I - SYSTEM/COMPONENT TESTS - 5.

0L

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xxxx 1997 Chevrolet Pickup K1500

ARTICLE BEGINNING

1997 ENGINE PERFORMANCE

General Motors Corp. - System & Component Testing - 5.0L

Chevrolet; Pickup
GMC; Pickup

INTRODUCTION

Before testing separate components or systems, perform

procedures in the F - BASIC TESTING - 5.0L article. Since many
computer-controlled and monitored components set a trouble code if
they malfunction, also perform procedures in the
G - TESTS W/CODES - 5.0L article.

NOTE: Testing individual components does not isolate shorts or

opens. Perform all voltage tests with a Digital
Volt-Ohmmeter (DVOM) with a minimum 10-megohm input
impedance, unless stated otherwise in test procedure. Use
ohmmeter and refer to the L - WIRING DIAGRAMS article to
isolate wiring harness shorts or opens.

The following table provides the location of commonly used

diagnostic information.

GENERAL MOTORS REFERENCE TABLE

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System Or Component Diagnostic Information Location

Malfunction Indicator Light (MIL) .... See ON-BOARD DIAGNOSTIC (OBD)

SYSTEM CHECK in F - BASIC TESTING - 5.0L article
DLC & MIL On Steady ..... See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK
in F - BASIC TESTING - 5.0L article
No Scan Tool Data .... See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK in
F - BASIC TESTING - 5.0L article
No-Start Diagnosis ... See appropriate NO-START - ENGINE CRANKS OKAY
in F - BASIC TESTING - 5.0L article
Injector Circuit Diagnosis ......... See BASIC FUEL SYSTEM CHECKS in
F - BASIC TESTING - 5.0L article
Fuel Pump Relay ............ See MODULES, MOTORS, RELAYS & SOLENOIDS
Fuel System Diagnosis ..... See appropriate BASIC FUEL SYSTEM CHECKS
in F - BASIC TESTING - 5.0L article
Injector Balance Test .............................. See FUEL SYSTEM
MAP Sensor ........................... See ENGINE SENSORS & SWITCHES
Transmission Range Switch ............ See ENGINE SENSORS & SWITCHES
IAC Valve ................ See IDLE CONTROL SYSTEM under FUEL SYSTEM
Fuel Evaporation Control ........ See EMISSION SYSTEMS & SUB-SYSTEMS
Ignition Control Circuit ....................... See IGNITION SYSTEM
Knock Sensor Check ............................. See IGNITION SYSTEM
EGR System ...................... See EMISSION SYSTEMS & SUB-SYSTEMS

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Torque Converter Clutch ......... See MISCELLANEOUS PCM/VCM CONTROLS

Manual Transmission
Shift Lights .................. See MISCELLANEOUS PCM/VCM CONTROLS
A/C Clutch Control .......... (1) See MISCELLANEOUS PCM/VCM CONTROLS
Electric Cooling Fan
Control ................... (1) See MISCELLANEOUS PCM/VCM CONTROLS

(1) - Complete coverage in the A/C-HEATER SYSTEM article in the

AIR CONDITIONING & HEAT section.
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COMPUTERIZED ENGINE CONTROLS

CONTROL UNIT

NOTE: To perform the following ground and power tests, use

appropriate wiring diagram in the L - WIRING DIAGRAMS
article.

Ground Circuits
1) Using an ohmmeter, check for continuity to ground on
PCM/VCM ground terminals. Resistance should be zero ohms. If not,
repair open to ground.
2) Using a voltmeter, touch negative lead of voltmeter to a
good ground. Touch positive lead of voltmeter to each ground terminal.
With vehicle running, voltmeter should indicate less than one volt. If
voltmeter reading is more than one volt, check for open, corrosion or
loose connection on ground circuit.

Power Circuits
1) Using a voltmeter, check for battery voltage between
PCM/VCM continuous power terminal(s) and ground. If battery voltage is
not present, check for blown fuse or open fusible link. If okay, check
for open in wire between PCM/VCM terminal and power source.
2) Turn ignition on. Using a voltmeter, check for battery
voltage between PCM/VCM ignition power terminals and ground. If
battery voltage is not present, check IGN fuse. If fuse is okay, check
for an open in wire between battery and ignition switch, and between
ignition switch and PCM/VCM terminal. If okay, check for a defective
ignition switch.
3) Connect voltmeter between ground and PCM/VCM starter
(crank) signal terminal. On vehicles with manual
transmission/transaxle, depress clutch pedal. Turn ignition switch to
START position. Battery voltage should be present ONLY when ignition
switch is in START position.
4) If voltage is not present, check CRANK fuse or fusible
link between ignition switch and PCM/VCM terminal. If fuse or fusible
link is okay, check for an open in wire between ignition switch and
PCM/VCM terminal, or check for a defective ignition switch.

ENGINE SENSORS & SWITCHES

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A/C ON Switch/System Test

1) Turn ignition switch to RUN position. Move mode selector
switch to any position other than OFF position. With A/C control
assembly connected, measure voltage between mode selector switch Light
Green wire and ground. For wiring schematics, see the L - WIRING
DIAGRAMS article.
2) Battery voltage should be present. If battery voltage is
present, mode selector switch is operating normally. If battery
voltage is not present, check wire from mode selector switch to fuse
for an open circuit. Also check A/C high and low pressure switches for
open.
3) Check voltage between mode selector switch Dark
Green/White wire or Light Green wire and ground. Voltage should not be
present. If voltage is present, replace mode selector switch.

Brake Switch
Disconnect brake switch harness connector. Using an ohmmeter,
check continuity between brake switch terminals. Continuity should be
present. Depress brake pedal or activate brake switch, continuity
should not be present.

Engine Coolant Temperature (ECT) Sensor

If a coolant sensor-related code is present, see the
G - TESTS W/CODES - 5.0L article. An out-of-calibration sensor may not
set a trouble code. Use following procedure to test sensor
calibration. Disconnect ECT sensor connector. Measure resistance
between sensor terminals. Resistance should be high when engine is
cold and drop as engine warms. See ECT SENSOR RESISTANCE VALUES table.

ECT SENSOR RESISTANCE VALUES TABLE

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Temperature øF (øC) Resistance (Ohms)

212 (100) ............................................ 177

158 (70) ............................................. 467
100 (38) ............................................ 1800
68 (20) ............................................. 3520
23 (-5) ........................................... 12,300
0 (-18) ........................................... 25,000
-40 (-40) ........................................ 100,700
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NOTE: Intake Air Temperature (IAT) sensor is also referred to as

Manifold Air Temperature (MAT) sensor.

Intake Air Temperature (IAT) Sensor

If an IAT sensor-related code is present, see the
G - TESTS W/CODES - 5.0L article. An out-of-calibration sensor may not
set a trouble code. Use following procedure to test calibration.
Disconnect IAT sensor harness connector. Connect ohmmeter between
sensor terminals. Sensor resistance should be as specified. See IAT

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SENSOR RESISTANCE table. With vehicle sitting overnight, IAT sensor

and coolant sensor should have close to the same resistance reading.

IAT SENSOR RESISTANCE TABLE

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Temperature øF (øC) Resistance (Ohms)

212 (100) ............................................ 185

158 (70) ............................................. 450
100 (38) ............................................ 1800
68 (20) ............................................. 3400
40 (4) .............................................. 7500
20 (-7) ........................................... 13,500
0 (-18) ........................................... 25,000
-40 (-40) ........................................ 100,700
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Knock Sensor
1) Disconnect knock sensor harness connector. Using an
ohmmeter, measure knock sensor resistance between sensor terminal and
engine block. Resistance should be 3300-4500 ohms. Connect voltmeter
between sensor terminal and ground. Set voltmeter to 2-volt AC scale.
2) Start and idle engine. Tap on engine block near sensor. A
signal should be indicated on voltmeter. If no signal is indicated,
replace knock sensor. Also see TIMING CONTROL SYSTEMS under IGNITION
SYSTEM and the G - TESTS W/CODES - 5.0L article.

