A marine fuel sending unit is the device responsible for telling the fuel gauge what level to display, thereby informing the boat driver of how much fuel is left in the tank. The unit is generally made up of a float connected to a thin metal arm. The floater sinks as the tank loses gas, causing the metal rod to move a wiper across a resistor. The level of resistance encountered by the wiper dictates the fuel gauge reading.
Marine fuel sending units can encounter transmission problems that can cause incorrect gas gauge readings. To confirm that the problem is in the fuel sending unit, est the device using a standard digital voltmeter or DVM. Switch the DVM to the ohm setting, and connect it to the center stud and ground reference.
For U.S. fuel gauges made by companies such as Faria, Moeller, Tempo and Universal, the standard DVM reading is between 33 and 240 ohms. If the DVM's readings are outside the ideal ohm range, there is a problem with the sender. Clean the center and ground reference terminals with a stainless steel brush, and test the sender again. If the problem persists, it's time to replace the sender.
If the reading is within range but the fuel gauge still reads incorrectly, that could mean a failure somewhere in the connections from the tank to the gauge, which may require the involvement of a certified boat technician.