How a Reed Switch Marine Fuel Sending unit Work ?

The reed switch fuel sender unit, or boat fuel tank sending unit or RV tank level sensor, is wired to the marine fuel level gauge by a sending unit wire.Working like a toilet tank float, the attached float moves with the fuel level, allowing the sender to adjust its resistance (ohms) as the float descends. This change moves the gauge needle, showing the tank's fullness on the boat fuel gauge.
 It’s a good idea to familiarize yourself with these parts before they malfunction.You’ll typically find a small, round plate at the fuel tank’s top (boat, truck, or camper), usually reachable via a deck plate at the aft end.
Your fuel & water level sensor has either two or three connections. In the three-wire type: one to the gauge, one to power, and one to ground/battery negative. In the two-wire type: one to the gauge (for power and signal) and one to ground.