Pontiac Fiero 1988 Service Manual page 653

6E3-C1-4 DRIVEABILITY AND EMISSIONS - FUEL INJECTION (PORT)
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"
• Disconnect 12 volt pigtail connector located
near the battery.
• Refer to ECM Quad Driver Check (CHART C-
1) before replacing ECM.
Since the electronic control module (ECM) can
have a failure which may effect 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 indicated 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:
• There is a problem with the ECM terminal
connections. 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 ECM or PROM 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.
Solenoid and relay coil resistance must measure
more than 20 ohms. Less resistance will cause early
failure of the ECM "driver".
Before replacing an ECM, be sure to check the coil
resistance of all solenoids and relays controlled by the
ECM. See ECM wiring diagram for the solenoid(s)
and relay(s) and the coil terminal identification.
J34636 or BT 8405 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 the PROM
rarely fails, it operates as part of the ECM. Therefore,
it could be the cause of the problem.
• The replacement ECM may be faulty. After the
ECM is replaced, the system should be rechecked for
proper operation. If the diagnostic chart again
indicates the ECM is the problem, substitute a known
good ECM. Although this is a rare condition, it could
happen.
ECM
A faulty ECM will be determined in the diagnostic
charts or by a Code 55.
PROM
An incorrect or faulty PROM, which is part of the
ECM, may set a Code 51.
ECM INPUTS
All of the sensors and input switches can be
diagnosed by the use of a "Scan" tool. Following is a
short description of how the sensors and switches can
be diagnosed by the use of "Scan". The "Scan" can
also be used to compare the values for a normal
running engine with the engine you're diagnosing.
Coolant Temperature Sensor
A "Scan" tool displays engine temperature in
degrees centigrade. After engine is started, the
temperature should rise steadily to about 9 0 ° C then
stabilize when thermostat opens. If the engine has not
been run for several hours (overnight), the coolant
temperature and manifold air temperature (MAT)
temperatures should read close to each other. A fault
in the coolant sensor circuit should set a Code 14 or 15.
The code charts also contain a chart to check for sensor
resistance values relative to temperature.
MAT
Sensor
A "Scan" tool displays temperature of the air
entering the engine and should read close to ambient
air temperature when engine is cold, and rise as
underhood temperature increases. If the engine has
not been run for several hours (overnight) the MAT
sensor temperature and coolant temperature should
read close to each other. A failure in the MAT sensor
circuit should set a Code 23 or 25. The code charts also
contain a chart to check for sensor resistance values
relative to temperature.
Oxygen O2Sensor
The "Scan" has several positions that will indicate
the state of the exhaust gases , O2 voltage, integrator,
and block learn. See "Scan" position information in
Introduction, Section "BE".