Has anyone had any problems associated with the fuel gauge/sender unit on a 2000 AU Ford supercab ute? On start-up gauge indicates full but after running for about 30 secs drops to empty

gead short in the wire from the gauge to the sender unit
the gauge works by the variable resistance in the tank unit so when the gauge is full there is low resistance so the gauge reads full
reading full all the time indicates that there is power flowing through the gauge to a ground before the sender unit in the tank

Not much point replacing the fuel pump until the supply and ground has been checked and found to be ok.
Gauge sender might share the same ground...

If the fuel gauge sender and fuel pump share the same space, replacing the pump provides the ideal opportunity to test the gauge unit with an analogue ohm meter.

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 Fuel level FLOAT, in the tank is defective. Possible that it "holed", and has sunk to the bottom of the tank.
To check, separate the harness plug to the Tank sender unit, and ground the supply to a suitable place on the frame. The dash gauge should record "Full", therefore the sender unit is suspect.
regards. DT

The sender is more likely to fail. If you disconnect the sender electrical connector near the tank, you can ground the sender lead to see if the gauge goes to empty, indicating that the gauge and wiring are OK.

Fuel Level Indication System The fuel level indication system is a magnetic gauge system. It consists of a magnetic fuel gauge (9280) mounted in the instrument cluster (10849) and a sending unit located in the fuel tank (9002) with an instrument cluster gauge amplifier (10E849) in series between the two. No instrument voltage regulator is used with this system. Fuel Gauge
The magnetic gauge movement consists of three primary coils, one of which is wound at a 90-degree angle to the other two. The coils form a magnetic field which varies in direction according to the variable resistance of the sender unit which is connected between two of them. A primary magnet, to which a shaft and pointer are attached, rotates to align to this primary field, resulting in pointer position. The bobbin/coil assembly is pressed into a metal housing which has two holes for dial mounting. There is no adjustment, calibration or maintenance required for these gauges. The instrument cluster amplifier is calibrated to the fuel gauge. The instrument cluster amplifier and fuel gauge must be replaced as a system. Instrument Cluster Gauge Amplifier/Check Gauge Module
The instrument cluster gauge amplifier electrically averages fuel sender readings so the fuel gauge displays actual fuel level and not fluctuations due to sloshing in fuel tank. The instrument cluster gauge amplifier is a small printed circuit board which latches into a pocket on the back of the instrument cluster. The electrical connections for ignition, ground, input from fuel sender and output to fuel gauge are made through a spring-type connector on the instrument cluster gauge amplifier to the instrument cluster printed circuit (10K843) on the instrument cluster. There are no provisions for calibration or adjustment of the instrument cluster gauge amplifier.
Fuel Sending Unit
The fuel sending unit consists of a variable screened resistor made up of a ceramic substrate. It is controlled by the action of an attached float in the fuel tank. When the fuel level is low, resistance in the sending unit is low. When the fuel level is high, the resistance in the sending unit is high. As the float moves from empty to full, the resistance will gradually and continuously increase.

You're either looking at a short in the wire that goes to the fuel sender, or a bad fuel sender. If it was just a broken wire, it would read empty instead. The fuel sender is a permanent part of the fuel pump, so that has to be replaced as a unit.

Ford has lots of problems with the sender reliability. Typically, the sender (and expensive fuel pump) replacement resolves the issue. To test, you need to take the connection off of the sending unit, at the tank, turn on ignition, and then ground and un ground it and view results at the gauge. If the gauge responds, the sender needs to be replaced.

There are aftermarket senders that cost less than OEM.

the buffer plates in the tank seem to be loosely held! so sometimes they fall on the sender unit,causing the gauge to drop!!!!! when the vehicle is shaken!!!! remove that tank and have the plates checked!!!!

The float on the sending unit could be jammed of your have a short in the harness.

Check the harness first and pay attention to the connections, if there ok then it's the float on the sending unit on the gas tank.

Sorry for the bad news but it make start to work again any time if it is the float.

Good luck, and hope this helps