A fuel guage has two units, the guage in the dashboard and the unit at the tank side as shown in figure. The tank unit is like a Rheostat whose one terminal is connected to ground and the other to the terminal marked T on the Guage. As float position moves downward (tank empty), the resistance becomes low and as float moves upward (full tank), the resistance becomes high. This arrangement can also be reverse (empty tank-high Resistance, Full tank--> Low resistance).
The battery positive is connected to terminal B on the guage and the ground to terminal marked ground. 12V and 6V guages exist.
Two coils at 90 degree orientation are arranged (one between T & G in left to right and other between T & B in up -down direction). The pointer is attached to a magnet pivoted at the centre, which rotates as magnetic field generated by coils changes. One coil tries to move the magnet to Full position while the other tries to move the magnet to Empty position. When the magnet rotates,so does the pointer.
In old cars, the B connection is through the ignition key. So, turning the key to on, only shows the fuel level
Troubleshooting:-
Assuming the guage to be type where full tank indicates high resistance, following are trouble-shooting tips
1. Guage Always Reads full.
Disconnect the tank connection. Connect the T terminal to Ground, if the guage should move to empty, indicates good guage and disconnected wire to tank side or disconnected ground wire at tank side. If connecting T terminal to ground does not make the guage go empty, then guage is faulty.
2. Guage Always reads empty
Disconnected B or G terminal at Guage.
