Fuel gauge stays on full mark when empty, but the low fuel indicator light comes on. working the fuel sending unit by hand, I raise up the float and the low fuel light goe off

sending unit is stuck its in the tank

The fuel level indicating system is a magnetic type system, which consists of the sending unit located in the fuel tank, an anti-slosh module located on the back of the instrument cluster, and a fuel gauge located in the instrument cluster. The sending unit changes resistance according to the level of fuel in the fuel tank, which varies the current flow through the gauge.The pointer position varies proportionately to the current flow. In this system, the sending unit resistance is low (22 ohms) when the fuel level is low and high( 145 ohms) when the fuel level is high. The pointer of the magnetic gauge remains in position when the ignition is turned to the Off position. On some models, an anti-slosh module is used to dampen out fluctuating fuel signals from the sender. Some vehicles are equipped with a low fuel warning indicator. The anti-slosh module will also actuate the low fuel indicator when the fuel level in the tank reaches 1/16 to 1/8 full.


Do your other gauges/warning indicators work properly with the ignition on? If not, check for power at the fuse panel/fuses.

If they do then partially remove your cluster with connector still installed and check for voltage at cluster connector and gauge. Check continuity of cluster and gauge for ground.

Sometimes the anti-slosh module (if installed) goes bad in the Grand Marquis but not likely.

Sender troubleshooting:

1. Perform test with resistors as follows:

1. Disconnect wiring connector at sender unit, then connect resistor between gauge lead and a suitable ground.
2. Turn ignition switch to On position.
3. With 145 ohm resistor, the gauge pointer should contact the Full mark.
4. With 22 ohm resistor, the gauge pointer should contact the Empty mark.

1. Perform test with fuel sender of known quality as follows:

1. Turn ignition to Off position.
2. Disconnect wiring connector from sender and connect it to test sender.
3. Move float rod away from fuel filter against Full stop position. Wait approximately 30 seconds, then turn ignition switch to On position. The gauge should read on or above the Full mark.
4. Move float rod toward fuel filter against Empty stop position. Turn ignition switch to Off position. Wait approximately 30 seconds, then turn ignition switch to On position. The gauge should read on or below the Empty mark. If gauge works properly replace sender, which involves dropping fuel tank.
   

The problem is in the fuel sender unit in the tank. It sounds like the float part of the sender unit is binding when it's at the top (when tank is full) and hangs up there till it falls back down when tank is near empty. It will likely have to come out to be checked, fixed, or replaced. Good luck.

Ensure that all of the wire connections are clean, secure and corrosion-free.

2 Check your fuel gauge fuse if the gauge's indicator stays on empty when your turn on the ignition. If the fuse is good, join the sending unit connector to ground. Replace the sending unit if the gauge indictor moves to the full line.

3 Ground the terminal located on the back of the fuel gauge with a jumper wire if the indicator doesn't move. If it does move, consider replacing the wire between the fuel gauge and the sending unit. A poor ground is frequently responsible for fuel gauge problems.

4 Release the wire that connects to the fuel tank's sending unit. Confirm that there's no worn insulation that would permit wire contact to the body or frame. A fuel gauge reading of full generally indicates a problem with the tank unit, not the wiring.

very well sounds like the sending unit in the tank. Try running some octane booster and fuel injection cleaner in tank of the truck to loosen any deposits on the float.

When testing a fuel guage you should disconnect the wires to it,guage should read over full,ground the float wire,turn off key then turn back on should read empty.You have to cycle the key for guage to move.If still inop the wire is broken in the harness between the tank and the dash.Voltage on the float wire is low.When tested with a test light it will barely light the light and pulsate.This test indicates the wiring is good.

You might have a leak in the float in the fuel tank sender unit (the float should sit on top of the fuel and feedback info to the gauge - the fuel gauge staying on empty might mean that the float is either leaking or it is stuck).

Got the same problem, when empty fuel gauge shows full but fuel indicator light lights on.
Problem was the gauge itself and replaced by a second hand one.
How to remove: Remove the front glass (8 screws), remove the black frame (free), pull out both gauges (Fuel & Temp) they come togheter with their frame using a little screewdriver under their corners. Push the new unit in.That's it. Michel

OK, your fuel indicator is wired to a sending unit that is located inside the fuel tank. this assembly is part of the fuel pump assembly. the fuel indicator sending unit is housed in the same assembly as the fuel pump. The fuel indicator will not function properly if there is a bad ground or, if there is a fault within the sending unit. there is a floater device attached on this assembly and, it raises with the fuel level and lowers when the fuel level is low, sending resistance signals to the indicator gauge on your dash board. these signals can be disrupted if the ground is corroded or, if the sending unit is damaged. In most cases, replacing the sending unit will fix this issue. if the fuel indicator is stuck on the full mark, check the ground. if the fuel gauge is laying on empty all the time, replace the sending unit.

You don;t say whether the fuel gauge is reading accurately, or whether it's always on empty. If the gauge is always showing empty, it's most likely a problem with the sending unit in the fuel tank. Check the resistance across the two terminals of the sending unit.

On a C900, the fuel sender level unit is separate from the fuel pump and located on the right side top of the tank, accessible by removing the trunk floor lift up panel (the pump is located on the left side top of the tank.
Possible problems are: electrical line off or broken, float not working (it's fairly delicate); or gauge not working.

Test the resistance of the sending unit with the lines disconnected, the resistance should be 2.6 to 3.8 ohms full tank, 63.3 to 67.5 ohms empty tank; and test for voltage across the top of the sender unit, if no voltage, that's likely the problem!

If the gauge is otherwise reading accurately (showing 'full' for a full tank, etc) then the problem is likely a gauge malfunction and can be simply replaced.

Good luck!

Saabreur