The factory Fuel gauge is 73 ohms Empty and 13 ohms Full. 25 ohms will move the needle to around half full.
The only difference between the Fuel, Water and Oil gauges is the face.
If you can get your hands on a multimeter then you can measure the resistance at the sender. Once you have confirmed that one of the senders is working then you can use that sender to test the gauges.
This is an explanation I found for the standard LS alternator when I was sorting out my alternator which was not charging at all.
Alternator Wiring Description
The L-terminal circuit from the generator is a discrete circuit (a discrete circuit has no splices and only one source and destination) into the PCM. The PCM applies ignition voltage to the generator L-terminal circuit. A small amount of current flows from this circuit through the generator windings to ground to create a magnetic field which starts the generator process. When the generator is at operating speed and producing voltage, a solid state switch for the L-terminal circuit in the generator opens and the PCM detects that the initial startup current flow has stopped.
The PCM expects to detect low voltage on the L-terminal circuit prior to the generator rotating at operating speed and conversely expects the circuit to be at ignition voltage potential when the generator is operational. When the PCM detects a fault (circuit shorted to ground, or circuit shorted to voltage), the Driver Information Centre will display Charging System Fault.
The generator has an input to the PCM called the F Terminal to indicate the percentage of total capacity that the generator is producing. This signal is detected by the PCM as a duty cycle from the generator and displayed on the scan tool as a percentage. The PCM can monitor the generators output under all conditions to determine if it is functioning normally.
When there is low demand from the electrical system on the generator, a low duty cycle percentage will be displayed. As more accessory load is placed on the generator, the duty cycle output detected by the PCM will approach 100 percent. A normally functioning generating system will never reach 100 percent as indicated on the scan tool.
L terminal is orange/red
F terminal is grey
ECU Bypass/Standard Dash Light
L-Terminal Orange/Red
The L terminal is disconnected from the PCM and connected to a globe on the dash and then +12v on the ignition switch. When you turn the key to on the globe will light. Once the alternator is working the switch in the alternator opens and the globe will switch off. If the alternator fails the light switches/stays on.
F-Terminal Grey
This can be left connected to the ECU so the scan tool can read alternator load.
Alternatively it can be connected to +12v ignition