Fuel gage not working - Cars & Trucks

What year , make ,an model vehicle

I didn't see an s15 in that model year, in my database?
If the speedometer works but the tach doesn't, I think something is wrong with instrument cluster?
GM has had a problem with fuel gage sending units. The sending units are not hard wired to instrument cluster, they send signal to pcm, the pcm sends serial data to instrument cluster, telling the cluster the gage settings.
It may be possible to use different size resistors in place of the fuel level sending unit, to see if fuel gage reacts properly to different resistors, if the gage reacts properly, then I think the problem is the fuel gage sending unit.

Maybe a problem with instrument cluster?
Usually the circuit works off resistance to ground. The sending unit in gas tank provides resistance to ground. The signal wire for fuel gage is a purple wire, if goes to fuel pump wiring connector. With key on, use digital multimeter, check for voltage. If no voltage, either problem with wiring circuit or instrument cluster. W/0 looking at instrument cluster, I don't know if you can replace gage itself w/o replacing instrument cluster?

If voltage on the purple wire, ground it, while a helper watches the gage, if the gage starts reacting, then check the grounds for fuel pump wiring connector, if that checks ok, suspect the fuel pump sending unit has a problem.

did you set the fuel in right .sound like the sending unit is not right place

The fuel gage shares the same fuse as all the gages, so if the fuel gage is inop and all other gages are working then you have another problem. The most common cause is that the sending unit for the fuel gage inside the fuel tank is defective, that is true 99% of the time in my 35 years of repair work.

The most common cause of a erratic or incorrect fuel gage reading is a defective fuel gage sending unit inside the fuel tank. To replace the sending unit requires that the fuel tank be removed from the car and then the fuel pump module which contains the sender be replaced.

Testing the fuel gage requires accessing the harness from the fuel pump and grounding the yellow / white wire with 160 ohms resistance this should cause the gage to read full, and with the circuit open you should have the gage reading empty. If the gage works in this order then the likely failure is the sending unit in the tank , and will require replacement.

u will need to replace the fuel gauge sending unit.

check your fuel pump wireing

sending unit or float in gas tank, check if any broken wires or well plugged in and then check the float for the gas.