Manifold Absolute Pressure (MAP) Sensor

1) MAP sensor circuit malfunction should set a related code
in PCM/VCM memory. If a code is present, see the
G - TESTS W/CODES - 5.0L article. An out-of-calibration sensor may not
set a trouble code. Use following procedure to test sensor
calibration. If driveability problems exist, MAP sensor failure is
suspected and no MAP code is present, disconnect MAP sensor connector.
If driveability condition improves, check MAP vacuum hose for splits,
kinks, proper routing and blockage. If no problems are found, replace
MAP sensor.
2) With ignition on and engine off, check MAP sensor
parameter using a scan tool connected to Data Link Connector (DLC).
Voltage should be as specified in MAP SENSOR VOLTAGE RANGE table.
3) If MAP sensor voltage is as specified, go to next step. If
voltage is not as specified, check 5-volt reference supply to sensor.
Check harness integrity. If no problems are evident, replace MAP
sensor.
4) Using a hand-held vacuum pump, apply 10 in. Hg to MAP
sensor and note voltage change. Voltage should drop to about 1.0-2.5
volts less than specified in table. If voltage is not as specified or
voltage reading does not immediately follow vacuum change, MAP sensor
is faulty.

MAP SENSOR VOLTAGE RANGE TABLE

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Altitude (Ft.) Range (Volts)

Below 1000 ....................................... 3.8-5.5

1000-2000 ........................................ 3.6-5.3
2000-3000 ........................................ 3.5-5.1
3000-4000 ........................................ 3.3-5.0
4000-5000 ........................................ 3.2-4.8
5000-6000 ........................................ 3.0-4.6
6000-7000 ........................................ 2.9-4.5
7000-8000 ........................................ 2.8-4.3
8000-9000 ........................................ 2.6-4.2
9000-10,000 ...................................... 2.5-4.0
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Oxygen Sensor (O2S)

1) Start engine and warm to operating temperature. Disconnect
oxygen sensor. Connect DVOM between Purple wire of oxygen sensor and
ground. Place DVOM on 2-volt scale.
2) Using another DVOM on 20-volt scale, connect DVOM in
series between Purple wire from PCM/VCM and positive post of battery.
This will simulate a rich condition, causing PCM/VCM to respond by
leaning mixture. Reading on DVOM connected to oxygen sensor should
decrease to less than .3 volt.
3) Move DVOM lead from positive battery post to negative
battery post. This will simulate a lean condition, causing PCM/VCM to
respond by richening mixture. Reading on DVOM connected to oxygen
sensor should increase to greater than .8 volt. If reading does not
change as specified, replace oxygen sensor.
4) If a second DVOM is not available, connect a jumper in
Purple wire from PCM/VCM. Hold jumper in one hand and touch positive
post of battery with other hand to simulate a rich condition. Touch
negative post of battery to simulate a lean condition. For additional
testing procedures, see the G - TESTS W/CODES - 5.0L article.

Oxygen Sensor Heating Element

On models with oxygen sensor heating elements, disconnect 3-
wire connector at oxygen sensor. Measure resistance between White wire
terminals on sensor side of connector. Resistance should be 3.5-14
ohms at 68øF (20øC). If resistance is not 3.5-14 ohms, replace oxygen
sensor.

Throttle Position (TP) Sensor

1) Install jumper wires to enable connection of a DVOM in
parallel between TP sensor harness connectors. Connect DVOM positive
lead to Dark Blue wire terminal. Connect negative lead to Black wire
terminal. See Fig. 1.
2) Turn ignition on, engine off. Slowly depress accelerator
pedal. Signal voltage should gradually change from less than one volt
at closed throttle to about 5.0 volts at wide open throttle position.
If reading is not as specified, replace TP sensor.
3) TP sensor circuit malfunction should set a related trouble
code. For further information, see the G - TESTS W/CODES - 5.0L

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article. Also see TP SENSOR ADJUSTMENT in the D - ADJUSTMENTS - 5.0L

article.

Fig. 1: Typical Throttle Position (TP) Sensor Circuit

Courtesy of General Motors Corp.

Transmission Range Switch

A problem in transmission range switch circuit will set
related diagnostic trouble code. See the G - TESTS W/CODES - 5.0L
article.

Vehicle Speed Sensor (PM Generator)

Disconnect vehicle speed sensor harness connector (located in
transmission/transaxle). Place gear selector in Neutral. Raise vehicle
drive wheels off the ground. Turn drive wheels by hand (more than 3
MPH). Measure AC signal voltage between sensor terminals. Voltage
reading should vary from 0.1 to 0.5 volt AC as wheel is turned. If
reading is not as specified, replace vehicle speed sensor. If a code
is set, refer to the G - TESTS W/CODES - 5.0L article.

MODULES, MOTORS, RELAYS & SOLENOIDS

RELAYS

NOTE: To perform the following tests, use the L - WIRING DIAGRAMS

article.

A/C Clutch Relay

See MISCELLANEOUS PCM/VCM CONTROLS.

Fuel Pump Relay

1) If a prolonged crank is required to start vehicle, fuel
pump relay may be faulty. To verify, start engine. With engine
running, disconnect oil pressure switch (fuel pump back-up circuit).
If engine stalls, fuel pump relay is faulty. If vehicle continues to
run, relay is okay. Check for other causes of prolonged crank.
2) To test fuel pump relay, disconnect fuel pump relay. Refer
to COMPONENT LOCATIONS. Apply battery voltage and ground to fuel pump
relay winding terminals (control and ground).
3) Using an ohmmeter, check continuity between fuel pump
relay control and ground terminals. Continuity should exist. If
continuity does not exist, fuel pump relay is defective.

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4) To by-pass fuel pump relay on vehicle (fuel pump not

operating), turn ignition off. Disconnect fuel pump relay connector.
Using a fused jumper wire, connect fuel pump test connector to
positive side of battery. Fuel pump should run.
5) If fuel pump runs, check for faulty connections to relay
or replace defective relay. To locate fuel pump test connector, refer
to COMPONENT LOCATIONS.

SOLENOIDS

NOTE: All PCM/VCM-controlled solenoids should have at least 20

ohms of resistance (except fuel injectors).

Canister Purge Solenoid

See EMISSION SYSTEMS & SUB-SYSTEMS.

Idle Air Control (IAC) Valve

See IDLE CONTROL SYSTEM under FUEL SYSTEM.

Torque Converter Clutch (TCC) Solenoid

See MISCELLANEOUS PCM/VCM CONTROLS.

FUEL SYSTEM

FUEL DELIVERY

NOTE: For fuel system pressure testing, see the

F - BASIC TESTING - 5.0L article.

Fuel Pressure Regulator (CSI)

Fuel pressure regulator is mechanically controlled by
internal spring pressure. Regulator is adjusted at factory and is not
serviceable. If fuel pressure is too low, check for restricted
delivery line. Also, check fuel pump pressure and volume. If fuel
pressure is too high, check for restricted fuel tank return line or
fuel filter. If no faults are found and pressure is too high or too
low, replace fuel pressure regulator.

Fuel Pump Oil Pressure Switch (Back-Up Circuit)

To test fuel pump oil pressure switch (fuel pump back-up
circuit), start engine. With engine running, disconnect fuel pump
relay. If engine stalls, fuel pump oil pressure switch is faulty. If
vehicle continues to run, switch is okay.

Fuel Pump Relay

See FUEL PUMP RELAY under MODULES, MOTORS, RELAYS & SOLENOIDS
.

Fuel Pump Relay By-Pass Procedure

If fuel pump will not energize, relay may be by-passed to
test fuel pump. Turn ignition off. Using a fused jumper wire, apply
battery voltage to fuel pump test connector. Fuel pump should turn on.

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For fuel pump test connector location, refer to COMPONENT LOCATIONS.

FUEL CONTROL

NOTE: To check Central Sequential Port Injectors (CSI), see

Central Sequential Port Injection Balance Test (CSI) and
Injector Coil Test (CSI).

Oxygen Sensor (O2S)

See ENGINE SENSORS & SWITCHES.

