1999 Chevy Express Fuel gauge shows full tank when empty

1999 CHEVROLET EXPRESS
250,000 MILES • V8 • 2WD • AUTOMATIC
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MBECKER123
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Today I ran out of gas when the gauge showed a full tank. I put in 10 gallons and the truck started right up and drove no problem, but the gauge still shows full when it should be a quarter. Help!

The gauge has been a blur for well over a year now. This seems to be a very common but benign issue. About 2 months ago the pump went bad, so I replaced the pump/ sender unit. The old unit did not have a pressure sensor but new one does. This new pump/ sender has about 2k miles and the gauge was accurate until today. The gauge still has that blur. The gauge goes to 3/4 with key out, past F with key in.
Jul 4, 2010 at 12:50 PM
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INTERNETMECHANIC
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Check the wiring at the fuel pump connector, if you can reach it. The purple wire around 5 volts, key on, no crank. Black wire is ground. If you use a 40 ohm resistor between those two wires, key on, the gage should read empty. If you check voltage on purple wire use digital multimeter. The database wiring may have been for older pump, didn't notice pressure sensor?
Jul 4, 2010 at 2:18 PM
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MBECKER123
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thanks, I'll test the float resistance. As for the pressure sensor the pump/ sender unit was available with and without, didn't know which I needed so I got the unit with pressure sensor. My truck does not use that sensor. The new pump unit has about 2000 miles now.
Jul 5, 2010 at 9:19 AM
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INTERNETMECHANIC
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You use the resistor on the wiring connector not the pump assembly.
Jul 5, 2010 at 10:14 AM
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MBECKER123
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My sender unit checks out. I also swapped the oil and gas gauges but fuel gauge still behaves the same. Where can I find the "Fuel Sender Buffer Module"? The Blazer forums talk about this problem but I cant find it on my van.
Aug 8, 2010 at 9:02 PM