When your fuel gauge gives you erratic readings or none at all, there may be a few things going on. The problem may result from the following: a blown fuse, a loose ground wire from the fuel gauge or the sending unit to the common ground, a loose wire on the S-post of the fuel gauge or the fuel sending unit, a bad fuel gauge or a bad sending unit.
Use a screwdriver to disconnect the sending unit wire from the boat's common ground. Disconnect the wire from the post on the sending unit marked "S" that leads to the fuel gauge. Isolate the wire by tagging it with a piece of electrical tape. Remove the screws with the screwdriver--or use a wrench if the sending unit is mounted with bolts--from the tabs on the side of the sending unit. Lift the sending unit out of the tank.
Stuff a clean rag into the fuel sending unit opening. If there's any gasket material from the old sending unit around the opening, scrape it away with a putty knife. Wipe the area with a clean rag dipped in acetone to remove any remaining gasket material.
Take the rag out of the opening. Put the gasket supplied with the sending unit onto the bottom of the new sending unit. Set the new sending unit into the tank and adjust the float arm as necessary. You can bend the arm sideways by hand to avoid baffles in the fuel tank.
Dip the sending unit mounting screws in a gasoline-proof thread-locking compound. Push them through the holes in the flange of the sending unit and thread them into the sending unit mounting holes in the fuel tank. Tighten them with the screwdriver if they are screws, or a wrench if they are bolts. If the manufacturer specifies a torque for the bolts, use a torque wrench to tighten them to the appropriate torque.
Connect the fuel gauge wire that you tagged with a piece of tape to the S-post on the back of the new gauge. Connect the sending unit ground wire (marked "-") to the boat's common ground.
Avoid fire, open flame or lit cigarettes during installation. Working on fuel tanks makes your work area a no-smoking one.