NOTE: If injectors are dirty, they should be cleaned using

approved injector cleaning procedure before performing fuel
INJECTOR BALANCE TEST.

Central Sequential Port Injector Balance Test (CSI)

The injector balance test is used to pulse the injector for a
precise amount of time, spraying a measured amount of fuel in the
intake manifold. As each injector is pulsed, a drop in fuel rail
pressure occurs. This pressure drop can be recorded and compared to
other injectors. All injector should have the same pressure drop of 1.
5 psi (.11 kg/cmý).

NOTE: Allow engine to cool to avoid irregular readings due to "hot

soak" fuel boiling. To prevent flooding, INJECTOR BALANCE
TEST should not be repeated more than once without starting
and running engine.

CAUTION: To avoid possible vehicle fire, wrap a shop towel around

fitting to avoid fuel spillage.

1) Perform on-board diagnostics. Check for Diagnostic Trouble

Code (DTC). If DTC(s) are present, diagnose DTC first. If DTC(s) are
not present, go to next step.
2) Turn ignition off. Connect Fuel Pressure Gauge (J-39021-
301) to pressure tap. Unplug injector harness connector. Connect
Injector Switch Box (J-39021-210). Turn ignition on. Monitor fuel
gauge. Fuel pressure should be 60-66 psi (4.2-4.6 kg/cmý). If pressure
is as specified, go to step 3). If pressure is not as specified, check
fuel pump and fuel lines for restriction.
3) Run fuel pump for at least 2 seconds after ignition is
turned on. Energize injectors one at a time. Note fuel pressure for
each injector. Fuel pressure should drop about 1.5 psi (.11 kg/cmý) on
each injector. If fuel pressure drop is as specified, go to next step.
If fuel pressure drop is not as specified, perform injector coil test.
See INJECTOR COIL TEST (CSI).
4) Remove upper manifold assembly. Connect Poppet Nozzle
Tester (J-34730-230). Connect a fused jumper wire (10-amp) to fuel
pump test connector. Energize each injector one at a time, fuel should
spray from each poppet nozzle tester. If fuel did not spray from an
injector, go to next step.
5) Replace faulty injector and poppet nozzle assembly. After

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replacing injector and poppet nozzle assembly, go to next step.

6) Install Tech 1 scan tool. Select DTC, CLEAR INFO. Start
engine. If engine starts and continues to run, go to next step. If
engine does not start or starts and dies, repeat step 2).
7) Warm engine to normal operating temperature. Select DTC,
FAILED THIS IGN. If additional DTC(s) are displayed, diagnose DTC. If
DTC(s) are not present, go to next step.
8) Using scan tool, select CAPTURE INFO, REVIEW INFO. If
additional DTC(s) are present, diagnose DTC. If DTC(s) are not present
at this time, system is okay.

CAUTION: To avoid possible vehicle fire, wrap a shop towel around

fitting to avoid fuel spillage.

Injector Coil Test (CSI)

1) Perform on-board diagnostic test. Diagnose any Diagnostic
Trouble Code (DTC) present. If DTC is not present, go to next step.
2) Turn ignition off. Relieve fuel pressure. Connect Fuel
Injector Tester (J-39021) to battery positive. Connect Injector Switch
Box (J-39021-210). Set amperage on fuel injector tester to 0.5 amp.
Connect DVOM to fuel injector tester. Connect Tech 1 scan tool and
monitor engine coolant temperature. If coolant temperature is 50-95øF
(10-35øC), go to step 4). If coolant temperature is not as specified,
go to next step.
3) Allow engine to warm or cool as necessary. If engine
coolant temperature is now 50-95øF (10-35øC), go to next step.
4) Using fuel injector tester, press PUSH TO START TEST
button and monitor DVOM reading. Perform this test to each injector.
Voltage reading should be 5.44-7.53 volts. If voltage reading is as
specified, go to next step. If voltage reading is not as specified,
perform injector balance test. See CENTRAL SEQUENTIAL PORT INJECTOR
BALANCE TEST (CSI).
5) Replace faulty injector nozzle and poppet assembly and
then proceed to next step.
6) Using Tech 1 scan tool, select DTC, CLEAR INFO. Start
engine. If engine starts and continues to run, go to next step. If
engine does not start or starts and dies, repeat step 2).
7) Warm engine to normal operating temperature. Using scan
tool, select DTC, FAILED THIS IGN. If DTC(s) are present, diagnose
DTC(s). If DTC(s) are not present, go to next step.
8) Using scan tool, select CAPTURE INFO, REVIEW INFO. If
additional DTC(s) are present, diagnose DTC(s). If DTC(s) are not
present, system is okay.

IDLE CONTROL SYSTEM

Idle Air Control (IAC) Valve

1) Disconnect harness connector to motor. Check resistance
across IAC coil terminals "A" to "B" and "C" to "D". See Fig. 2.
Resistance should be 40-80 ohms. If okay, go to next step. If
resistance is not as specified, replace IAC valve.
2) Check resistance between IAC terminals "B" to "C" and "A"

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to "D". Resistance should be infinite. If resistance is not as

specified, replace IAC valve.

NOTE: Functional testing of Idle Air Control (IAC) valve requires

a bidirectional scan tool capable of cycling PCM/VCM output
devices or a special IAC Driver and Noid Light Set (222L or
J-37027). Text in G - TESTS W/CODES - 5.0L article may refer
to Tech 1 tester, General Motor's bidirectional scan tool.

Fig. 2: Identifying IAC Valve Circuits (Typical)

Courtesy of General Motors Corp.

IGNITION SYSTEM

CIRCUIT DESCRIPTION

System includes a distributor, ignition coil, ignition

control module, secondary wires, spark plugs, VCM and crank sensor.
Ignition system is controlled by Vehicle Control Module (VCM). The VCM
monitors the information from various engine sensors, computes the
desired spark timing and controls the dwell and firing of the ignition
coil via an (IC) line to the ignition control module.

ENHANCED IGNITION SYSTEM CHECK

NOTE: Before clearing DTCs, use scan tool Capture Info to save
Freeze Frame and Failure Records for reference. Control
module's data is deleted once Clear Info function is used.

1) Perform Powertrain On-Board Diagnostic (OBD) System Check.

2) Check spark plug wires for open circuits, cracks or
improper seating of terminals at spark plugs, distributor and ignition
coil before proceeding with test. Check spark at plug with J-26792
spark tester or equivalent while cranking. (If there is no spark on
one wire, check a second wire). A few sparks then nothing is
considered no spark. If adequate spark is present, go to HARD START
diagnosis. If adequate spark is not present, go to step 3).
3) Remove coil wire from distributor cap. Insert J-26792

