PONTIAC FIERO 1988 Service Repair Manual

6E3-A-28 2.8L (VIN S) DRIVEABILITY AND EMISSIONS
4-WAY CONNECTOR
(REAR OF ENGINE) COOLANT SENSOR
AND MAT
CODE 15
C0OLAN"FT"EMPERAWURE SENSOR CIRCUIT
(LOW TEMPERATURE INDICATED)
2.8L (VIN S) 'Tf3SEWlES (PORT)
Circuit Description:
The coolant temperature sensor uses a thermistor to control the signal voltage to the ECM. The ECM
applies a voltage on CKT 410 to the sensor. When the engine is cold the sensor (thermistor) resistance is high,
therefore the ECM will see high signal voltage.
As the engine warms, the sensor resistance becomes less, and the voltage drops. At normal engine operating
temperature
(85OC to 95°C) the voltage will measure about 1.5 to 2.0 volts at the ECM.
Test Description : Numbers below refer to circled
numbers on the diagnostic chart.
1. Code 15 will set if:
@ Signal voltage indicates a coolant temperature
less than
-44" C (-47" F) for 3 seconds.
2. This test simulates a Code 14. If the
ECM
recognizes the low signal voltage, (high
temperature) and the "Scan" reads
130°C, the ECM
and wiring are OK.
3. This test will determine if CKT 410 is open. There
should be 5 volts present at sensor connector if
measured with
a DVM.
Diagnostic Aids:
A "SCAN" tool reads engine temperature in
degrees centigrade. After engine is started the
temperature should rise steadily to about 90°C then
stabilize when thermostat opens.
A faulty connection, or an open in CKT 410 or 452
will result in a Code
15.
If Code 23 or 63 is also set, check CK'I' 452 for faulty
wiring or connections. Check terminals at sensor for
good contact. Refer to "intermittents" in Section
"B".

DRIVEABILITY AND EMISSIONS 2.8L (WIN S) 6E3-A-29
" SHOULD DISPLAY OVER 130°C.
DIAGNOSTIC AID
COOLANT SENSOR
TEMPERATURE TO RESISTANCE VALUES
(APPROXIMATE)
OF "C OHMS
210 100 185
160 70 450
100 3 8 1,800
70 20 3,400
40 4 7,500
20 -7 13,500
0 -1 8 25,000
-40 -40 100,700

6E3-A-38 2.8b (VIM S) DRIVEABILITY AND EMISSIONS
FRONT VIEW OF CONNECTOR
MODE SIGNAL
5V REFERENCE
THRBmLE POSiT10N SENSOR (TPS) CIRCUIT
(SIGNAL VOLTAGE HIGH)
2.8L (VIN S) ""F-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:
@ Engine is running
@ TPS signal voltage is greater than 2.5 volts
@ Air flow is less than 12 gmlsec.
All conditions met for 5 seconds.
OR
@ TPS signal voltage over 4.5 volts with ignition
"ON".
The TPS has an auto zeroing feature. If the
voltage reading is within the range of 0.35 to 0.7 volts,
the ECM will use that value as closed throttle.
If the
voltage reading is out of the auto zero range at closed
throttle, refer to "TPS Adjustment" in Section
"6E3-
C1".
2. With the TPS sensor disconnected, the TPS
voltage should go low if the
ECM and wiring is
OK.
3. Probing CKT 452 with a test light checks the 5volt
return circuit, because a faulty 5volt return will
cause a Code 21.
Diagnostic Aids:
A "SCAN" tool reads throttle position in volts.
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".

DRlVEABlLlTV AND EMISSIONS 2.8L (VIN S) 6E3-A-31
CODE 21 IS INTERMITTENT. IF NO
ADDITIONAL CODES WERE STORED,
2 VOLTS (200mV). EFER TO "DIAGNOSTIC AIDS" ON
CLEAR CODES AND CONFIRM "CLOSED LOOP" OPERATION AND NO "SERVICE ENGINE SOON" LIGHT.

6E3-A-32 2.8L (VIN S) DRIVEABILITY AND EMISSIONS
FRONT VIEW OF CONNECTOR
MODE SIGNAL
5V REFERENCE
CODE 22
f HRO"I$E"LE POSlTlON SENSOR ("BPS) ClRCUlT
(SIGNAL VOLTAGE LOW)
2.8L (VIN 9) "F" "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 22 will set if:
@ Engine running
@ TPS signal voltage is less than about .2 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. The TPS has an auto zeroing feature. If the
voltage reading is within the range of
0.35 to 0.7
volts, the ECM will use that value as closed
throttle.
If the voltage reading is out of the auto
zero range at closed throttle, refer to "TPS
Adjustment" in Section
"6E3-Cl".
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.
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 2.8L (VIN %) 6E3-A-33
@ JUMPER CKTS416 &417 TOGETHER. IF
NO ADDITIONAL CODES WERE STORED, REFER TO
CONNECTED TO
12 VOLTS.
"SCAN" TOOL SHOULD DISPLAY TPS OVER
CKT416 OPEN. CKT 417 OPEN.
SHORTED TO GROUND. SHORTED TO
GROUND.
FAULTY CONNECTION. SHORTED TO
SENSOR GROUND CIRCUIT.
FAULTY ECM CONNECTION.
FAULTY ECM.
CLEAR CODES AND CONFIRM "CLOSED LOOP" OPERATION AND NO "SERVICE ENGINE SOON" LIGHT.

6E3-8-34 2.8L (VIN S) DRIVEABILITY AND EMISSIONS
MAT SENSOR
MAT SENSOR SIGNAL
TO TPS AND COOLANT
SENSORS
CODE 23
MANIFOLD AIR TEMPERAWRE (MAq SENSOR CIRCUIT
(LOW TEMPERATURE INDICATED)
2.8L (VIN S) "F"" SERIES (PORT)
Circuit Description:
The MAT 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 the air is cold the sensor (thermistor) resistance is high,
therefore the ECM 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 a manifold air
temperature below
-35°C (-32°F) for 3
seconds.
@ Time since engine start is 8 minutes or longer.
@ NoVSS.
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.
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 tetnperature when engine is cold, and rises as
underhood temperature increases.
A faulty connection, or an open in CKT 472 or 452
will result in a Code 23.
Refer to "Intermittents" in Section
"B".

DRlWEABlLllV AND EMISSIONS 2.8L (WIN 5) 6E3-A-45
DIAGNOSTIC AID
MAT SENSOR
TEMPERATURE TO RESISTANCE VALUES
(APPROXIMATE)

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.