1988 PONTIAC FIERO Service Repair Manual

6E3-A-36 2.8L (VlN 5) DRIVEABILIW YW EMISSIONS
- PARK OR NEUTRAL
CODE 24
VEHICLE SPEED SENSOR (VSS) CIRCUIT
2.8L (VIN 5) ""FYMIES (PORT)
Circuit Description:
The ECM applies and monitors 12 volts on CKT 437. CKT 437 connects to the vehicle speed sensor buffer
which alternately grounds CKT
437 when drive wheels are turning. This pulsing action takes place about 2000
times per mile and the ECM will calculate vehicle speed based on the time between "pulses".
A "SCAN" tool reading should closely match with speedometer reading with drive wheels turning.
Test Description: Numbers below refer to circled
numbers on the diagnostic chart.
1. Code
24 will set if:
@ CKT 437 voltage is constant.
@ Engine speed between 1400 and 3600 rpm.
o Less than 2% throttle opening.
Low load condition (low air flow).
@ Not in park or neutral.
e All conditions must be met for 3 seconds.
These conditions are met during a road load
deceleration.
2. A voltage of less than 1 volt at the 15-way
connector indicates that the CKT
437 wire may be
shorted to ground. Disconnect CKT
437 at the
VSS buffer.
If voltage now reads above 10 volts,
the VSS buffer is faulty. If
voltage remains less than
10 volt, then CKT 437
wire is grounded or open. If 437 is not grounded or
open,
check for a faulty ECM connector or ECM.
Diagnostic Aids:
If "Scan" displays vehicle speed, check
paruneutral switch CHART C-1A on vehicle with
auto trans.
If switch is OK, check for intermittent
connections. An open or short to ground in CKT
437
will result in a Code 24. If the customer also
complained about
a loss of mph on the I.P., check the
P.M. generator circuit. Refer to Section "8A" for
complete wiring diagram.
Refer to "Intermittents" in Section
"B".

DRIVEABILITY AND EMISSIONS 2.8L (VIN O) 6E3-A-37
NOTE: TO
PREVENT MISDIAGNOSIS, THE TECHNICIAN SHOULD REVIEW ELECTRICAL SECTION "8A"
OR THE ELECTRICAL TROUBLESHOOTING MANUAL AND IDENTIFY THE TYPE OF VEHICLE
SPEED SENSOR USED PRIOR TO USING
THIS CHART. DISREGARD CODE 24 IF SET WHEN
DRIVE
WHEELS ARE MOT TURNING.
IN GEAR, "SCAN" SHOULD
CTOR TERMINAL
"K"
IGNITION "ON."
BACK PROBE 15-WAY I.P. CONNECTOR
SHOULD DISPLAY
10 VOLTS OR MORE.

6E3-A-38 2.8L (WIN S) DRIVEABILITY AND EMISSIONS
MAT SENSOR
MAT SENSOR SIGNAL
SENSORS
CODE 25
MANIFOLD AIR TEMPERATURE (MAT) SENSOR CIRCUIT
(HIGH TEMPERATURE INDICATED)
2.8L (VIN S) "F" SERIES (PORT)
Circuit Description:
The manifold air temperature sensor uses a thermistor to control the signal-voltage to the ECM. The ECM
applies a voltage (about
5 volts) on CKT 472 to the sensor. When manifold air is cold, the sensor (thermistor)
resistance is high, therefore the ECM will see a high signal voltage. As the air warms, the sensor resistance
becomes less, and the voltage drops.
Test Description: Numbers below refer to circled
numbers on the diagnostic chart.
1. Code 25 will set if:
@ Signal voltage indicates a manifold air
temperature greater than 145°C
(293" F) for 3
seconds.
@ Time since engine start is 8 minutes or longer.
@ A vehicle speed is present.
Diagnostic Aids:
A "SCAN" tool reads temperature of the air
entering the engine and should read close to ambient
air temperature, when engine is cold, and rises as
underhood temperature increases.
A short to ground in CKT 472 will result in a Code
25.
Refer to "Intermittents" in Section
"B".

