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Your guage is wired so that it gets voltage from the bus behind the dash. Basically, there is usually a purple wire that feeds all the dash guages. The 12 volts goes through the guage and out to the sender. The sender has a variable resistor that varies according to the level of fuel in the tank. The circuit is completed by the sender being grounded either directly to the tank or a separate ground wire. You can take the wire that goes to the sender off and hook up an ohmmeter to it and ground to see if you have the sender grounded. You should read some resistance of up to a few hundred ohms if there is fuel in the tank. If you don't then look for a bad ground. Hope this helps. Here is a generic code for marine wiring. It's old and now the ground wire is coded yellow. I don't know what year it changed. Good luck
FUNCTION -WIRE COLOR(S)
Ground circuit - Black ( Now Yellow)
Positive circuit - Red
Switched positive - Purple
Fused positive - Orange
Tachometer sender - Gray
Fuel gauge sender - Pink
Oil warning sender - Varies with engine manufacturer
Temperature sender - Tan (also horn hot lead)
Trim gauge sender - White w/tan stripe
Instrument lights - Dark blue
Bow and stern lights - Dark gray w/white stripe
Bilge pump - Brown
Engine kill circuit - Black w/yellow stripe
Neutral start circuit - Yellow w/red stripe
Power trim main circuit - Red w/blue or white stripe (Mercury; purple)
Power trim (down) - Light green w/white stripe
Power trim (up) - Light blue w/white stripe
Hull bonding ground - Dark green
Bilge blower - Yellow ( This one definitely had to change but I
don't have the new color)
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