

You can try shorting the sense wire to ground to see if the gauge goes to full. You can do this right at the gauge or at the sending unit. If the gauge still doesn't move much, check the power, ground, and sense connections at the gauge. Use a multimeter to verify that you have 12v at the gauge and ohm out the ground to make sure it is good. Also, there should be a constant output voltage on the sense connection. I don't recall the voltage, but I think it is around 2 volts.

To check out the sending unit, you will have to remove it from the tank. The sending unit is simply a rheostat. You can use a multimeter to test the resistance from the sense post through the sending unit to the ground post. When the float is in the full position, you should read a low ohm value, around 22 ohms. In the empty position, you should a higher ohm value, around 220 ohms. As you slowly move float between full and empty, you should see the ohm value change relative to the direction, but never see it go to 0 ohms.