DRIVEABILITY AND EMISSIONS 2.8b (VIN S) 6E3-A-39
DISCONNECT SENSOR.
"SCAM" SHOULD DISPLAY TEMPERATURE
STORED, REFER TO "DIAGNOSTIC
CKT
472 SHOUTED TO SENSOR GROUND CIRCUIT.
DIAGNOSTIC AID
MAT SENSOR
TEMPERATURE TO RESISTANCE VALUES
(APPROXIMATE)

6E3-A-40 2.8L (VIN 5) DRIVEABILITY AND EMISSIONS
VENT FILTER
450 BLWHT ITCH SIGNAL -
CODE 32
EGR SYSTEM FAILURE
2.8L (VIN S) "F" "SERIES (PORT)
Circuit Description:
The EGR vacuum control uses an ECM controlled solenoid. The solenoid is normally closed and the vacuum
source is a ported signal. The ECM will turn the EGR "ON" and "OFF" (Duty Cycle) by grounding CKT
435.
The duty cycle is calculated by the ECM based on information from the coolant and mass airflow sensor and
engine rpm. The duty cycle should be
0% (no EGR) when in park or neutral, TPS input below a specified value,
or TPS indicating WOT
With the ignition
"ON", engine stopped, the EGR solenoid is de-energized unless the diagnostic terminal is
grounded. Code 32 means that the EGR vacuum diagnostic switch was closed during start-up, or that the switch was
not detected closed under the following conditions.
@ Coolant temperature greater than 80" C (176°F).
@ EGR duty cycle commanded by the ECM is greater than 55%.
@ TPS less than half throttle, but not at idle.
@ All conditions above must be met for 5 seconds.
If the switch is detected closed during start-up, or
, if the switch is detected open when the above conditions
are met, the "Service Engine Soon" light will remain "ON" unless the switch changes state.
Test Description: Numbers below refer to circled 3. 'Po this point the EGR solenoid and valve are OK
numbers on the diagnostic chart. and the following check will check the diagnostic
1. If the first step caused Code 32 to set, then the vacuutn switch portion of the system.
ECM has recognized a closed vacuum switch on
4. The diagnostic switch should close at about 2" of
start-up. This test will determine whether the
vacuum. With vacuum applied, the switch should
EGR vacuum diagnostic switch is the cause or if close and resistance go to near zero ohms and the
the wiring or the ECM is the cause. vacuum should hold.
2. With the ignition "ON", the solenoid should not be
energized and vacuum should not pass to the EGR
valve.

DRIVEABILITY AND EMISSIONS 2.8L (VIN S) 6E3-A-41
SOLENOID, ALSO CHECK HOSES FOR LEAKS OR RESTRICTIONS.
SHOULD BE ATLEAST23.64
kPa (7'3 HG VACUUM AT2000 RPM.
NOT SHORTED IT IS A
BETWEEN "A" AND

6E3-A-42 2.8L (VIN S) DRIVEABILITY AND EMISSIONS
MAF SENSOR CONNECTOR
MAF SENSOR
940
ORNIBLK BRNMIHT 528
450 BLWHT
MAF RELAY
CODE 33
MASS AIR FLOW (MAF) SENSOR CIRCUIT
(GMISEC HIGH)
2.8L (VIN S) ""FXERIES (PORT)
Circuit Description:
The MAF sensor measures the flow of air entering the engine. The sensor produces a frequency output
between 32 and 150 hertz
(3gdsec to 150gmlsec). A large quantity (high frequency) indicates acceleration, and
a small quantity (low frequency) indicates deceleration or idle. This information is used by the
ECM for fuel
control and is converted by a "Scan" tool to read out the air flow in grams per second.
A normal reading is about
4-7 grams per second at idle and increases with rpm.
The MAF sensor is powered up by the MAF sensor relay and the sensor should have power supplied to it
anytime the ignition is "ON".
Test Description: Numbers below refer to circled Diagnostic Aids:
numbers on the diagnostic chart.
1. Code 33 will set if:
The "Scan" tool is not of much use in diagnosing
@ Ign. "ON" and air flow exceeds 20gdsec. this code because when the code sets gmlsec will be
OR displaying the default value. However, the "Scan"
@ Engine is running less than 1300 rpm. may be useful in comparing the signal of a problem
@ TPS is 8% or less.
vehicle with that of a known good running one.
@ Air flow greater than 20 grams per second Refer to "Intermittents" in Section "B".
(high frequency).
(9 All of the above are met for 2 seconds.

DRIVEABILITY AND EMISSIONS 2.8L (VIN S) 6E3-A-43
CLEAR CODES AND CONFIRM "CLOSED LOOP" OPERATION AND NO "SERVICE ENGINE SOON" LIGHT.
OR
1 MINUTE
OR UNTIL CODE 33 SETS
REPLACE MAF SENSOR CODE
33
IS INTERMITTENT.
REFER TO "DIAGNOSTIC
AIDS"
ON FACING PAGE.