1988 PONTIAC FIERO trip computer

842-2 INSTRUMENT PANEL. GAGES & CONSOLE
Pinion Gear
The PINION GEAR is attached to the
traaasmission/transaxle output shaft and rotates in
proportion to the speed of the car. This rotation is
transferred from the pinion gear to the speedometer
head by the speedometer cable.
Photo Speed Sensor
On vehicles that use a mechanical drag-cup
speedometer, the PHOTO SPEED SENSOR is
inserted into the frame of the mechanical speedometer
to provide an electrical feedback to the ECM that
represents vehicle speed. The ECM needs to know how
fast the car is traveling in order to control and operate
the cruise control, cooling fan, and transmission and
evaporative systems.
The photo speed sensor is made up of two special
electronic devices: a Eight-Emitting Diode (LED) and
a photo transistor (a light-sensitive amplifying device).
In the mechanical speedometer, there is a reflective
blade attached to the rotating magnet that is polished
to
reflect light from the LED back to the photo
transistor. Whenever the light strikes the photo
transistor, it conducts electricity. The rate that the
transistor conducts and does not conduct is
proportional to the speed of the magnet, which reflects
the speed of the vehicle. This voltage signal from the
photo transistor is sent to a buffer amplifier (part of the
speed sensor) to be conditioned to a signal the ECM
can understand and use.
PM Generator
The PM (Permanent Magnet) GENERATOR is
a small
AC generator used to sense vehicle speed. The
shaft of the generator fits into a pinion gear in the
transmission/transaxle output shaft (as does the cable
in
a mechanical system).
When the output shaft rotates, the magnet rotates
and generates a voltage. Except for the permanent
magnet, the
PM Generator is exactly like a miniature
alternator. The PM generator is constructed to provide
a voltage whose frequency is about
1.1 cycles per
second for every mile per hour of vehicle speed. This
signal is sent to a buffer amplifier, and then to the
speedometer and the
EGM.
SPEEDOMETER
The speedometer is a road speed indicator with
an odometer to record total mileage, and, on some cars,
a resettable trip odometer.
The major types of speedometers in use are
mechanical instruments and electronic instruments.
Mechanical speedometers use a dial needle to indicate
road speed. Electronic speedometers include
instruments that use a dial indicator and those using
bar-graph
LCD's (Liquid Crystal Displays) or VTF
(Vacuum Tube Fluorescent) displays.
Meehanice! Speedometers
A mechanical speedometer uses a cable driven
(through a pinion gear) by the transmission output
shaft. The cable connects to a magnetic drag-cup inside the
speedometer, which rotates the speedometer
needle. The end of the rotating cable causes a small bar
magnet to rotate within a metal cup. As the magnet
rotates within the cup, it magnetically attracts (drags)
the metal cup along behind it. Two things work to
prevent the cup from rotating as quickly as the magnet.
1. The distance of the magnet from the cup reduces
its effect on the cup.
2. A counterspring is wound around the shaft of the
cup in such a way as to oppose the normal
rotation of the cup. The counterspring loads the
drag-cup to give correct indication of the speed,
prevent needle overshoot, and also to return the
drag cup to a zero point.
Mechanical speedometers require a photo speed
sensor to provide road speed information for the ECM
and other systems, such as Cruise Control and the TCC
(Torque Converter Clutch).
The odometer on these instruments consists of
numbered wheels that are rotated by the speedometer
cable through worm gears.
Quartz Speedometer
The quartz speedometer is an electrically driven
instrument. The indicator needle is driven by a
precision DC motor, and is countersprung to provide
a mechanical load, prevent overshoot of the needle, and
return the indicator to zero when the road speed is
zero.
The source of speed information for a quartz
speedometer is the PM generator. From the PM
generator, speed information goes to the buffer
amplifier to be converted to digital voltage, and then
to the cluster circuitry, which interprets the speed of
the vehicle and produces small voltage to apply to the
speedometer motor.
The odometer on this instrument consists of
numbered wheels that are electrically driven by a
special precision DC motor called a stepper motor.
Digital Speedometer
Digital clusters utilize two types of displays: LCD
(Liquid Crystal Display) and VTF (Vacuum Tube
Fluorescent). They are used in digital speedometers
and bar-graph tachometers, fuel gages, etc.
Speed information entering the cluster from the
buffer amplifier is interpreted by a microcomputer
which controls the speed indication, the tachometer
display and the odometer reading.
The odometers associated with these instruments
utilize either numbered wheels driven by a small motor
or electronic displays. With an electronic display, the
mileage reading is stored in a computer chip (called a
non-volatile RAM chip; NVRAM) that does not
become 'erased' when the vehicle is turned off,
as the
display does not retain the information.
FUEL GAGE
An electrical fuel gage is used on all models,
consisting of an instrument panel gage and a fuel tank
pick-up. The fuel gage indicates the quantity of fuel in

96.4 MISCELLANEOUS ACCESSORIES
DASH AND CONSOLE MOUNTED ACCESSORY
SWITCHES
Installation for various dash and console
mounted accessory switches is shown in Section
8C.
GENERAL DESCRIPIIOM
ELECTRONIC GLARE CONTROL MIRROR
Service procedures regarding this mirror are
found in the Body Section of this manual.
Fig. 601 EGC Mirror
The EGC mirror has light sensors which monitor
activity in front and back of the mirror and compensate
for brightly lit or extremely dark driving conditions.
After glare is no longer detected in the night mode, a
short time delay is automatically activated, preventing
unnecessary changing as traffic moves in and out
behind the car.
As an added feature, the Automatic
Day/Night
Mirror will hold in the normal viewing mode whenever
the shift selector is placed in reverse. This assures you
of a bright, clear view while backing up.
The Automatic Mirror with EGC (Electornic
Glare Control) is a technological breakthrough using
advanced computer circuitry. It is powered by the
vehicle electrical system and requires no batteries or
scheduled maintenance. Power is drawn only when the
ignition is on.
Illuminated Distance Dial provides a full
range of adjustments to match your eye sensitivity.
Night Mode Indicator illuminates green when
mirror switches to the non-glare position.
Light Sensors monitor ambient lighting
conditions and
headlamp glare, then adjust the mirror
accordingly.
Auto-Off Switch allows fully automatic
operation or manual selection.
ON-CAR SERVICE
RALLY GAGES, TACH REAR WINDOW DEFOGGER
Remove
and Replace
The electric defogger system, available on all
models, is covered in Section
8A. Electrical diagnosis
and grid repair is covered in Body Service Manual.
Removal of the entire gage cluster assembly is not
POWER RmOTE CONTROL REARVIEW
required to service the individual rally gages and/or MIRROR
tachometer.
Electric powered remote control mirrors are
available with a console mounted control that allows
To replace the rally gages, remove the lower and the
mirrors to be adjusted from the driver's seat. The
upper trimplates. Remove the trip odometer reset knob repair
and mounting of the mirror assemblies are
(if so equipped) and gage cluster lens from the front covered in
the Body Service Manual. Location and
(see Section
8C). installation of the control switch is shown in Figure
603.
To replace the tachometer by itself, remove the
lower trimplates, the trip odometer reset knob (if so
equipped) and the instrument cluster lens. Remove the
gage from the cluster. Refer to Section
8C for
hstrument panel cluster removal.