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spark tester into the coil wire and clamp the tester onto a ground.
Crank engine. If adequate spark is present, go to step 13). If
adequate spark is not present, go to step 4).
4) Measure coil wire resistance using a DVOM. The resistance
should be approximately 1000 ohms/inch. If resistance is within
specified value, go to step 5). If resistance is not within specified
value, go to step 32).
5) Disconnect ignition coil harness connector. Probe ignition
coil connector terminal C with a test lamp connected to battery
positive terminal. Crank engine. If test lamp is flashing while
cranking engine, go to step 7). If test lamp is not flashing while
cranking engine, go to step 6).
6) Turn OFF ignition. Probe ignition coil harness connector
terminal C with a test lamp connected to battery positive terminal. If
test lamp is illuminated, go to step 28). If not, go to step 9).
7) Probe ignition coil harness connector terminal B with a
test lamp connected to battery positive. If test lamp is illuminated,
go to step 29). If not, go to step 8).
8) Ignition ON, engine OFF. Measure terminal A voltage with a
DVOM connected to a ground. If DVOM measures above 10V, go to step
21). If not, go to step 18).
9) Turn OFF ignition. Disconnect ignition control module
harness connector. Check for an open circuit between ignition coil
harness connector terminal C and ignition control module harness
connector terminal D. If circuit is open, go to step 27). If circuit
is not open, go to next step.
10) Disconnect ignition control module harness connector.
Ignition ON, engine OFF. Measure voltage on terminal A at ignition
control module harness connector with DVOM connected to ground. If
DVOM measures above 10V, go to next step. If not, go to step 19).
11) Probe ignition control module hamess connector terminal C
with a test lamp connected to battery positive terminal. If test lamp
is illuminated, go to next step. If not, go to step 23).
12) Disconnect ignition control module harness connector.
Probe ignition control module harness connector terminal B with DVOM
set to AC scale connected to ground. Crank engine. Observe voltage
while engine is being cranked. Voltage should be between 1-4 VAC. If
voltage is as specified, go to step 20). If voltage is not as
specified, go to step 15).
13) Remove distributor cap. Check cap for cracks, moisture,
carbon tracks or physical damage. If any of these conditions are
present, go to step 34). If cap is okay, go to next step.
14) Crank engine. Observe distributor rotor while engine is
being cranked. If rotor did not turn, refer to appropriate N -
REMOVE/INSTALL/OVERHAUL article. If rotor did turn, go to step 33).
15) Turn OFF ignition. Disconnect VCM C3 connector. Check
ignition timing signal circuit for an open between the VCM C3 harness
connector and ignition control module harness connector terminal B. If
circuit is open, go to step 24). If circuit is okay, go to next step.
16) Probe ignition control (IC) circuit at VCM C3 connector
with a test lamp connected to battery positive terminal. If test lamp
is illuminated, go to step 25). If not, go to next step.

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17) Ignition ON, engine OFF. Probe ignition control (IC)

circuit at VCM C3 connector with a test lamp connected to ground. If
test lamp is illuminated, go to step 26). If not, go to step 22).
18) Check for an open or shorted to ground ignition positive
voltage circuit at terminal A of ignition coil. Repair as necessary.
Go to step 37).
19) Check for an open or shorted to ground ignition positive
voltage circuit at terminal A of the ignition control module. Repair
as necessary. Go to step 37).
20) Check for poor ignition control module connection. If
problem was found, go to step 30). If problem was not found, go to
step 35).
21) Check for a poor coil connection. If problem was found,
go to step 30). If problem was not found, go to step 31).
22) Check for a poor VCM connection. If problem was found, go
to step 30). If problem was not found, go to step 36).
23) Repair open ignition control module ground circuit. Go to
step 37).
24) Repair open IC circuit between VCM and ignition control
module. Go to step 37).
25) Repair grounded IC circuit between VCM and ignition
control module. Go to step 37).
26) Repair short to voltage in the IC circuit between the VCM
and the ignition control module. Go to step 37).
27) Repair open in tachometer signal circuit between ignition
coil and ignition control module. Go to step 37).
28) Repair short to ground in tachometer signal circuit
between ignition coil and ignition control module. Go to step 37).
29) Repair short to ground in tachometer input circuit
between ignition coil and instrument cluster. Go to step 37).
30) Repair circuit as necessary. Go to step 37).
31) Replace ignition coil. Refer to appropriate N -
REMOVE/INSTALL/OVERHAUL article. Go to step 37).
32) Replace coil wire. Go to step 37).
33) Replace distributor rotor. Go to step 37).
34) Replace distributor cap. Go to step 37).
35) Replace ignition control module. Refer to appropriate N -
REMOVE/INSTALL/OVERHAUL article. Go to step 37).
36) Replace VCM. Go to step 37).
37) Operate vehicle within conditions under which original
symptom was noted. If system is not functioning properly, repeat
testing from step 1).

Diagnostic Aids

NOTE: Numbers below refer to the step numbers in diagnostic test.

NOTE: Battery should be fully charged prior to any tests.

1) Perform the OBD system check before proceeding unless it

was already performed.
2) Checks for proper output from the enhanced ignition

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system. The spark tester requires a minimum of 25,000 volts to

operate. This check can be used in case of an ignition miss, because
the system may provicle enough voltage to run the engine but not
enough to operate a spark plug under heavy load.
3) This test separates distributor cap, rotor and ignition
wires from ignition coil in order to help identify a secondary
ignition system problem.
5) This test checks ignition control module, connections, and
wiring.
12) This test begins to determine if the VCM is providing a
signal to the ignition control module. If the VCM is not providing a
signal to the ignition control module, the problem exists between the
ignition control module and the VCM.
14) This test checks for a basic engine mechanical problem.

TIMING CONTROL SYSTEMS

Ignition Control Circuit

An open or short to ground in Ignition Control (IC) or by-
pass circuit will cause PCM/VCM to turn on Malfunction Indicator Light
(MIL) and confirm fault by setting a related trouble code. Refer to
the G - TESTS W/CODES - 5.0L article.

Knock Sensor Circuit (Models Using External Spark Controller

Module)
1) An open or short circuit on IC module control wire to
PCM/VCM will cause a loss of 12-volt IC module signal. This will cause
PCM/VCM to fully retard ignition timing.
2) If a scan tool is available, connect tester to Data Link
Connector (DLC). Using a metal object, tap on engine next to knock
sensor and note knock parameter. Knock should be indicated on scan
tool.
3) If a scan tool is not available, backprobe PCM/VCM knock
sensor signal terminal with a DVOM. With engine idling, 8-12 volts
should be present at this terminal. Using a metal object, tap on
engine close to knock sensor. Voltage signal at PCM/VCM terminal
should drop to zero volts, and return to original voltage when knock
signal ceases.
4) If voltage signal does not respond as described, check
knock sensor-to-module-signal. On vehicles equipped with automatic
transmission, it may be necessary to place transmission in Drive for
timing change to occur. See KNOCK SENSOR under
ENGINE SENSORS & SWITCHES.

Knock Sensor Circuit (Models Using Knock Sensor With Internal

Spark Controller Module)
1) An open or short circuit on knock sensor wire to PCM/VCM
will set a related trouble code. A false detonation signal will not
cause PCM/VCM to set a code.
2) If a scan tool is available, connect it to Data Link
Connector (DLC). Tap on engine next to knock sensor and note "knock"
parameter. Knock should be indicated on scan tool.

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3) If a scan tool is not available, connect tachometer to

engine. Start engine and hold RPM above idle. Using a metal object,
tap on engine close to knock sensor. A noticeable decrease in engine
RPM should occur. If no RPM decrease occurred, check knock sensor to
PCM/VCM circuit.
4) On vehicles equipped with automatic transmission, it may
be necessary to place transmission in Drive for timing change to
occur. See KNOCK SENSOR under ENGINE SENSORS & SWITCHES.

EMISSION SYSTEMS & SUB-SYSTEMS

EXHAUST GAS RECIRCULATION

Linear EGR Valve (Digital Valve)

1) Install scan tool. Ensure transmission range switch is
operating properly. See ENGINE SENSORS & SWITCHES. With engine at
normal operating temperature, command EGR pintle position to zero
percent. Increase engine speed to 2000 RPM. If scan tool reads actual
EGR pintle position at greater than 3 percent, EGR valve is stuck
open. Replace EGR valve.
2) If scan tool reads actual EGR pintle position at 3 percent
or less, command a 25 percent position step increase (i.e. 0-25
percent, 25-50 percent, 50-75 percent, etc.). Observe MAP reading and
actual EGR pintle position for 3 seconds. EGR should increase by about
25 percent position and MAP reading should also increase.
3) If actual EGR pintle position is stable and within 10
percent of desired EGR pintle position command after 2 seconds, go to
next step. If actual EGR pintle position is not as specified, go to
step 5).
4) MAP reading should have increased when EGR pintle
responded. If MAP did not respond, check EGR passages and EGR valve
for blockage. If MAP responded, set desired EGR pintle position to 100
percent. If EGR pintle position sets to 100 percent, EGR is okay. If
not, replace EGR valve.
5) Turn engine off. Check EGR electrical circuit and
connecting components. Turn ignition on, check for 5-volt reference
voltage on harness connector terminal "D" (Gray wire). If 5-volt
reference voltage is not present, check PCM/VCM. See CONTROL UNIT
under COMPUTERIZED ENGINE CONTROLS. If circuits are okay, replace EGR
valve.

