service ISUZU TF SERIES 2004 Workshop Manual

3.5L ENGINE DRIVEABILITY AND EMISSIONS 6E-1
ENGINE
3.5L ENGINE DRIVEABILITY AND EMISSIONS
CONTENTS


ABBREVIATION CHARTS ................................ 6E-5
COMPONENT LOCATOR ................................. 6E-6
ENGINE COMPONENT LOCATOR TABLE .. 6E-6
ECM CIRCUIT DIAGRAM ................................. 6E-10
GROUND POINT CHART G.EXP (LHD)
WITHOUT EUROPE, ISRAEL, TURKEY (1/4). 6E-13
GROUND POINT CHART G.EXP (LHD) (2/4) .. 6E-14
GROUND POINT CHART G.EXP (LHD) (3/4) .. 6E-15
GROUND POINT CHART G.EXP (LHD) (4/4) .. 6E-16
GROUND POINT CHART G.EXP (RHD) (1/4) . 6E-17
GROUND POINT CHART G.EXP (RHD) (2/4) . 6E-18
GROUND POINT CHART G.EXP (RHD) (3/4) . 6E-19
GROUND POINT CHART G.EXP (RHD) (4/4) . 6E-20
LOCATION ........................................................ 6E-21
CABLE HARNESS & CONNECTOR
LOCATION ....................................................... 6E-22
CONNECTOR LIST ........................................... 6E-27
RELAY AND FUSE ............................................ 6E-30
RELAY AND FUSE BOX LOCATION
(LHD&RHD) ................................................... 6E-30
RELAY AND FUSE BOX LOCATION
(LHD&RHD) ................................................... 6E-31
FUSE AND RELAY LOCATION
(LHD&RHD) ................................................... 6E-32
ECM WIRING DIAGRAM (1/10) ........................ 6E-33
ECM WIRING DIAGRAM (2/10) ........................ 6E-34
ECM WIRING DIAGRAM (3/10) ........................ 6E-35
ECM WIRING DIAGRAM (4/10) ........................ 6E-36
ECM WIRING DIAGRAM (5/10) ........................ 6E-37
ECM WIRING DIAGRAM (6/10) ........................ 6E-38
ECM WIRING DIAGRAM (7/10) ........................ 6E-39
ECM WIRING DIAGRAM (8/10) ........................ 6E-40
ECM WIRING DIAGRAM (9/10) ........................ 6E-41
ECM WIRING DIAGRAM (10/10) ...................... 6E-42
ECM CONNECTOR PIN ASSIGNMENT
& OUTPUT SIGNAL ......................................... 6E-43
GENERAL DESCRIPTION FOR ECM AND
SENSORS ........................................................ 6E-51

Mass Air Flow (MAF) Sensor & Intake Air
Temperature (IAT) Sensor ............................. 6E-52
Throttle Position Sensor (TPS)....................... 6E-52
Idle Air Control (IAC) Valve ............................ 6E-53
Camshaft Position (CMP) Sensor .................. 6E-53
Crankshaft Position (CKP) Sensor ................. 6E-54
Engine Coolant Temperature (ECT) Sensor .. 6E-54
Vehicle Speed Sensor (VSS) ......................... 6E-55
Heated Oxygen (O
2) Sensor........................... 6E-55
GENERAL DESCRIPTION FOR FUEL
METERING....................................................... 6E-56
GENERAL DESCRIPTION FOR ELECTRONIC
IGNITION SYSTEM IGNITION
COILS & CONTROL ......................................... 6E-58
GENERAL DESCRIPTION FOR EVAPORATIVE
EMISSION SYSTEM ........................................ 6E-61
GENERAL DESCRIPRION FOR EXHAUST
GAS RECIRCULATION (EGR) SYSTEM......... 6E-62
ISUZU STRATEGY BASED DIAGNOSTICS .... 6E-63
Diagnostic Thought Process .......................... 6E-64
1. Verify the Complaint ................................... 6E-64
2. Perform Preliminary Checks....................... 6E-64
3. Check Bulletins and Troubleshooting
Hints ........................................................... 6E-65
4. Perform Service Manual Diagnostic
Checks ....................................................... 6E-65
5a and 5b. Perform Service Manual
Diagnostic Procedures ............................... 6E-65
5c. Technician Self Diagnoses ....................... 6E-65
5d. Intermittent Diagnosis............................ 6E-66
Symptom Simulation Tests.......................... 6E-67
5e. Vehicle Operates as Designed ................. 6E-68
6. Re-Examine the Complaint ........................ 6E-68
7. Repair and Verify Fix .................................. 6E-68
GENERAL SERVICE INFORMATION .............. 6E-69
Aftermarket Electrical and Vacuum
Equipment ..................................................... 6E-69
Electrostatic Discharge Damage .................... 6E-69

