Lesson 4/Learning Event 3
When the fuel tank is empty, the float lays on the bottom. In this position, the arm-operated cam
exerts no pressure on the grounded contact. When the ignition is switched on, current flows from
the ground, through the heating coil in the sending unit, through the heating coil in the instrument
panel gage, and to the battery. The heating of the bimetallic strip in the sending unit causes it to
deflect, opening the contact points. The opening of the points will allow the bimetallic strip to cool
and return to its original position, again closing the points. The cycle of opening and closing the
points will continue, supplying current pulses to the heating element in the fuel gage. The length of
the pulses from the sending unit when the tank is empty will only heat the gage bimetallic strip
enough to cause deflection that will move the pointer to the empty position on the gage face.
When the fuel tank contains fuel, the float will raise the arm, causing the cam to push the
grounded contact tighter against the bimetallic strip contact. This will cause an increase in the
amount of heat required to open the contact points in the sending unit. The result will be longer
current pulses to the instrument panel gage, causing higher gage readings. The gage reading will
increase proportionally with the float level in the fuel tank.
The tank unit will compensate for variations in electrical system voltage automatically. High
voltage will increase heating, causing the points to cycle faster, and low voltage will decrease
heating, causing slower point cycling.
Because the gage pointer is moved by the heating and cooling of the bimetallic strip, the gage
reading will not react to sudden fuel level changes caused by fuel sloshing. This will prevent erratic
Thermostatic Fuel Gage - Externally Regulated
The externally regulated thermostatic fuel gage uses an instrument panel gage whose operation is
the same as the gage used with the self-regulating system. The differences in the system are the
use of a variable resistance fuel tank sending unit and an external voltage-limiting device. The
sending unit controls the gage through the use of a rheostat. A rheostat is a wire-wound resistance
unit whose value varies with its effective length. The effective length of the rheostat is controlled
in the sending unit by a sliding brush that is operated by the float arm. The power supply to the
gage is kept constant through the use of a voltage limiter. The voltage limiter consists of a set of
contact points that are controlled by an electrically heated bimetallic arm.