Lesson 4/Learning Event 3
When the fuel tank is empty, the float lays on the bottom. The float arm will position the contact
brush so that the full length of the rheostat resistor will be utilized. The resulting high resistance
will drop voltage to the gage sufficiently so that the pointer will rise only to empty.
As the fuel level rises in the tank, the float will raise the float arm, which, in turn, will move the
contact brush on the rheostat resistor. As the float arm moves upward, the resistance will decrease
proportionally, resulting in a proportional voltage increase to the gage. The gage readings will
increase proportionally, resulting in accurate readings of the fuel level in the tank.
The voltage limiter effectively will ensure a constant supply of current regulated to the equivalent of
5 volts, to provide accurate gage readings regardless of electrical system voltage variations.
Thermostatic Fuel Gage - Differential Type
The differential-type fuel gage system uses an instrument panel gage whose operating principles are
much the same as the thermostatic gage. The differential-type thermostatic gage, however, uses
two electrically heated bimetallic strips that share equally in operating and supporting the gage
pointer. The pointer position is obtained by dividing the available voltage between the two strips
(differential). The tank unit is a rheostat type whose operating principles are much the same as the
tank unit described in the paragraph above titled "Thermostatic Fuel Gage - Externally Regulated."
The tank unit in this system, however, contains a wire-wound resistor that is connected between
two external terminals. Each one of the external terminals connects to one of the instrument panel
gage bimetallic strips. The float arm moves a grounded brush that raises resistance progressively to
one terminal, while lowering the resistance to the other. This causes the voltage division and
resulting heat differential to the gage strips that formulate the gage readings. Two additional
bimetallic strips are provided for temperature compensation. In addition, one of these blades
operates contact points to limit voltage to approximately 5 volts.
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