6E-2 3.5L ENGINE DRIVEABILITY AND EMISSIONS
Visual/Physical Engine Compartment
Inspection ...................................................... 6E-74
Basic Knowledge of Tools Required............... 6E-71
Serial Data Communications .......................... 6E-71
On-Board Diagnostic (OBD) ........................... 6E-71
Comprehensive Component Monitor
Diagnostic Operation ..................................... 6E-71
The Diagnostic Executive ............................... 6E-72
Verifying Vehicle Repair ................................. 6E-73
Reading Flash Diagnostic Trouble Codes ...... 6E-73
Reading Diagnostic Trouble Codes Using
a Tech 2......................................................... 6E-73
On-Board Diagnosis (Self-Diagnosis)............. 6E-74
Diagnosis with Tech 2 .................................... 6E-75
TYPICAL SCAN DATA & DEFINITIONS
(ENGINE DATA) ............................................... 6E-79
TYPICAL SCAN DATA & DEFINITIONS
(O2 SENSOR DATA)........................................ 6E-81
MISCELLANEOUS TEST .................................. 6E-83
PLOTTING SNAPSHOT GRAPH ...................... 6E-85
Plotting Graph Flow Chart (Plotting graph
after obtaining vehicle information)................ 6E-86
Flow Chart for Snapshot Replay
(Plotting Graph) ............................................. 6E-87
SNAPSHOT DISPLAY WITH TIS2000.............. 6E-88
SERVICE PROGRAMMING SYSTEM (SPS).... 6E-91
HOW TO USE BREAKER BOX ........................ 6E-94
ON-BOARD DIAGNOSTIC (OBD) SYSTEM
CHECK ............................................................. 6E-97
NO CHECK ENGINE LAMP (MIL)..................... 6E-101
CHECK ENGINE LAMP (MIL) "ON" STEADY ... 6E-104
FUEL INJECTOR COIL TEST PROCEDURE
AND FUEL INJECTOR BALANCE
TEST PROCEDURE ......................................... 6E-106
FUEL SYSTEM ELECTRICAL TEST ................ 6E-111
FUEL SYSTEM DIAGNOSIS ............................. 6E-116
A/C SYSTEM CIRCUIT DIAGNOSIS ................ 6E-122
ECM DIAGNOSTIC TROUBLE CODES (DTC) 6E-130
MULTIPLE DTC SETS TROUBLESHOOTING
AIDS ................................................................. 6E-142
DTC P0101 (FLASH CODE 61) MASS
AIR FLOW SENSOR CIRCUIT
RANGE/PERFORMANCE ................................ 6E-146
DTC P0102 (FLASH CODE 61) MASS
AIR FLOW SENSOR CIRCUIT LOW INPUT ... 6E-149
DTC P0103 (FLASH CODE 61) MASS
AIR FLOW SENSOR CIRCUIT HIGH INPUT .. 6E-154
DTC P0112 (FLASH CODE 23) INTAKE AIR
TEMPERATURE (IAT) SENSOR LOW
INPUT ............................................................... 6E-158 DTC P0113 (FLASH CODE 23) INTAKE AIR
TEMPERATURE (IAT) SENSOR HIGH
INPUT ............................................................... 6E-163
DTC P0117 (FLASH CODE 14) ENGINE
COOLANT TEMPERATURER (ECT)
SENSOR LOW INPUT ..................................... 6E-168
DTC P0118 (FLASH CODE 14) ENGINE
COOLANT TEMPERATURER (ECT)
SENSOR HIGH INPUT..................................... 6E-173
DTC P0121 (FLASH CODE 21) THROTTLE
POSITION SENSOR (TPS) CIRCUIT
RANGE/PERFORMANCE ................................ 6E-179
DTC P0122 (FLASH CODE 21) THROTTLE
POSITION SENSOR (TPS) CIRCUIT
LOW INPUT ..................................................... 6E-183
DTC P0123 (FLASH CODE 21) THROTTLE
POSITION SENSOR (TPS) CIRCUIT
HIGH INPUT ..................................................... 6E-188
DTC P0131 (FLASH CODE 15) O2 SENSOR
CIRCUIT LOW VOLTAGE
(BANK 1 SENSOR 1) ....................................... 6E-193
DTC P0151 (FLASH CODE 15) O2 SENSOR
CIRCUIT LOW VOLTAGE
(BANK 2 SENSOR 1) ....................................... 6E-193
DTC P0132 (FLASH CODE 15) O2 SENSOR
CIRCUIT HIGH VOLTAGE
(BANK 1 SENSOR 1) ....................................... 6E-201
DTC P0152 (FLASH CODE 15) O2
SENSOR CIRCUIT HIGH VOLTAGE
(BANK 2 SENSOR 1) ....................................... 6E-201
DTC P0134 (FLASH CODE 15) O2 SENSOR
CIRCUIT NO ACTIVITY DETECTED
(BANK 1 SENSOR 1) ....................................... 6E-207
DTC P0154 (FLASH CODE 15) O2 SENSOR
CIRCUIT NO ACTIVITY
DETECTED (BANK 2 SENSOR 1) ................... 6E-207
DTC P0171 (FLASH CODE 44) O2 SENSOR
SYSTEM TOO LEAN (BANK 1) ....................... 6E-211
DTC P0174 (FLASH CODE 44) O2 SENSOR
SYSTEM TOO LEAN (BANK 2) ....................... 6E-211
DTC P0172 (FLASH CODE 45) O2 SENSOR
SYSTEM TOO RICH (BANK 1) ........................ 6E-216
DTC P0175 (FLASH CODE 45) O2 SENSOR
SYSTEM TOO RICH (BANK 2) ........................ 6E-216
DTC P1171 (FLASH CODE 44) FUEL
SUPPLY SYSTEM LEAN DURING POWER
ENRICHMENT (TYPE A) ................................. 6E-221
DTC P1172 (FLASH CODE 44) FUEL SUPPLY
SYSTEM LEAN DURING POWER
ENRICHMENT (TYPE B) ................................. 6E-221

