Pontiac Fiero 1988 Service Manual page 521

6E2-C1-4 DRIVEABILITY AND EMISSIONS - FUEL INJECTION (TBI)
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THROTTLE BODY ASSEMBLY
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THROTTLE POSITION SENSOR
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S C R E W A S S E M B L Y
Figure C1-6 - T h r o t t l e Position Sensor
[T] i m p o r t a n t
Vehicle should not be driven with park/neutral
switch disconnected as idle quality will be affected
and a possible false Code 24 (VSS).
See Section "8A" for more information on the P/N
switch, which is part of the neutral/start and backup
light switch assembly.
A / C Request Signal
This signal tells the ECM that the A/C selector
switch is turned "ON", and that the low pressure
switches is closed. The ECM uses this to turn "ON"
A/C and adjust the idle speed when the air
conditioning is working.
Vehicle Speed Sensor
The vehicle speed sensor (VSS) sends a pulsing
voltage signal to the ECM, which the ECM converts to
miles per hour. This sensor mainly controls the
operation of the TCC system. See "TCC System" for
more information.
Crankshaft Sensor
The crankshaft sensor sends a signal, through the
DIS module, to the ECM. The ECM uses this
reference signal to calculate rpm and crankshaft
position. See Section "C4" for further Information.
DIAGNOSIS
To read the codes, ground the diagnostic terminal
with the engine not running and the ignition "ON"
The "Service Engine Soon" light will flash Code 12
three times and then flash each code stored in memory
three times. All codes stored in memory would have
been read when Code 12 was flashed again. No new
codes can be stored when in the diagnostics mode
(diagnostics lead grounded). This eliminates
confusion while the system Is being worked on.
To clear the codes from memory:
• Ignition "OFF."
• Open pigtail connector at battery.
Since the ECM can have a failure, which may
affect only one circuit, following the Diagnostic
Procedures in this section will determine which circuit
has a problem and where It Is.
If a diagnostic chart Indicates that the ECM
connections or ECM is the cause of a problem, and the
ECM is replaced, but does not correct the problem, one
of the following may be the reason:
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There is a problem with the ECM terminal
i onnections.-The diagnostic chart will say ECM
connections or ECM. The terminals may have to be
removed from the connector in order to check them
properly.
• The ECM or PROM is not correct for the
application.- The incorrect components may cause a
malfunction and may, or may not, set a code.
• The problem is intermittent - This means that the
problem is not present at the time the system is being
checked. In this case, refer to the "Symptoms" portion
of the manual and make a careful physical inspection
of all portions of the system involved.
• Shorted solenoid, relay coil, or harness - Solenoids
and relays are turned "ON" and "OFF" by the ECM,
using internal electronic switches, called "Drivers".
Each driver Is part of a group of four called "Quad-
drivers". Failure of one driver can damage any other
driver in the set.
A shorted solenoid, relay coil, or harness in a
GMP4 computer will not damage the ECM, but
will cause the circuit and controlled component
to be inoperative* When the circuit fault Is not
present or has been repaired, the "Quad Driver"
will again operate in a normal manner due to it's
fault protected design. If a fault h a s been
repaired in a circuit controlled by a "Quad
Driver", the original ECM should be reinstalled
and the circuit checked for proper operation,
ECM replacement will not be necessary if the
repaired circuit or component now operates
correctly.
J34636 or BT8405 testers, or equivalent, provide a
fast, accurate means of checking for a shorted coil, or a
short to battery voltage.
• The PROM may be faulty - Although these rarely
fail, it operates as part of the ECM. Therefore, it could
be the cause of the problem. Substitute a known good
PROM.
• The replacement ECM may be faulty - After the
ECM is replaced, the system should be rechecked for
proper operation. If the diagnostic chart again