FUEL EVAPORATION CONTROL

Canister Purge Solenoid

1) Set parking brake and block drive wheels. Warm engine to
normal operating temperature. Ensure engine coolant temperature is
greater than 113øF (45øC). Check vacuum source to solenoid. Repair if
vacuum is not present. Disconnect purge outlet hose from solenoid and
connect vacuum gauge. Connect scan tool and select EVAP PURGE SOLENOID
CONTROL.
2) Increase engine speed to 2000 RPM. Using scan tool,
command evap purge solenoid ON. Vacuum should pass through solenoid

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when solenoid is energized. If vacuum passes through solenoid, check

purge vacuum hose to canister. Repair as necessary. If vacuum is not
present, go to next step.
3) Disconnect purge solenoid harness connector. Connect test
light between canister purge control terminal and ignition feed
circuit harness connector terminal (Pink wire). Command evap purge
solenoid on. If test light illuminates, check for poor connection at
purge solenoid or for faulty purge solenoid. If test light does not
illuminate, go to next step.
4) Connect test light to an alternate ground and probe
ignition feed circuit harness connector terminal (Pink wire). If test
light illuminates, go to next step. If test light does not illuminate,
repair open or short in ignition feed circuit.

NOTE: When replacing PCM/VCM, new PCM/VCM needs to be programmed

when installed.

5) Connect test light to battery positive. Backprobe canister

purge control circuit PCM/VCM harness connector (Dark Green/White
wire). Using scan tool, command evap purge solenoid ON. If test light
illuminates, repair open in PCM/VCM harness connector circuit. If test
light does not illuminate, check for poor connection at PCM/VCM
connector or check for faulty PCM/VCM.

POSITIVE CRANKCASE VENTILATION

Required Service
The PCV system may require service for obstructions if any of
the following conditions exist:

* Rough Idle
* Stalling or Low Idle Speed
* Oil Leaks
* Oil in Air Cleaner
* Sludge in Engine

A leaking PCV valve or hose could cause:

* Rough Idle
* Stalling
* High Idle Speed

If engine idles roughly, check for clogged PCV valve and for
plugged or broken PCV hoses BEFORE adjusting idle. Check for correct
PCV valve application to ensure the correct valve is fitted. Replace
PCV valve if required.

Checking PCV Valve Function

1) Remove PCV valve from rocker cover. Run engine at idle.
Place thumb over open end of valve to check for vacuum. If there is no
vacuum at valve, check for obstruction in manifold port, hoses or PCV
valve. Repair or replace as necessary.

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2) Turn engine off. Remove PCV valve. Shake valve and listen
for rattle of check valve inside PCV valve. If a clear rattle is not
heard, replace PCV valve.
3) Visually inspect valve for varnish or deposits that may
make PCV valve sticky, restricted or incompletely seated. Replace if
necessary.
4) Engine must be sealed for PCV system to function as
designed. If leakage, sludging or dilution of oil is noted and PCV
system is functioning properly, check engine for cause and repair as
required to ensure PCV system will continue to function properly.
5) Since an engine operating without any crankcase
ventilation can be damaged, it is important to replace PCV valve and
air cleaner breather at regular intervals (at least every 30,000
miles). Check all hoses and clamps for failure or deterioration.

MISCELLANEOUS PCM/VCM CONTROLS

NOTE: Although not considered true engine performance-related

systems, some controlled devices may affect driveability if
they malfunction.

TRANSMISSION

NOTE: To perform the following tests, use appropriate wiring

diagram in the L - WIRING DIAGRAMS article.

Torque Converter Clutch (TCC) Solenoid

Disconnect harness connector to TCC solenoid. Measure
resistance between TCC solenoid terminals. Solenoid resistance should
be 10-15 ohms at 68øF (20øC).

NOTE: Some solenoids have an internal pressure switch in series

with solenoid winding and will not show continuity until
transmission hydraulic pressure is applied.

Converter Lock-Up Signal At Transmission

1) Warm engine to operating temperature. Raise vehicle and
support drive wheels. Support suspension where necessary to prevent
damage to drive axles.
2) Disconnect converter clutch connector at transmission.
Connect a test light across converter clutch harness terminals. Start
engine and place transmission in Drive. Accelerate vehicle to 45 MPH
and note test light.
3) If test light is not on, check solenoid power supply wire
of harness for open or short to ground. Check ground circuit for open
between harness connector and PCM/VCM. If harness is okay, see
CONVERTER LOCK-UP SIGNAL FROM PCM/VCM.

Converter Lock-Up Signal From PCM/VCM

1) Warm engine to operating temperature. Raise vehicle and
support drive wheels. Support suspension where necessary to prevent
damage to drive axles.

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2) Connect a test light to battery voltage. Touch TCC control

driver terminal with test light. Accelerate vehicle to 45 MPH and note
test light. If test light does not illuminate, problem is a faulty
PCM/VCM connector or PCM/VCM.

Shift Light (Manual Transmission)

1) These tests assume a shift light problem exists. Use this
procedure only if the light will not illuminate, or illuminates all
the time.
2) Turn ignition on, with engine off. Note shift light. Shift
light should not be on. If shift light is on, check for a short to
ground between bulb and PCM/VCM or for bad PCM/VCM.
3) Turn ignition on, with engine off, ground test terminal
No. 2 (Purple wire) of Data Link Connector (DLC). Malfunction
Indicator Light (MIL) should flash and shift light should come on. If
MIL illuminates, go to next step. If MIL does not flash, perform ON-
BOARD DIAGNOSTIC (OBD) SYSTEM CHECK in the F - BASIC TESTING - 5.0L
article.
4) If shift light does not come on, ground Tan/Black light
driver wire at PCM/VCM terminal using a jumper wire. For circuit
identification, see SHIFT LIGHT CIRCUIT IDENTIFICATION table. See
Fig. 3.
5) If shift light still does not come on, check for blown
GAGES fuse, blown bulb or open circuit between fuse and PCM/VCM. If
shift light illuminates when grounding PCM/VCM terminal with a jumper
wire, problem is a bad PCM/VCM connection or bad PCM/VCM.

SHIFT LIGHT CIRCUIT IDENTIFICATION TABLE

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
Application VCM Connector
(Terminal No.)

"C" & "K" Series ................................. C4 (15)

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

Fig. 3: Typical Shift Light Circuit

Courtesy of General Motors Corp.

PCM CONTROLLED WARNING LIGHTS & GAUGES

Warning Light Diagnosis

1) Perform On-Board Diagnostics (OBD) system check. See ON-
BOARD DIAGNOSTIC (OBD) SYSTEM CHECK in the F - BASIC TESTING - 5.0L

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article. After performing OBD system check, go to next step.