3.5L ENGINE DRIVEABILITY AND EMISSIONS 6E-3
DTC P0201 (FLASH CODE 31) INJECTOR 1
CONTROL CIRCUIT ........................................ 6E-226
DTC P0202 (FLASH CODE 31) INJECTOR 2
CONTROL CIRCUIT ........................................ 6E-226
DTC P0203 (FLASH CODE 31) INJECTOR 3
CONTROL CIRCUIT ........................................ 6E-226
DTC P0204 (FLASH CODE 31) INJECTOR 4
CONTROL CIRCUIT ........................................ 6E-226
DTC P0205 (FLASH CODE 31) INJECTOR 5
CONTROL CIRCUIT ........................................ 6E-226
DTC P0206 (FLASH CODE 31) INJECTOR 6
CONTROL CIRCUIT ........................................ 6E-226
DTC P0336 (FLASH CODE 29) CRANKSHAFT
POSITION SENSOR CIRCUIT
RANGE/PERFORMANCE (58X) ...................... 6E-234
DTC P0337 (FLASH CODE 29) CRANKSHAFT
POSITION SENSOR CIRCUIT NO SIGNAL
(58X) ................................................................. 6E-234
DTC P0341 (FLASH CODE 41) CAMSHAFT
POSITION SENSOR CIRCUIT
RANGE/PERFORMANCE ............................... 6E-243
DTC P0342 (FLASH CODE 41) CAMSHAFT
POSITION SENSOR CIRCUIT NO SIGNAL .... 6E-243
DTC P0351 (FLASH CODE 42) IGNITION 1
CONTROL CIRCUIT ........................................ 6E-249
DTC P0352 (FLASH CODE 42) IGNITION 2
CONTROL CIRCUIT ........................................ 6E-249
DTC P0353 (FLASH CODE 42) IGNITION 3
CONTROL CIRCUIT ........................................ 6E-249
DTC P0354 (FLASH CODE 42) IGNITION 4
CONTROL CIRCUIT ........................................ 6E-249
DTC P0355 (FLASH CODE 42) IGNITION 5
CONTROL CIRCUIT ........................................ 6E-249
DTC P0356 (FLASH CODE 42) IGNITION 6
CONTROL CIRCUIT ........................................ 6E-249
DTC P0404 (FLASH CODE 32) EGR CIRCUIT
RANGE/PERFORMANCE (OPEN VALVE)....... 6E-258
DTC P1404 (FLASH CODE 32) EGR CIRCUIT
RANGE/PERFORMANCE (CLOSED VALVE) .. 6E-258
DTC P0405 (FLASH CODE 32) EGR
CIRCUIT LOW.................................................. 6E-263
DTC P0406 (FLASH CODE 32) EGR
CIRCUIT HIGH ................................................. 6E-268
DTC P0444 EVAP PURGE SOLENOID
VALVE CIRCUIT LOW VOLTAGE ................... 6E-274
DTC P0445 EVAP PURGE SOLENOID
VALVE CIRCUIT HIGH VOLTAGE .................. 6E-274
DTC P0500 (FLASH CODE 24) VEHICLE
SPEED SENSOR (VSS) CIRCUIT
RANGE/PERFORMANCE ................................ 6E-279 DTC P0562 (FLASH CODE 66) SYSTEM
