Figure 7 shows the components of an electromechanical level gauge. The electromechanical sensors use the contact at the bottom of the float to shortcircuit the fixed contact on the lower end of the stem (Fig. 5), and this switches on the light.
An optional electronic device can be used to check the reserve indicator for a few seconds during ignition. During normal operation it introduces a delay to prevent the light from flashing when the vehicle pitches or rolls. These very inexpensive sensors are designed to indicate the reserve level in small tanks containing non-conductive fluids, such as oil or fuel.

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