Boat fuel gauges can be troublesome. Here is a quick method of determining whether the fault lies in your boat's fuel gauge or in the fuel gauge sending unit on the tank. You may need a partner to complete this simple task, depending upon how you proceed.
Locate the fuel sending unit. This is often accessed through a deckplate placed over the after end of the tank. What you will see is a small, round plate fastened to the tank with five screws and with two wires connected to it.
Identify the sender( Pink is the ABYC-designated wire color for the conductor leading from sender to fuel gauge) conductor. It should be attached to the center of the sending unit.
Identify the ground conductor. In most cases this will be black.
Turn the ignition key to on, or otherwise energize your gauge panel so that the fuel gauge is operable.
Using the barrel of a screwdriver or a jumper wire, connect ( "jump") the sender's connection to the ground connection. Now, have your helper read the gauge for you. If you used a jumper wire, you can leave it in place and go read the gauge yourself.
If after " grounding out" the sender the gauge reads Full, then the problem is with the sender; if the gauge does not respond then the problem is with the gauge. If the problem is with the gauge or its wiring, start by performing a continuity check of the gauge wiring.