VOLTAGE LOW .............................................. 6E-288
DTC P0563 (FLASH CODE 66) SYSTEM
VOLTAGE HIGH .............................................. 6E-321
DTC P0601 (FLASH CODE 51) ENGINE
CONTROL MODULE (ECM) MEMORY
CHECKSUM ..................................................... 6E-293
DTC P0602 PROGRAMMING ERROR............. 6E-295
DTC P1508 (FLASH CODE 22) IDLE AIR
CONTROL SYSTEM LOW/CLOSED ............... 6E-296
DTC P1509 (FLASH CODE 22) IDLE AIR
CONTROL SYSTEM HIGH/OPEN ................... 6E-296
DTC P1601 (FLASH CODE 65) CAN BUS
OFF .................................................................. 6E-304
DTC U2104 (FLASH CODE 67) CAN BUS
RESET COUNTER OVER-RUN ...................... 6E-310
DTC P1626 IMMOBILIZER NO SIGNAL ........... 6E-317
DTC P1631 IMMOBILIZER WRONG
SIGNAL ............................................................ 6E-323
DTC P1648 IMMOBILIZER WRONG
SECURITY CODE ENTERED .......................... 6E-325
DTC P1649 IMMOBILIZER FUNCTION NOT
PROGRAMMED ................................................ 6E-327
SYMPTOM DIAGNOSIS ................................... 6E-329
PRELIMINARY CHECKS .............................. 6E-329
VISUAL/PHYSICAL CHECK .......................... 6E-329
INTERMITTENT ............................................. 6E-329
ENGINE CRANKS BUT WILL NOT RUN ......... 6E-331
HARD START SYMPTOM ................................ 6E-334
ROUGH, UNSTABLE, OR INCORRECT
IDLE, STALLING SYMPTOM ........................... 6E-337
SURGES AND/OR CHUGS SYMPTOM ........... 6E-341
HESITATION, SAG, STUMBLE SYMPTOM ..... 6E-345
CUTS OUT, MISSES SYMPTOM ..................... 6E-347
LACK OF POWER, SLUGGISH OR SPONGY
SYMPTOM ....................................................... 6E-352
DETONATION/SPARK KNOCK SYMPTOM..... 6E-356
POOR FUEL ECONOMY SYMPTOM ............... 6E-359
EXCESSIVE EXHAUST EMISSIONS OR
ODORS SYMPTOM ......................................... 6E-362
DIESELING, RUN-ON SYMPTOM .................... 6E-365
BACKFIRE SYMPTOM ..................................... 6E-366
ON-VEHICLE SERVICE PROCEDURE ........... 6E-368
ENGINE CONTROL MODULE (ECM) .............. 6E-368
CRANKSHAFT POSITION (CKP) SENSOR ..... 6E-369
CAMSHAFT POSITION (CMP) SENSOR ......... 6E-369
ENGINE COOLANT TEMPERATURE (ECT)
SENSOR .......................................................... 6E-370
MASS AIR FLOW (MAF) SENSOR & INTAKE AIR
TEMPERATURE (IAT) SENSOR ...................... 6E-370

