INSTRUMENTS
MOTOR TRUCK SERVICE MANUAL
Fig. 22. Fuel Gauge Circuit
Fuel Level Gauge:
When the fuel tank is empty, the tank unit has a
resistance of 0 ohms. Since this resistance is smaller
The electric fuel gauge system consists of two basic
than the combined resistance of the bucking and full
components the dash gauge and the gas tank sending
coils, the current flows through the tank unit. The current
unit. The dash gauge indicates the quantity of fuel in the
flow through the empty coil creates a magnetic field
gas tank, while the tank unit controls the gauge. The two
around the coil. This field attracts the magnet which is
units are connected electrically as shown in Fig. 22.
on the same spindle as the indicating pointer and rotates
the pointer toward the empty scale reading. When the
The fuel gauge is of the air core type. The air core
fuel tank is full, the tank unit has a resistance of 90
gauge consists of three (3) copper wire coils that are
ohms. As the current reaches point B, it seeks a path
wound around a plastic bobbin containing a magnet and
through the series connection of the bucking coil and the
spindle assembly. Attached to the magnet and spindle
full coil to ground, since their combined resistance is less
assembly is a pointer which indicates the fuel level. The
than that of the tank unit. All of the three coils now have
fuel gauge requires a 0 to 90 ohm resistance sender to
a magnetic field induced by the current flow; however,
operate. The sender is the tank unit. The tank unit
the empty coil and the bucking coil are wound in opposite
consists of a float and arm assembly and a variable
directions. This causes their respective magnetic fields
resistor. The sender's resistance is controlled by the
to cancel each other. The remaining effective field
position of the float and arm assembly. A full fuel tank
around the full coil attracts the magnet, thus rotating the
raises the float to its highest position. At this position the
pointer to the full scale reading.
variable resistor has a resistance of 88 to 92 ohms. With
an empty fuel tank the float assumes its lowest position,
creating a sender resistance of less than 1 ohm.
Electrical current, supplied by the battery, travels
through the ignition switch and enters the fuel gauge at
point A, Fig. 22. At point A the current divides between
the "calibrating resistor'[ and the "empty coil" as it flows
to point B. From point B two possible paths exist to
complete the circuit to ground. The current flows through
the series connection of the "Bucking Coil" and the "Full
Coil" to ground or it can travel through the tank unit.
Since current will travel through the path of least
Fig. 23. Fuel Level Gauge
resistance, the flow is determined by the resistance of
the sender.
576 A
