1988 PONTIAC FIERO Service Repair Manual

6E3-A-30 5.OL (VIN F) & 5.7L (VIN 8) DRIVEABILITY AND EMISSIONS
FRONT VIEW TO MAT AND
OF CONNECTOR
COOLANT
452
ELK D2
416 GRY - C14
CODE 21
THROmLE POSITION SENSOR (TPS) CIRCUIT
(SIGNAL VOLTAGE HIGH)
5.OL (VIN I") & 5.7L (VIN 8) 'T" SERIES (PORT)
Circuit Description:
The throttle position sensor (TPS) provides a voltage signal that changes relative to the throttle blade.
Signal voltage will vary from about
.5 at idle to about 5 volts at wide open throttle.
The TPS signal is one of the most important inputs used by the ECM for fuel control and for most of the
ECM control outputs.
Test Description: Numbers below refer to circled
numbers on the diagnostic chart.
1. Code 21 will set if:
@ TPS signal voltage is greater than 2.5 volts
@ Engine is running
@ Air flow is less than 12 GMIsec.
e All conditions met for 3 seconds.
OR
@ TPS signal voltage over about 4.8 volts with
ignition "ON".
With throttle closed, the TPS should read less
than .62 volts. If it doesn't check adjustment.
2. With the TPS sensor disconnected, the TPS
voltage should go low if the ECM and wiring is
OK.
Diagnostic Aids:
A "SCAN" tool reads throttle position in volts.
Should read
.54V f .08V with throttle closed and
ignition "ON" or at idle. Voltage should increase at a
steady rate as throttle is moved toward WOT.
An open in CKT 452 will result in
a Code 21.
Refer to "Intermittents" in Section
"B".
3. Probing CKT 452 with a test light checks the 5V
return CKT, because
a faulty 5V return will cause
a Code 21.

DRIVEABILITY AND EMISSIONS 5.OL (VIN F) & 5.7L (VIN 8) 6E3-A-31
DDlTlONAL CODES WERE STORED,
CLEAR CODES AND CONFIRM "CLOSED LOOP" OPERATION AND NO "SERVICE ENGINE SOON" LIGHT.

6E3-A-32 5.OL (VIN F) & 5.7L (VIN 8) DRIVEABILITY AND EMISSIONS
FRONT VIEW TO MAT AND OF CONNECTOR COOLANT
SENSORS
452 BLK - D2 .
416 GUY - C14
CODE 22
THROTLE POSITION SENSOR (WS) CIRCUIT
(SIGNAL VOLTAGE LOW)
5.OL (VIN F) & 5.7L (VIN 8) "F'YSERIES (PORT)
Circuit Description:
The throttle position sensor (TPS) provides a voltage signal that changes relative to the throttle blade.
Signal voltage will vary from about
.5 at idle to about 5 volts at wide open throttle.
The TPS signal is one of the most important inputs used by the
ECM for fuel control and for most of the
ECM control outputs.
Test Description: Numbers below refer to circled
numbers on the diagnostic chart.
1. Code22willsetiE
@ Engine running
TPS signal voltage is less than about
.% volt for
3 seconds.
2. Simulates
Code
21: (high voltage) If the ECM
recognizes the high signal voltage the ECM and
wiring are
OK.
3. TPS adjustment: With throttle closed, the TPS
voltage reading should be
54V f .08V.
4. This simulates a high signal voltage to check for
an open in CKT
417.
Diagnostic Aids:
A "Scan" tool reads throttle position in volts.
Should read
54V + .08V with throttle closed and
ignition
"ON" or at idle. Voltage should increase at a
steady rate as throttle is moved toward WOT.
An open or short to ground in
CKTs 416 or 417
will result in a Code 22.
Refer to "Intermittents" in Section
"B".

DRIVEABILITY AND EMISSIONS 5.OL (VIN F) & 5.7L (VIN 8) 6E3-A-33
(SIGNAL VOLTAGE LOW)
JUMPER CKTS 416 & 417 TOGETHER.
"SCAN" SHOULD DISPLAY TPS OVER
4.0 V (4000 mV
"SCAN" TOOL SHOULD DISPLAY TPS OVER
SHORTED TO GROUND
FAULTY CONNECTION. ORTED
TO SENSOR GROUND CIRCUIT.
FAULTY ECM CONNECTION.

6E3-A-34 5.OL (VIN F) & 5.7L (VIN 8) DRIVEABILITY AND EMISSIONS
MANIFOLD AIR
TEMPERATURE
MAT SENSOR SIGNAL
NSOR GROUND
COOLANT
TEMPERATURE
SENSOR
CODE 23
MANIFOLD AIR TEMPERATURE (MAT) SENSOR CIRCUIT
(LOW TEMPERATURE INDICATED)
5.OL (VIN F) & 5.7L (VIN 8) "F" SERIES (PORT)
Circuit Description:
'I'he MAT sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage (4-6
volts) on CKT 472 to the sensor. When the air is cold, the sensor (thermistor) resistance is high, therefore, the
ECY will see a high signal voltage. If the air is warm, the sensor resistance is low, therefore, the ECM will see
a low voltage.
Test Description: Numbers below refer to circled
numbers on the diagnostic chart.
1. Code 23 will set if:
@ A signal voltage indicates u manifold air
temperature below -30°C (-22°F) for
12
seconds.
@ Time since engine start is 1 minute or longer.
@ No VSS (vehicle not moving)
2. A Code 23 will set, due to an open sensor, wire, or
connection. This test will determine if the wiring
and ECM are OK. The MAT sensor is difficult to
reach and this test can be performed
by
disconnecting the MAT jumper harness connector.
If the "Scan" indicates a temperature of over
130°C the jumper harness to the sensor should be
checked before replacing the sensor.
3. This will determine if the signal CKT 472 or the
5V return CKT 452 is open.
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.
Carefully check harness
ancl connections for
possible open CKT
472 or 452.
Refer to "Intermittents" in Section
"H".

DRIWEABILIW AND EMISSIONS 5.OL (WIN F) & 5.7L (VIM 8) 6E3-A-35
MANIFOLD AIR TEMPERATURE (MAT) SENSOR
DIAGNOSTIC AID
MAT SENSOR
TEMPERATURE TO RESISTANCE VALUES
(APPROXIMATE)

6E3-A-36 5.OL (VIN F) & 5.7L (VIN 8) DRIVEABILITY AND EMISSIONS
CONNECTOR
15- WAY .I.P. CONNECTOR - - PARK OR NEUTRAL
CODE 24
VEHICLE SPEED SENSOR (VSS) CIRCUIT
5.0L (VIN F) & S.SL (VIN 8) "F" SERIES (PORT)
rcuit Description:
'I'he ECM applies and monitors 12 volts on CKT 437. CK'I' 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 it
@ CKT 437 voltage is constant.
@ Engine speed between 1400 and 3600 rpm.
@ Less than 2% throttle opening, about .10V
( 100mV) above close throttle.
@ Low load condition (low air flow).
@ Not in park or neutral.
@ All conditions must be met for 4 seconds.
'I'hese 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
parWneutra1 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 5.OL (WIN F) & 5.7L (WIN 8) 6E3-A-37
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 NOT TURNING.
MPH ABOVE
0.
REFER TO " DIAGNOSTIC AIDS "
0 DISCONNECT VSS BUFFER 0 IGNITION "ON." 0 BACK PROBE 15-WAY I.P. CONNECTOR
SHOULD DISPLAY
10 VOLTS OR MORE.