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.
- Perform test with resistors as follows:
- Disconnect wiring connector at sender unit, then connect resistor between gauge lead and a suitable ground.
- Turn ignition switch to On position.
- With 145 ohm resistor, the gauge pointer should contact the Full mark.
- With 22 ohm resistor, the gauge pointer should contact the Empty mark.
- Perform test with fuel sender of known quality as follows:
- Turn ignition to Off position.
- Disconnect wiring connector from sender and connect it to test sender.
- 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.
- 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.