The MGA With An Attitude
FUEL GAUGE CALIBRATION -- FG-102d
Now for another curiosity. When the switch is changed to the "OFF" position it disconnects the signal wire from the gauge. Higher resistance at the sender unit causes higher readings on the gauge. In this case the signal resistance is off the scale on the high side (infinity). But the right side magnet in the gauge is still connected to a ground connection, so now all of the current flowing through the gauge will be flowing through the right side magnet, which causes the indicator needle to move hard against the right side peg. If the gauge should do this in your car, that could be an indication of too much resistance in the sender circuit, a bad connection for the sensor wire, or possibly a bad ground for the fuel tank.
Gauge off scale with signal wire disconnected.
The last condition is another normal occurrence. When you turn off the ignition switch in your car it removes power from the fuel gauge. No voltage, no current, and no magnetic field anywhere in the fuel gauge. This allows the needle to move to the left by virtue of gravity acting on the small off center weight on the moving armature. The needle will park gently against the left side peg when the power is off.
Power to the gauge is disconnected.
Well okay, now we know all about how the fuel gauge works, and some of the things that can cause it not to work, or to behave badly. But still, just how do you go about getting it calibrated properly? Time to turn the page again.