3.5L ENGINE DRIVEABILITY AND EMISSIONS 6E-57
Starting Mode
When the ignition is first turned "ON," the ECM
energizes the fuel pump relay for two seconds to allo
w
the fuel pump to build up pressure. The ECM then
checks the engine coolant temperature (ECT) senso
r
and the throttle position sensor to determine the proper
air/fuel ratio for starting.
The ECM controls the amount of fuel delivered in the
starting mode by adjusting how long the fuel injectors
are energized by pulsing the injectors for very short
times.

Fuel Metering System Components
The fuel metering system is made up of the following
parts.

Fuel injector

Throttle Body

Fuel Rail

Fuel Pressure regulator

ECM

Crankshaft position (CKP) sensor

Camshaft position (CMP) sensor

Idle air control valve

Fuel pump

Fuel Injector
The sequential multi-port fuel injection fuel injector is a
solenoid operated device controlled by the ECM. The
ECM energizes the solenoid, which opens a valve to
allow fuel delivery.
The fuel is injected under pressure in a conical spray
pattern at the opening of the intake valve. Excess fuel
not used by the injectors passes through the fuel
pressure regulator before being returned to the fuel
tank.

Fuel Pressure Regulator
The fuel pressure regulator is a diaphragm-operated
relief valve mounted on the fuel rail with fuel pump
pressure on one side and manifold pressure on the
other side. The fuel pressure regulator maintains the
fuel pressure available to the injector at three times
barometric pressure adjusted for engine load. It may be
serviced separate.
If the pressure is too low, poor performance and a DTC
P0131, P0151, P0171, P0174, P1171 or P1174 will be
the result. If the pressure is too high, excessive odo
r
and/or a DTC P0132, P0152, P0172 or P0175 will be
the result. Refer to Fuel System Diagnosisfo
r
information on diagnosing fuel pressure conditions.
Fuel Rail
The fuel rail is mounted to the top of the engine and
distributes fuel to the individual injectors. Fuel is
delivered to the fuel inlet tube of the fuel rail by the fuel
lines. The fuel goes through the fuel rail to the fuel
pressure regulator. The fuel pressure regulato
r
maintains a constant fuel pressure at the injectors.
Remaining fuel is then returned to the fuel tank.







055RV009
Fuel Pump Electrical Circuit
When the key is first turned "ON," the ECM energizes
the fuel pump relay for two seconds to build up the fuel
pressure quickly. If the engine is not started within two
seconds, the ECM shuts the fuel pump off and waits
until the engine is cranked. When the engine is cranked
and the 58 X crankshaft position signal has been
detected by the ECM, the ECM supplies 12 volts to the
fuel pump relay to energize the electric in-tank fuel
pump.
An inoperative fuel pump will cause a "no-start"
condition. A fuel pump which does not provide enough
pressure will result in poor performance.

Camshaft Position (CMP) Sensor Signal
The ECM uses this signal to determine the position o
f
the number 1 piston during its power stroke, allowing
the ECM to calculate true sequential multiport fuel
injection. Loss of this signal will set a DTC P0341. If the
CMP signal is lost while the engine is running, the fuel
injection system will shift to a calculated sequential fuel
injection based on the last fuel injection pulse, and the
engine will continue to run. The engine can be restarted
and will run in the calculated sequential mode as long
as the fault is present, with a 1-in-6 chance of being
correct.

6E-58 3.5L ENGINE DRIVEABILITY AND EMISSIONS
GENERAL DESCRIPTION FOR
ELECTRONIC IGNITION SYSTEM IGNITION
COILS & CONTROL
A separate coil-at-plug module is located at each spark
plug.
The coil-at-plug module is attached to the engine with
two screws. It is installed directly to the spark plug by an
electrical contact inside a rubber boot.
A three way connector provides 12 volts primary supply
from the ignition coil fuse, a ground switching trigge
r
line from the ECM, and ground.
The ignition control spark timing is the ECM's method o
f
controlling the spark advance and the ignition dwell.
The ignition control spark advance and the ignition dwell
are calculated by the ECM using the following inputs.

