The gas tank is in the center of the floor, and the sending unit is normally at the back end of the tank. Look for a deck plate under your rear seats, or possibly under your engine well.
Usually, the first thing to check is the wiring connections (including the grounds). Also, the sending unit is usually less reliable then the gage, however:
When your ignition switch is off, the gage should drop to "E", so that makes it sound like you have an issue with the gage itself.
Also, you can test the sending unit without accessing the tank in the floor. You just need a digital electrical meter for ohm readings:
. Disconnect the "S" lead from the back of the gage.
. Check the resistance ("Ohm" reading) of the sending unit...check between the "S" wire and ground.
. According to Faria, the Ohms should be:
--- 240ohm at Empty
--- 203ohm at Half Tank
--- 33.5ohm at Full
When you rock the boat, you should also see a fluctuation in the ohms because of the sloshing.
If the sending unit seems to check OK, and the gage reads wrong even with cleaned up wires, then replace the gage.
http://faria-instruments.com/site_manuals/IS0085E.pdfRay
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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings