My fuel gage recently started to behave in a weird way. It can drop rapidly like in 2 seconds to a position below the Empty mark regards of the fuel amount in the tank. Sometimes it just shoots up back to the actually level but comes back to the same below Empty position with a couple of minutes. This is a C220, 72K km. Can somebody help?

Question edited to add year make and model.

The information is here by pinout, (pin 12)

http://orangecowboyhat.com/s10.html

https://www.google.com/search?q=98+s10+instrument+cluster+wiring+diagram

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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

I would say that you have a bad battery . If the alternator is charging ok but the battery is bad all the current will go to the dying cell . Have a load test done on the battery to eliminate it as a source of a problem. Next have the alternator bench tested as there may be a problem in the rectifier pack

Map sensor is bad

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.
   

Sounds like you have a weak fuel pump and a faulty fuel sending unit.

COULD BE THE FUEL SENDER OR YOU HAVE A POOR GROUND POINT IN THE CABIN.
WEIRD AND MULIPLE MALFUNCTIONS ARE USALLY A POOR GROUND.

Fuel pump is bad. At idle pressure should read 50 PSI. Replace with original manufacture fuel pump after market pumps will burn out fast and are unreliable from my experience. Also could have a bad fuel regulator but that only works while driving down the road full throttle. You can unhook the fuel rail pressure regulator and that will give it full fuel and the pressure reading should read 60 PSI while it is running. If you turn the engine off and turn the key to run position it should read 50 PSI and hold for 60 seconds, if it drops below 30 PSI after 30 seconds Pump is defective. Many pumps are going bad here of late as people have been running them less than ¼ tank and this kills the pump. The fuel cools and lubricates the pump so the lower the fuel level the worse it is on the pump below ¼ tank. Not to hard of a job to do if you want instructions let me know.

Is......the problem--"the gas gage reads empty when there is fuel in the tank. OR, The gage moves when the key to turned off ?

If it's the first,
You will need to test the gage and through process of elimination, determin if the Sending unit inside the fuel tank has gone Bad.

If its the second ? - Electrical gauges will move when powered down. Normal

Testing the gauge may not be easy - just depends on your ability.
It is a simple 2-wire gauge circuit, when disconnect (usually need the fuel tank) the gage reads in one direction (either full or empty)
then when shorted-the gage reads in oppisite direction.

Of course you must select the correct wires or you may short out the wrong circuits!! Most GM vehicles use a purple wire & black wire.

A quick method I have used to check the in-take part of the gauge is this.
1st. know there is gas - 1/2 tank works best.
2nd driver's door open, key in run position, engine not running.
3rd gently lift up on the door then let if come back down, gently.
just enough to cause the gas to slosh around.
4th - keep an eye on the gas gage, a bad sending unit in the take will show up as a "twitchy" needle.

Of course, if the gage never leaves empty - sloshing won't work.
I have a known good sending unit I can plug into the vehicle and simulate different levels of fuel - if the gas gage works, I know the problem is in the tank.

Let us know.

Dave

need to know how to change the fuel pump, do i have to drop the fuel tank or can i get to it from under the back seat