Fuel gauge not working properly. Says fuel tank full when fuel tank empty

You could use two resistors, Two different ohms specs for full and empty. Place resistor between proper wire colors in diagram. Turn on the key, If gage reacts properly to full and empty resistors, then I think the problem is the sending unit in gas tank. If not, the problem could be almost anywhere, wiring circuit--pcm--instrument cluster. I did see trouble codes relating to fuel gage sending unit. I hesitate to get into that? good luck

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.
   

If the trouble with the gauge started after you worked on it then you likely damaged or created a restriction to the movement of the sending unit. You will have to remove the tank and double-check your work.

Repair is a major job involving dropping the fuel tank, a messy and dirty and dangerous job. It will take you at least one full Saturday to do this job if everything goes right and you have the proper parts. My free advice to you is to fill your tank every 200 miles and forget about the fuel gauge.

you have 2 recalls
pcm update fuel pump replacement
fuel pump module replacement and pcm software

It sounds like the fuel sender is bad in the fuel tank.

Remove the sender wire from the fuel tank and short it to ground (metal part of tank).

If the fuel gauge goes to full then your sender unit is faulty and replace it.If not ,please come back to me .

Please leave positive feedback if you feel I have helped you.

Thank you

the only thing that will cause this symptom is the fuel tank sending unit.

Replace fuel tank sending unit.

Sounds like the Fuel sender unit in the fuel tank needs replacing however you may want to check that the electrical plugs are contacting properly,Check that the gauge is working by putting a piece of wire from the sender unit wire to the chassis,at the fuel tank sender unit plug,Turn ignition on while watching gauge if it moves toward fill turn ignition off and remove wire to chassis.If gauge moves it means from the fuel tank to fuel gauge is intact and not your problem.