Lesson 4/Learning Event 3
Operation
When the tank is half full, the float arm positions the contact brush midway of the rheostat. This
causes equal resistance values to each sending unit circuit, resulting in equal heating of the gage
bimetallic strips, causing the gage to read one-half.
Fuel levels above or below half will cause the tank unit to divide the voltage to the gage bimetallic
strips in the correct proportions to create correct gage readings. An example would be a one-quarter
full fuel tank. At this level, the tank unit would divide the voltage so that 75 percent of the current
would flow through the right-side stip and 25 percent would flow through the left-side strip. This
would produce a one-quarter gage reading.
Magnetic Fuel Gage
The basic instrument panel gage consists of a pointer that is mounted on an armature. Depending
on the gage design, the armature may contain either one or two poles. The gage is motivated by a
magnetic field that is created by two separate magnetic coils that are contained within the gage.
One of these coils is connected directly to the battery, producing a constant magnetic field. The
other coil produces a variable field whose strength is determined by a rheostat-type tank unit whose
operation is the same as the one described in the paragraph above titled "Thermostatic Fuel Gage -
Externally Regulated." The coils usually are placed 90 apart.
Operation
When the tank is empty, the tank unit creates a very high resistance. This causes the variable
magnetic coil to produce almost no magnetic field. Therefore, the armature poles on the gage
pointer will be attracted to the constant magnetic coil. The pointer will point to empty when the
armature is in this position.
When the tank is full, the tank unit will create no resistance. Therefore, the armature poles on the
gage pointer will be positioned midway between the coils. When the armature is in this position,
the pointer will indicate a full reading.
Variations in electrical system voltage will affect both magnetic coils equally and, therefore, will not
upset the differential created between them. Because of this, the magnetic gage is self-regulating
and requires no voltage-limiting devices.
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