Engine speed

Crankshaft position (CKP) sensor

Camshaft position (CMP) sensor

Engine coolant temperature (ECT) sensor

Throttle position sensor

Park or neutral position switch

Vehicle speed sensor

ECM and ignition system supply voltage

Based on these sensor signal and engine load
information, the ECM sends 5V to each ignition coil
requiring ignition. This signal sets in the powe
r
transistor of the ignition coil to establish a grounding
circuit for the primary coil, applying battery voltage to
the primary coil.
At the ignition timing, the ECM stops sending the 5V
signal voltage. Under this condition the power transistor
of the ignition coil is set off to cut the battery voltage to
the primary coil, thereby causing a magnetic field
generated in the primary coil to collapse.
On this moment a line of magnetic force flows to the
secondary coil, and when this magnetic line crosses the
coil, high voltage induced by the secondary ignition
circuit to flow through the spark plug to the ground.

Ignition Control ECM Output
The ECM provides a zero volt (actually about 100 mV to
200 mV) or a 5-volt output signal to the ignition control
(IC) module. Each spark plug has its own primary and
secondary coil module ("coil-at-plug") located at the
spark plug itself. When the ignition coil receives the
5-volt signal from the ECM, it provides a ground path fo
r
the B+ supply to the primary side of the coil-at -plug
module. This energizes the primary coil and creates a
magnetic field in the coil-at-plug module. When the
ECM shuts off the 5-volt signal to the ignition control
module, the ground path for the primary coil is broken.
The magnetic field collapses and induces a high voltage
secondary impulse which fires the spark plug and
ignites the air/fuel mixture.
The circuit between the ECM and the ignition coil is
monitored for open circuits, shorts to voltage, and
shorts to ground. If the ECM detects one of these
events, it will set one of the following DTCs:

P0351: Ignition coil Fault on Cylinder #1

P0352: Ignition coil Fault on Cylinder #2

P0353: Ignition coil Fault on Cylinder #3

P0354: Ignition coil Fault on Cylinder #4

P0355: Ignition coil Fault on Cylinder #5

P0356: Ignition coil Fault on Cylinder #6

Spark Plug
Although worn or dirty spark plugs may give satisfactory
operation at idling speed, they frequency fail at highe
r
engine speeds. Faulty spark plugs may cause poor fuel
economy, power loss, loss of speed, hard starting and
generally poor engine performance. Follow the
scheduled maintenance service recommendations to
ensure satisfactory spark plug performance. Refer to
Maintenance and Lubrication.
Normal spark plug operation will result in brown to
grayish-tan deposits appearing on the insulator portion
of the spark plug. A small amount of red-brown, yellow,
and white powdery material may also be present on the
insulator tip around the center electrode. These
deposits are normal combustion by-products of fuels
and lubricating oils with additives. Some electrode wea
r
will also occur. Engines which are not running properly
are often referred to as “misfiring." This means the
ignition spark is not igniting the air/fuel mixture at the
proper time.
Spark plugs may also misfire due to fouling, excessive
gap, or a cracked or broken insulator. If misfiring
occurs before the recommended replacement interval,
locate and correct the cause.

6E-60 3.5L ENGINE DRIVEABILITY AND EMISSIONS



TS2394
A broken or cracked lower insulator tip (around the
center electrode) may result from “heat shock" (spark
plug suddenly operating too hot).



TS23993

"Heat shock" breakage in the lower insulator tip
generally occurs during several engine operating
conditions (high speeds or heavy loading) and may
be caused by over-advanced timing or low grade
fuels. Heat shock refers to a rapid increase in the
tip temperature that causes the insulator material
to crack.
If there is any doubt about the serviceability of a spark
plug, replace it. Spark plugs with cracked or broken
insulators should always be replaced.

3.5L ENGINE DRIVEABILITY AND EMISSIONS 6E-63
ISUZU STRATEGY BASED DIAGNOSTICS
Overview
As a retail service technician, you are part of the ISUZU
service team. The team goal is FIX IT RIGHT THE
FIRST TIME for the satisfaction of every customer. You
are a very important member of the team as you
diagnose and repair customer vehicles. You have maximum efficiency in diagnosis when you
have an effective, organized plan for your work.
Strategy Based Diagnostics (refer to Figure 1) provides
you with guidance as you create and follow a plan of
action for each specific diagnostic situation.