2) Check instrument cluster. See the INSTRUMENT PANEL article
in the ACCESSORIES/SAFETY EQUIPMENT section. If instrument panel is
okay, go to next step.
3) Turn ignition off. Disconnect PCM connector. Turn ignition
on. Using a DVOM, check voltage between affected PCM output circuit at
PCM harness connector terminal and ground. See the L - WIRING DIAGRAMS
article. Battery voltage should be present. If battery voltage is
present, go to next step. If battery voltage was not present, go to
step 7).
4) Set DVOM to 10-amp scale. Check current between affected
PCM output circuit and ground. Monitor reading for about 2 minutes.
DVOM should read .05-1.50 amps. If reading is as specified, go to step
12). If reading is not as specified, go to next step.
5) Disconnect instrument cluster, leaving PCM connector
disconnected. Using DVOM, check voltage between affected PCM output
circuit and ground. DVOM should read zero volts. If voltage reading is
as specified, go to step 15). If voltage reading is not as specified,
go to next step.
6) Locate and repair short to voltage in affected PCM output
circuit. After repairs, go to step 17).
7) Check ignition feed fuse for instrument cluster indicator
lights. If fuse is blown, go to next step. If fuse is okay, go to step
9).
8) Locate and repair short to ground in ignition feed circuit
for instrument cluster indicator lights. Replace fuse and go to step
17).
9) Disconnect instrument cluster connector. Turn ignition on.
Check voltage between ignition feed circuit for instrument cluster
indicator lights and ground. Battery voltage should be present. If
battery voltage is present, go to next step. If battery voltage was
not present, go to step 14).
10) Check affected PCM output circuit for an open or shorted
circuit to ground. If circuit is open or shorted, go to step 17). If
circuit was okay, go to next step.
11) Check affected PCM output circuit and ignition feed
circuit for poor connection at instrument cluster and at PCM. If
problem is present, repair as necessary and then go to step 17). If
problem was not found, go to step 15).
12) Turn ignition off. Reconnect PCM harness connector.
Disconnect instrument cluster connector. Turn ignition on. Connect a
test light between affected PCM output circuit and ignition feed
circuit at instrument panel harness connector. Using a scan tool,
perform OUTPUTS TEST function to cycle affected warning light on and
off. If test light flashes on and off, check for shorted component or
circuit in the output driver circuit. Check for faulty instrument
cluster. If test light does not flash on and off, go to next step.
13) Check affected PCM output circuit for poor connection to
PCM. Replace defective terminal and then go to step 17). If terminal
is okay, go to step 16).
14) Repair open in ignition feed circuit to instrument
cluster indicator lights. After repairs, go to step 17).

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15) Replace instrument cluster and then go to step 17).

16) Replace PCM and then go to next step.
17) Using scan tool, operate affected warning light. If
warning light does not operate properly, go to step 3).

Tachometer Control Circuit Diagnosis

1) Diagnose instrument cluster. See the INSTRUMENT PANEL
article in the ACCESSORIES/SAFETY EQUIPMENT section. After diagnosis,
go to next step.
2) Perform On-Board Diagnostic (OBD) system check. See ON-
BOARD DIAGNOSTIC (OBD) SYSTEM CHECK in the F - BASIC TESTING - 5.0L
article. After performing OBD system check, go to next step.
3) Turn ignition off. Disconnect PCM connector. Turn ignition
on. Using a DVOM, check voltage between tachometer control circuit at
PCM harness connector terminal and ground. See the L - WIRING DIAGRAMS
article. Battery voltage should be present. If battery voltage is
present, go to next step. If battery voltage was not present, go to
step 7).
4) Set DVOM to 10-amp scale. Check current between tachometer
control circuit and ground. Monitor reading for about 2 minutes. DVOM
should read .05-1.50 amps. If reading is not as specified, go to next
step. If reading is as specified, go to step 12).
5) Disconnect instrument cluster, leaving PCM connector
disconnected. Using DVOM, check voltage between tachometer control
circuit and ground. DVOM should read zero volts. If voltage reading is
as specified, go to step 15). If voltage reading is not as specified,
go to next step.
6) Locate and repair short to voltage in tachometer control
circuit. After repairs, go to step 17).
7) Check ignition feed fuse for instrument cluster. If fuse
is blown, go to next step. If fuse is okay, go to step 9).
8) Locate and repair short to ground in ignition feed circuit
to instrument cluster. Replace fuse and go to step 17).
9) Disconnect instrument cluster connector. Turn ignition on.
Check voltage between ignition feed circuit for instrument cluster and
ground. Battery voltage should be present. If battery voltage was not
present, go to step 14). If battery voltage is present, go to next
step.
10) Check tachometer control circuit for an open or shorted
circuit to ground. If circuit is open or shorted, go to step 17). If
circuit was okay, go to next step.
11) Check tachometer control circuit and ignition feed
circuit for poor connection at instrument cluster and at PCM. If
problem is present, repair as necessary and then go to step 17). If
problem was not found, go to step 15).
12) Turn ignition off. Reconnect PCM harness connector.
Disconnect instrument cluster connector. Turn ignition on. Connect a
test light between tachometer control circuit and ignition feed
circuit at instrument panel harness connector. Using a scan tool,
perform OUTPUTS TEST function to cycle tachometer control output light
on and off. If test light flashes on and off, check for shorted
component or circuit in the output driver circuit. Check for faulty

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instrument cluster. If test light does flash on and off, go to next

step.
13) Check tachometer control circuit for poor connection to
PCM. Replace defective terminal and then go to step 17). If terminal
is okay, go to step 16).
14) Repair open in ignition feed circuit to instrument
cluster. After repairs, go to step 17).
15) Replace instrument cluster and then go to step 17).
16) Replace PCM and then go to next step.
17) Start engine and observe tachometer. If tachometer does
not operate properly, diagnose instrument cluster. See the INSTRUMENT
PANEL article in the ACCESSORIES/SAFETY EQUIPMENT section.

A/C COMPRESSOR CLUTCH CONTROLS

The A/C compressor clutch relay is controlled by the PCM. The

PCM improves idle quality by delaying A/C compressor clutch engagement
until idle speed is increased, or disengages A/C compressor clutch
when idle speed is too low. A/C compressor clutch is cycled by PCM.
PCM smooths cycling of A/C compressor clutch by adding fuel the
instant A/C compressor clutch is applied.

NOTE: See the L - WIRING DIAGRAMS article for component location,

terminal and wire color identification.

RELAY LOCATION

A/C COMPRESSOR CLUTCH RELAY LOCATION TABLE

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
Application Location

"C" & "K" Series ......... In Underhood Fuse/Relay Center,

Near Anti-Lock Brake Control Module
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

WARNING: Vehicles may be equipped with a PCM using an Electronically

Erasable Programmable Read Only Memory (EEPROM). When
replacing PCM, the new PCM must be programmed.

NOTE: To help save diagnostic time, ALWAYS check for blown fuses
or fusible links before proceeding with any testing. If
fuses are blown, locate and repair short circuit before
replacing fuses. Ensure all related relay and wire harness
connections are clean and tight. Repair as necessary.

A/C CLUTCH CIRCUIT DIAGNOSIS

Description
A/C clutch relay is controlled by Vehicle Control Module
(VCM) to delay A/C clutch engagement after A/C is turned on. This
allows VCM to adjust engine RPM before A/C clutch engages. VCM will
engage A/C clutch any time A/C has been requested unless coolant

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temperature is high, A/C system pressure is low, A/C system pressure

is high, wide open throttle, or high engine RPM.
When A/C-heater control panel is placed in A/C mode, a 12-
volt signal is sent to VCM. When VCM receives this signal, VCM will
ground A/C clutch relay control circuit to energize A/C relay. This is
shown on scan tool as A/C REQUEST YES.
When an A/C request has been detected by VCM, VCM will ground
A/C clutch relay control circuit. As a result, relay contacts close,
allowing current to flow through relay to A/C compressor clutch. When
A/C REQUEST has been detected by PCM, cooling fans will be turned on
when A/C pressure is greater than a predetermined value.

Compressor Clutch Control Circuit Diagnosis

1) Install A/C manifold gauge set. Ensure A/C system is fully
charged. Charge system if necessary and go to next step.
2) Start engine and allow it to reach normal operating
temperature. Turn on A/C. If A/C compressor clutch engages, then
disengages within 60 seconds, diagnose A/C-heater system. If operation
is not as specified, go to next step.
3) Install scan tool. With A/C on, monitor A/C REQUEST. If
scan tool displays A/C REQUEST as YES, go to next step. If scan tool
does not display A/C REQUEST as YES, go to step 8).
4) Disconnect A/C compressor clutch harness connector.
Connect a test light between harness connector terminals. If test
light comes on, go to next step. If test light does not come on, go to
step 12).
5) Check for faulty A/C compressor clutch harness
connections. If a problem is found, go to next step. If no problem is
found, go to step 7).
6) Repair faulty A/C compressor clutch wire harness
connections. Go to step 27).
7) Replace A/C compressor clutch. Go to step 27).
8) Turn ignition off. Disconnect VCM harness connectors. Turn
ignition on. Using a test light connected to ground, probe A/C request
circuit (Dark Green/White wire) at VCM harness connector. If test
light comes on, go to next step. If test light does not come on, go to
step 11).
9) Check for poor VCM harness connections. If a problem is
found, go to next step. If no problem is found, go to step 26).
10) Repair VCM harness connector. Go to step 27).
11) Repair open or short to ground in Dark Green/White wire
between VCM and A/C control switch. Go to step 27).
12) Using a test light connected to ground, probe Dark Green
wire at A/C compressor clutch harness connector. If test light comes
on, go to next step. If test light does not come on, go to step 14).
13) Repair open in A/C compressor clutch ground circuit
(Black wire) between harness connector and ground. Go to step 27).
14) Turn ignition on, engine off. Using scan tool, command
A/C relay on. If A/C relay clicks, go to step 23). If A/C relay does
not click, go to next step.
15) Disconnect A/C relay. Using a test light connected to
ground, probe A/C relay cavity No. 86 (Pink wire), then cavity No. 87

