Lets start with the simple things, make sure there is a grounding wire from one of the securing screws of the fuel sender to a clean spot on the body.as my first project car i recently bought a 1970 VW beetle, the car is actually in a very good condition but the fuel gauge doesn't work but both the odometer and speedometer are working fine. so i wanted to know ways how i can identify why the fuel gauge is not working, plus for those who drive with non functional fuel gauges how do you keep track of the fuel you have left before you get stranded on the side of the road.
Most common place would be a multiple ground connector by dash center bracket.
The fuel gauge sender is mounted on a rubber gasket and if the gas tank has been replaced chances are that the unit is not grounded,
Next make sure there is a 12 volt source to the gauge. Some models have a built in resistor that contains a fuse leading to the gauge.
The system works by providing variable ground to the gauge.
If you have power to gauge and proper ground then you have a defective sender or gauge.
Applying ground to gauge will make it to max out. Sender can be tested with an OHM meter or turning it upside down while grounding it and reading dash gauge ( also method of adjusting unit)
Let me know if you need additional info.