STRATEGY BASED DIAGNOSTICS CHART

6E-64 3.5L ENGINE DRIVEABILITY AND EMISSIONS
Diagnostic Thought Process
As you follow a diagnostic plan, every box on the
Strategy Based Diagnostics chart requires you to use
the diagnostic thought process. This method of thinking
optimizes your diagnosis in the following ways:

Improves your understanding and definition of the
customer complaint

Saves time by avoiding testing and/or replacing
good parts


Allows you to look at the problem from different
perspectives

Guides you to determine what level o
f
understanding about system operation is needed:
 Owner’s manual level
 Service manual level
 In-depth (engineering) level
1. Verify the Complaint
What you should do
To verify the customer complaint, you need to know the
correct (normal) operating behavior of the system and
verify that the customer complaint is a valid failure o
f
the system.
The following information will help you verify the
complaint:

WHAT the vehicle model/options are

WHAT aftermarket and dealer-installed
accessories exist

WHAT related system(s) operate properly

WHEN the problem occurs

WHERE the problem occurs

HOW the problem occurs

HOW LONG the condition has existed (and if the
system ever worked correctly)

HOW OFTEN the problem occurs

Whether the severity of the problem has
increased, decreased or stayed the same
What resources you should use
Whenever possible, you should use the following
resources to assist you in verifying the complaint:
Service manual Theory or Circuit Description
sections

Service manual “System Performance Check”

Owner manual operational description

Technician experience

Identical vehicle for comparison


Circuit testing tools

Vehicle road tests

Complaint check sheet

Contact with the customer
2. Perform Preliminary Checks
NOTE: An estimated 10 percent of successful
vehicle repairs are diagnosed with this step!
What you should do
You perform preliminary checks for several reasons:

To detect if the cause of the complaint is
VISUALLY OBVIOUS

To identify parts of the system that work correctly

To accumulate enough data to correctly and
accurately search for a ISUZU Service Bulletin.
The initial checks may vary depending on the
complexity of the system and may include the following
actions:

Operate the suspect system

Make a visual inspection of harness routing and
accessible/visible power and ground circuits

Check for blown fuses

Make a visual inspection for separated connectors

Make a visual inspection of connectors (includes
checking terminals for damage and tightness)

Check for any DTCs stored by the on-board
computers

Sense unusual noises, smells, vibrations o
r
movements

Investigate the vehicle service history (call othe
r
dealerships, if appropriate)
What resources you should use
Whenever appropriate, you should use the following
resources for assistance in performing preliminary
checks:

Tech 2 or other technical equipment for viewing
DTCs

Service manual information:
 Component locations
 Harness routing
 Wiring schematics
 Procedures for viewing DTCs

Dealership service history file

Vehicle road test

Identical vehicle or system for comparison

3.5L ENGINE DRIVEABILITY AND EMISSIONS 6E-65
3. Check Bulletins and
Troubleshooting Hints
NOTE: As estimated 30 percent of successful
vehicle repairs are diagnosed with this step!
What you should do
You should have enough information gained from
preliminary checks to accurately search for a bulletin
and other related service information. Some service
manual sections provide troubleshooting hints that
match symptoms with specific complaints.
What resources you should use
You should use the following resources for assistance
in checking for bulletins and troubleshooting hints:

Printed bulletins

Access ISUZU Bulletin Web site

Videotapes

Service manual
4. Perform Service Manual
Diagnostic Checks
What you should do
The “System Checks” in most service manual sections
and in most cells of section 8A (electrical) provide you
with:


A systematic approach to narrowing down the
possible causes of a system fault

Direction to specific diagnostic procedures in the
service manual

Assistance to identify what systems work correctly
What resources you should use
Whenever possible, you should use the following
resources to perform service manual checks:

Service manual

Technical equipment (for viewing DTCs and
analyzing data)

Digital multimeter and circuit testing tools

Other tools as needed
5a and 5b. Perform Service Manual
Diagnostic Procedures
NOTE: An estimated 40 percent of successful
vehicle repairs are diagnosed with these steps!