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(Orange wire). If test light comes on for both wires, go to step 17).
If test light does not come on for both wires, go to next step.
16) Repair open or short to ground in affected circuit. Go to
step 27).
17) Connect a test light between A/C relay cavity No. 86
(Pink wire) and cavity No. 85 (Dark Green/White wire). Using scan
tool, command A/C relay on. If test light comes on, go to next step.
If test light does not come on, go to step 19).
18) Replace A/C relay. Go to step 27).
19) Check A/C relay control circuit (Dark Green/White wire)
for faulty connection at VCM harness connector. If a problem is found,
go to next step. If no problem is found, go to step 21).
20) Repair faulty connection at VCM. Go to step 27).
21) Check for open in Dark Green/White wire between VCM and
A/C relay. If a problem is found, go to next step. If no problem is
found, go to step 26).
22) Repair open in Dark Green/White wire. Go to step 27).
23) Remove A/C relay. Using a fused jumper wire, jumper A/C
relay cavity No. 87 (Orange wire) and cavity No. 30 (Dark Green wire)
together. If A/C compressor clutch engages, go to next step. If A/C
compressor clutch does not engage, go to step 25).
24) Replace A/C relay. Go to step 27).
25) Repair open or short to ground in A/C compressor clutch
control circuit (Dark Green wire). Go to step 27).
26) Replace VCM. Go to next step.
27) Using scan tool, select DTC CLEAR INFO. Start engine and
allow it to reach normal operating temperature. Select DTC SPECIFIC,
then enter DTC number that was set. Operate vehicle within conditions
that may have set this DTC. If DTC does not reset, go to next step. If
DTC resets, go to step 2).
28) Using scan tool, select CAPTURE INFO, REVIEW INFO. If any
DTCs are displayed that have not been diagnosed, perform diagnosis for
applicable DTC. See the G - TESTS W/CODES - 5.0L article. If no DTCs
are displayed, system is okay.

COOLING FAN

The electric cooling fan is used for radiator and A/C

condenser cooling. Cooling fan operates when A/C is on and when engine
coolant temperature exceeds a specific value. One or more cooling fan
relays may be used. Some RWD trucks, vans and motorhomes are equipped
with an auxiliary electric cooling fan. The auxiliary electric cooling
fan is not controlled by Powertrain Control Module (PCM).
To help save diagnostic time, ALWAYS check for blown fuses or
fusible links before proceeding with any testing. If fuses are blown,
locate and repair short circuit before replacing fuses. Ensure all
related relay and wire harness connections are clean and tight. Repair
as necessary. See WIRING DIAGRAM for component, terminal and wire
color identification.

WARNING: Vehicles may be equipped with a PCM using an Electronically

Erasable Programmable Read Only Memory (EEPROM). When

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replacing PCM, the new PCM must be programmed.

SCAN TESTER

A variety of information is transmitted through Data Link

Connector (DLC). This data is transmitted at a high frequency which
requires a Tech 1 Scan Tester (94-00101A), appropriate cartridge kit
and vehicle interface module kit, or other scan tester for
interpretation. Several scan testers (scan tools) are available for
diagnostic work. Scan testers other than Tech 1 scan tester will
function and provide information for diagnostic work.

COOLING FAN RELAY LOCATION TABLE

ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
Application Location

"C" & "K" Series ........ In Engine Compartment Fuse/Relay

Block, Near Anti-Lock Brake Control Module
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

ELECTRIC COOLING FAN CIRCUIT DIAGNOSIS

Description
Auxiliary cooling fan operating voltage is supplied by AUX
FAN fuse (30-amp). Cooling fan relay (coil-side) voltage is supplied
by A/C COMP fuse (10-amp). Both fuses are located in underhood
fuse/relay center. Voltage is provided from fuses directly to
auxiliary cooling fan relay. When engine coolant temperature reaches a
set point, or A/C operation is requested, Vehicle Control Module (VCM)
supplies a ground path for coil-side of cooling fan relay. This allows
relay contacts to close resulting in auxiliary cooling fan operation.

NOTE: Check VCM Diagnostic Trouble Codes (DTCs) that may affect
system. If any DTCs are present, perform testing for
applicable DTC. Some DTCs may cause auxiliary cooling fan to
come on at all times. Repair all DTCs that are set before
proceeding with testing. See G - TESTS W/CODES - 5.0L
article.

Cooling Fan Circuit Diagnosis

1) If cooling fan does not run, go to next step. If cooling
fan runs continuously, go to step 9).
2) Remove cooling fan relay from harness connector. Using a
test light connected to ground, probe cooling fan relay harness
connector terminal No. 30. If test light comes on, go to next step. If
test light does not come on, locate and repair open or short to ground
in circuit No. 1540 (Orange wire) between AUX FAN fuse (30-amp) and
cooling fan relay.
3) Turn ignition on. With test light still connected to
ground, probe cooling fan relay harness connector terminal No. 85. If
test light comes on, go to next step. If test light does not come on,
locate and repair open or short to ground in circuit No. 1240 (Orange

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wire) between A/C COMP fuse (10-amp) and cooling fan relay.
4) Using a fused jumper wire, jumper cooling fan relay
harness connector terminals No. 30 and 87 together. If cooling fan
does not run, go to next step. If cooling fan runs, go to step 6).
5) Turn ignition off. Disconnect cooling fan motor harness
connector. Using an ohmmeter, check for continuity between harness
connector terminal "A" and ground. If continuity is present, locate
and repair open in circuit No. 702 (Red wire) between cooling fan
relay and cooling fan motor. If continuity is not present, locate and
repair open or poor connection in circuit No. 250 (Black wire) between
cooling fan motor and ground.
6) Turn ignition off. Disconnect VCM harness connector C4.
Using an ohmmeter, check for continuity between VCM harness connector
C4, terminal No. 6 and cooling fan relay harness connector terminal
No. 86. If continuity is present, go to next step. If continuity is
not present, locate and repair open in circuit No. 473 (Dark Blue
wire) between cooling fan relay and VCM.
7) Reconnect cooling fan relay. Turn ignition on. Using a
fused jumper wire, jumper VCM harness connector C4, terminal No. 6 to
ground. If cooling fan runs, go to next step. If cooling fan does not
run, replace cooling fan relay.
8) Using a test light connected to battery positive, probe
VCM harness connector C4, terminal No. 7. Push REC button on A/C
control panel to ON position. If test light comes on, check VCM
harness connector for corroded, backed-out or poorly connected
terminal pins. Repair as necessary. If connections are okay, VCM is
faulty.
9) Turn ignition on. Disconnect cooling fan relay from
harness connector. If cooling fan motor stops, go to next step. If
cooling fan motor continues to run, locate and repair short to power
in circuit No. 702 (Red wire) between cooling fan relay and cooling
fan motor.
10) Turn ignition off. Disconnect VCM harness connector C4.
Using an ohmmeter, check for continuity between VCM harness connector
C4, terminal No. 6 and ground. If continuity is not present, go to
next step. If continuity is present, locate and repair short to ground
in circuit No. 473 (Dark Blue wire) between cooling fan relay and
ground.
11) Ensure ignition is off. Ensure REC button on A/C control
panel is in OFF position. Using an ohmmeter, check for continuity
between VCM harness connector C4, terminal No. 7 and ground. If
continuity is present, locate and repair short to ground in circuit
No. 1614 (Dark Green wire) between A/C control module and VCM. If
continuity is not present, replace cooling fan relay.

