gauge circuit so that the electrical fuel gauge operates
as the pointer. With no current flowing through the
only when the ignition switch is on. Operation of the
ammeter, the permanent magnet holds the armature so
electrical fuel gauges depends on coil (magnetic) or
that the pointer indicates zero.
thermostatic (bimetallic) action.
When current flows, it produces a secondary
magnetic field that opposes the magnetic field of the
COIL-OPERATED FUEL GAUGE.--The elec-
permanent magnet. Therefore, the secondary magnetic
trical circuit for a coil-type fuel gauge is shown in
field acts on the pointer shaft, causing it to deflect so
figure 6-75.
that the pointer moves away from zero.
Current from the battery has parallel paths of flow
The amount of deflection is determined by the
through the rheostat of the sending unit and the
strength of the secondary field, which is determined by
operating coil of the gauge. The parallel paths join at
the strength of the current flow through the ammeter.
the common connection between the two coils of the
When the current flow in figure 6-74, view B, is from
gauge. Current flows through the limiting coil, ignition
the battery to the lights, a discharge will show on the
switch, ammeter, and back to the battery.
ammeter. When the generator output voltage exceeds
Fuel level indications result from variations in the
that of the battery, the generator will provide the current
magnetic fields of two coils. The rising or falling float
to operate the lights. It will also be charging the battery.
in the fuel tank moves the arm of a rheostat (variable
The charging current will cause the ammeter indicator
resistor), causing the magnetic fields to vary in the coils
needle to deflect toward the charge scale.
when the tank is full, and the sending unit float will be
at the upper position. This will present maximum
NOTE: The ammeter is connected so that current
resistance at the top between the limiting coil and the
from the battery to the starting motor does not flow
operating coil. Maximum current flow will be through
through the ammeter when the vehicle engine is started.
both coils. The operating coil becomes dominant,
The starting motor may draw several hundred amperes
which draws the pointer to the "full" position.
during starting, and the range of the ammeter would be
greatly exceeded.
As the fuel level drops, the sending unit float level
also drops. This lowers the resistance of the sending
Fuel Gauge
unit. With less resistance, more current flows through
the limiting coil and less through the operating coil.
Most fuel gauges are electrically operated and are
The magnetic attraction of the limiting coil is becoming
composed of two units--the gauge mounted on the
dominant, drawing the pointer toward the "empty" side
instrument panel and the sending unit mounted in the
of the scale. The further the float falls, the less
fuel tank. The ignition switch is included in the fuel
AMMETER
IGNITION SWITCH
GAUGE
E
LIMITING
OPERATING
COIL
COIL
SENDING UNIT
RHEOSTAT
PARTLY FULL POSITION
BATTERY
FLOAT
EMPTY POSITION
ASf06075
Figure 6-75.--Coil operated fuel gauge circuit.
6-62
