Pontiac Fiero 1988 Service Manual page 1089

INSTRUMENT PANEL: INDICATORS C L U S T E R
(Continued from previous page)
CIRCUIT OPERATION
The operation of an individual Indicator is
described along with its circuit. Eefer to the
schematic and text for the circuit that is stated
below each of the Indicators.
Fuel Gage
The pointer of the Fuel Gage is moved by the
magnetic fields of two coils. The coils are at
right angles to each other. Battery voltage is
applied to the E coil and the circuit divides at
the opposite end of this coil. One path continues
through the F coil. Another goes to the variable
resistor of the Fuel Gage Sender.
When the tank is low, the resistance of the
Sender is low. A large flow of current passes
through the E coil and the Fuel Gage Sender
resistor. This moves the pointer toward E on
the scale. When the tank is full the Sender
resistance is high. More current now flows
through the F coil, moving the pointer toward F
on the scale.
With two coils operating the pointer, the
Gage is not affected by changes in the system's
battery voltage.
Coolant Temperature Indicator
The Coolant Temperature Indicator warns
the driver of high coolant temperature. With
the Ignition Switch in RUN, BULB TEST or
START, voltage from the GAGES Fuse is
applied to the Coolant Temperature Indicator.
In RUN, the bulb can be grounded through the
Coolant Temperature Switch. The Switch
closes when the coolant temperature exceeds
258 °F (126 °C). The indicator light glows.
Speedometer and Odometers
The Speedometer is operated by an electronic
circuit. The Vehicle Speed Sensor, located in the
transaxle, generates an AC voltage whose fre-
quency is proportional to the speed of the vehi-
cle. This voltage/frequency goes to the Vehicle
Speed Buffer and to the Speedometer in the
Instrument Cluster. The Solid State circuit
drives the pointer of the Speedometer. This is
no Speedometer cable in the vehicle.
The same speed signal from the Vehicle Speed
Buffer is processed to drive the Odometers.
They are operated by a stepper motor that
responds to pulses from the Speedometer cir-
cuit.
Tachometer
The Tachometer displays engine speed in
rpm. Voltage pulses are taken from the Ignition
System and sent to the Tachometer. The
Tachometer responds to the frequency of the
voltage pulses which increases with engine
speed. In coil equipped vehicles (VIN 9) there is
a Tachometer Filter in the circuit that rounds
off the pulses and removes voltage spikes.
Solid State circuits process these pulses into
a signal that drives the pointer of the meter.
Oil Pressure Gage
The engine oil pressure is displayed by the Oil
Pressure Gage. The pointer of the Gage is
moved by two coils, and its operation is similar
to that of the Fuel Gage.
The Oil Pressure Sender is connected to the
junction of the two coils. It has low resistance
when the oil pressure is low and 90 ohms
resistance when the oil pressure is high. This
changing resistance changes the current flow
through the coils. The magnetic fields of the
coils move the pointer from Low to High.
Coolant Temperature Gage
The Coolant Temperature Gage is also oper-
ated by two coils. Battery voltage is applied to
both coils. One is grounded directly and the
other is grounded through the Coolant Tem-
perature Sender. This has 55 ohms resistance
with hot coolant and its resistance becomes
greater at lower temperatures. It is approxi-
mately 1400 ohms with cold coolant. This
causes the current through the Sender and one
coil to increase as the coolant temperature
increases. This moves the pointer.
Voltmeter
The Voltmeter measures the electrical sys-
tem voltage with the Ignition Switch in RUN,
BULB TEST, or START. With the engine run-
ning, the Voltmeter indicates Charging System
operation. With the engine stopped, the Volt-
meter indicates battery condition.
Hood/Trunk/Door Ajar Indicator
With the Ignition Switch in RUN, BULB
TEST, or START, voltage is available through
the GAGES Fuse to the AJAR Indicator.
Switches mounted in the doors, trunk, and hood
complete current paths to ground.