ELECTRIC COOLING FAN CIRCUIT DIAGNOSIS

(WITHOUT AUXILIARY FAN CONTROL SWITCH)

Description
On models not equipped with Vehicle Control Module (VCM), ENG
1 fuse (20-amp) located in instrument panel fuse block supplies
voltage to coil side of auxiliary cooling fan relay with ignition

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switch in ON position. A ground path for cooling fan relay is supplied

by A/C high pressure switch, or auxiliary fan control switch (if
equipped). On models equipped with VCM, a ground path for fan control
relay is supplied by VCM. As a result, relay contacts close, allowing
current to flow to cooling fan motor through relay, resulting in
auxiliary cooling fan motor operation.

NOTE: A fully charged A/C system is required for proper auxiliary

cooling fan operation.

Cooling Fan Circuit Diagnosis

1) If cooling fan does not operate, or does not operate
properly, go to next step. If cooling fan runs continuously with
ignition off, go to step 12). If cooling fan runs continuously with
ignition on, go to step 13).
2) If cooling fan motor never operates, go to next step. If
cooling fan motor does not operate with high coolant temperature, go
to step 9). If cooling fan motor does not operate with A/C on, go to
step 10).
3) Turn ignition off. Disconnect fan control relay harness
connector. Turn ignition on. Using a test light connected to ground,
probe fan control relay harness connector terminal No. 30. If test
light comes on, go to next step. If test light does not come on,
locate and repair open in Red wire between fan control relay and
splice near fusible link.
4) Using a fused jumper wire, jumper fan control relay
harness connector terminals No. 30 and 87 together. If cooling fan
motor runs, go to next step. If cooling fan motor does not run, go to
step 6).
5) Disconnect jumper wire. Using a test light connected to
ground, probe fan control relay harness connector terminal No. 85. If
test light comes on, go to step 8). If test light does not come on,
locate and repair open in Pink wire between ENG 1 fuse and fan control
relay.
6) With jumper wire still in place, disconnect cooling fan
motor harness connector. Using a test light connected to ground, probe
cooling fan motor harness connector terminal "B". If test light comes
on, go to next step. If test light does not come on, locate and repair
open in Red wire between fan control relay and cooling fan motor.
7) Using a test light connected to battery positive, probe
cooling fan motor harness connector terminal "A". If test light comes
on, replace cooling fan motor. If test light does not come on, locate
and repair open in Black wire between cooling fan motor and ground.
8) Reconnect fan control relay. Using a voltmeter, measure
voltage (backprobe) between fan control relay harness connector
terminal No. 86 and ground. If reading is less than 10 volts, replace
fan control relay. If reading is 10 volts or greater, locate and
repair open in Dark Green wire between fan control relay and splice.
9) Disconnect auxiliary fan control switch harness connector.
Turn ignition on. Using a voltmeter, measure voltage between auxiliary
fan control switch harness connector terminal "A" and ground. If
reading is less than 10 volts, locate and repair open in Dark Green

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wire between auxiliary fan control switch and splice. If reading is 10

volts or greater, replace auxiliary fan control switch.
10) Disconnect A/C high pressure switch harness connector.
Using a voltmeter, measure voltage between auxiliary fan control
switch harness connector terminal "B" and ground. If reading is 10
volts or greater, go to next step. If reading is less than 10 volts,
locate and repair open in Dark Green wire between A/C high pressure
fan switch and splice.
11) Using a test light connected to battery positive, probe
A/C high pressure switch harness connector terminal "A". If test light
comes on, replace A/C high pressure fan switch. If test light does not
come on, locate and repair open in Black wire between A/C high
pressure switch harness connector terminal "A" and splice.
12) With ignition on, disconnect fan control relay harness
connector. If cooling fan motor stops, replace fan control relay. If
cooling fan motor continues to run, locate and repair short to power
in Red wire between fan control relay and cooling fan motor.
13) Turn ignition off. Disconnect fan control relay harness
connector. Using a test light connected to battery positive, probe
cooling fan relay harness connector terminal No. 86. If test light
comes on, go to next step. If test light does not come on, replace fan
control relay.
14) With test light still connected, disconnect auxiliary fan
control switch harness connector. If test light remains on, go to next
step. If test light does not remain on, replace auxiliary fan control
switch.
15) With test light still connected, disconnect A/C high
pressure fan switch harness connector. If test light remains on,
locate and repair short to ground in Dark Green wire between fan
control relay and either A/C high pressure fan switch or auxiliary fan
control switch.

COMPONENT LOCATIONS

COMPONENT LOCATIONS TABLE

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Component Location

A/C Compressor Clutch Cycling Switch ...............

In Low Pressure
Refrigerant Line
A/C Compressor Clutch Relay ....... In Low Pressure Refrigerant Line
A/C High Pressure Cut-Out Switch ......... On Rear Of A/C Compressor
Air Injection Reaction Relay ..... In Underhood Fuse/Relay Center At
Left Rear Of Engine Compartment On Wheelwell
Auxiliary Cooling Fan Relay ... Left Rear Side Of Engine Compartment
Camshaft Position (CMP) Sensor ...................... In Distributor
Crankshaft Position (CKP) Sensor ....... Lower Right Front Of Engine
Block, Near Crankshaft
Cruise Control Module ......... Left Rear Side Of Engine Compartment
On Bulkhead
Data Link Connector (DLC) ...... Under Left Side Of Instrument Panel
Distributor ........................... Top Of Engine, Near Bulkhead

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EGR Valve .................. Front Of Engine Near Thermostat Housing

Engine Coolant Temperature (ECT) Sensor ...... Right Front Of Engine
Near Thermostat Housing
EVAP Canister Purge Solenoid ....... Right Center On Intake Manifold
Near MAP Sensor
EVAP Canister Vent ... Near Master Cylinder, At Left Front Wheelwell
EVAP Purge Solenoid Vacuum Switch ........ Right Rear Top Of Engine,
Right Of Purge Solenoid
Fuel Injectors .................................. In Intake Manifold
Fuel Pump Oil Pressure Switch ............... Rear Center Of Engine,
Behind Distributor
Fuel Pump Prime Connector ........ At Instrument Panel Harness, Near
Fuse/Relay Center Breakout
Fuel Pump Relay ..................... In Underhood Fuse/Relay Center
Fuel Tank Pressure Sensor ............................. In Fuel Tank
Heater & A/C Control Module ............. Center Of Instrument Panel
At Heater Control
Idle Air Control (IAC) Valve ........... Right Rear Of Throttle Body
Ignition Coil ............................ Right Rear Side Of Engine
Ignition Control Module .................. Right Rear Side Of Engine
Intake Air Temperature (IAT) Sensor ............. On Air Intake Duct
Knock Sensor ......... Right Side Of Engine, Below Exhaust Manifold,
Forward Of Starter
Manifold Absolute Pressure (MAP) Sensor ....... Top Center Of Intake
Manifold Near Fuel Injectors
Mass Airflow (MAF) Sensor ....... On Air Intake Duct Near Air Filter
Oxygen Sensors .................. In Exhaust Pipe In Front & Rear Of
Catalytic Converter
Power Steering Control Module ..... Under Center Of Instrument Panel
Throttle Position (TP) Sensor ............. On Side Of Throttle Body
Transfer Case Relay .............. Right Rear Of Engine Compartment,
Near Center Of Cowl
Underhood Fuse/Relay Center ........ Left Rear Of Engine Compartment
On Wheelwell
Vehicle Control Module (VCM) ........... In Engine Compartment, Near
Master Cylinder At Left Front Wheelwell
Vehicle Speed Sensor ............... Left Rear Side Of Transmission,
(Left Side Of Transfer Case On Vehicles With 4WD)
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ

WIRING DIAGRAM

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Fig. 4: Cooling Fan System Wiring Diagram

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