What you should do
When directed by service manual diagnostic checks,
you must then carefully and accurately perform the
steps of diagnostic procedures to locate the fault related
to the customer complaint.
What resources you should use
Whenever appropriate, you should use the following
resources to perform service manual diagnostic
procedures:

Service manual

Technical equipment (for analyzing diagnostic
data)

Digital multimeter and circuit testing tools

Essential and special tools
5c. Technician Self Diagnoses
When there is no DTC stored and no matching
symptom for the condition identified in the service
manual, you must begin with a thorough understanding
of how the system(s) operates. Efficient use of the
service manual combined with you experience and a
good process of elimination will result in accurate
diagnosis of the condition.
What you should do
Step 1: Identify and understand the suspect
circuit(s)
Having completed steps 1 through 4 of the Strategy
Based Diagnostics chart, you should have enough
information to identify the system(s) or sub-system(s)
involved. Using the service manual, you should
determine and investigate the following circuit
characteristics:

Electrical:
 How is the circuit powered (power distribution
charts and/or fuse block details)?
 How is the circuit grounded (ground distribution
charts)?
 How is the circuit controlled or sensed (theory
of operation):
 If it is a switched circuit, is it normally open o
r
normally closed?
 Is the power switched or is the ground
switched?
 Is it a variable resistance circuit (ECT senso
r
or TPS, for example)?
 Is it a signal generating device (MAF senso
r
of VSS, for example)?

6E-66 3.5L ENGINE DRIVEABILITY AND EMISSIONS
 Does it rely on some mechanical/vacuum
device to operate?

Physical:
 Where are the circuit components (componen
t
locators and wire harness routing diagrams):

Are there areas where wires could be
chafed or pinched (brackets or frames)?

Are there areas subjected to extreme
temperatures?

Are there areas subjected to vibration or
movement (engine, transmission or
suspension)?

Are there areas exposed to moisture, road
salt or other corrosives (battery acid, oil o
r
other fluids)?

Are there common mounting areas with
other systems/components?
 Have previous repairs been performed to
wiring, connectors, components or mounting
areas (causing pinched wires between panels
and drivetrain or suspension components
without causing and immediate problem)?
 Does the vehicle have aftermarket or dealer-
installed equipment (radios, telephone, etc.)

Step 2: Isolate the problem
At this point, you should have a good idea of what could
cause the present condition, as well as could not cause
the condition. Actions to take include the following:

Divide (and separate, where possible) the system
or circuit into smaller sections

Confine the problem to a smaller area of the
vehicle (start with main harness connections while
removing panels and trim as necessary in order to
eliminate large vehicle sections from furthe
r
investigation)

For two or more circuits that do not share a
common power or ground, concentrate on areas
where harnesses are routed together o
r
connectors are shared (refer to the following hints)

Hints
Though the symptoms may vary, basic electrical failures
are generally caused by:

Loose connections:
 Open/high resistance in terminals, splices,
connectors or grounds

Incorrect connector/harness routing (usually in
new vehicles or after a repair has been made):

 Open/high resistance in terminals, splices,
connectors of grounds

Corrosion and wire damage:
 Open/high resistance in terminals, splices,
connectors of grounds

Component failure:
 Opens/short and high resistance in relays,
modules, switches or loads


Aftermarket equipment affecting normal operation
of other systems You may isolate circuits by:

Unplugging connectors or removing a fuse to
separate one part of the circuit from another part

Operating shared circuits and eliminating those
that function normally from the suspect circuit

If only one component fails to operate, begin
testing at the component

If a number of components do no operate, begin
tests at the area of commonality (such as powe
r
sources, ground circuits, switches or majo
r
connectors)
What resources you should use
Whenever appropriate, you should use the following
resources to assist in the diagnostic process:

Service manual

Technical equipment (for data analysis)

Experience

Technical Assistance

Circuit testing tools
5d. Intermittent Diagnosis
By definition, an intermittent problem is one that does
not occur continuously and will occur when certain
conditions are met. All these conditions, however, may
not be obvious or currently known. Generally,
intermittents are caused by:

Faulty electrical connections and wiring

Malfunctioning components (such as sticking
relays, solenoids, etc.)

EMI/RFI (Electromagnetic/radio frequency
interference)

Aftermarket equipment
Intermittent diagnosis requires careful analysis of
suspected systems to help prevent replacing good
parts. This may involve using creativity and ingenuity to
interpret customer complaints and simulating all
external and internal system conditions to duplicate the
problem.