fuel gauge

i have a 91' explorer xlt and for some reason the fuel gauge doesnt work when the car is running and driving. it works only when the ignition is in the acc position but gives and incorrect reading becasue it always just goes up to a quater of a tank. any suggestions you have for me im open to them.

The most common problem is the float in the tank sinks because of a pin hole. Sender is 170 ohms full and 15 ohms empty. Reverse of many older vehicles. The test is the following. Unplug the tank electrical connector, you will see the gauge go to full. 99% of the time it is the float.