power ISUZU TF SERIES 2004 Workshop Manual

6D – 14 ENGINE ELECTRICAL
RECTIFIER ASSEMBLY



RTW46DSF000301

1. Voltmeter
2. Switch 1
3. DC regulated power supply
4. Lamp 2
5. Lamp 1
6. Switch 3
7. Switch 2
8. Pulse generator
9. Output signal

Test circuit
Refer to the judgment criteria shown in the Table below.
Carefully check Items 1~5. If all the items are OK, the IC
regulator is normal.

Circuit components
1 DC regulated power supply 0~20 volts variable with output of 1 ampere or more
2 Lamps (2) 12 volts, 1.4~3.4 watts
3 Switches (3) -----
4 DC voltmeter 0~30 volts, 0.5 grade
5 Pulse generator (Oscillator) 5~30 volt output at a frequency of 1kHz

Judgment criteria
Lamp condition
No. Switch
1 Switch
2 Switch
3 Voltmeter
reading
Lamp 1 Lamp 2Remarks
1 ON OFF OFF On
(dim) ON Initial excitation check
2 ON ON OFF 12V
On or
flashingFull excitation check
3 ON ON OFF 16V Off or
on (dim)OFF
Lamp 1 off or dimly lit when the
voltmeter shows less than 12 volts or
16 volts
4 OFF ON OFF 12V On or
flashingStator and brush separation check
5 ON ON ON 18V On ON
Excess voltage check

6E–8 4JA1/4JH1 ENGINE DRIVEABILITY AND EMISSIONS
(SYMPTOM CODE B) (FLASH CODE 76) ECM
MAIN RELAY SWITCHED OFF TOO LATE 6E-337
Circuit Description ..................................... 6E-337
Diagnostic Aids .......................................... 6E-337
Diagnostic Trouble Code (DTC) P1625
(Symptom Code A) (Flash Code 76) ECM
Main Relay Switched Off Too Early ......... 6E-338
Diagnostic Trouble Code (DTC) P1625
(Symptom Code B) (Flash Code 76) ECM
Main Relay Switched Off Too Late .......... 6E-339
DIAGNOSTIC TROUBLE CODE (DTC) P1630
(SYMPTOM CODE A) (FLASH CODE 51)
FUEL INJECTION QUANTITY CIRCUIT
MALFUNCTION ....................................... 6E-340
DIAGNOSTIC TROUBLE CODE (DTC) P1630
(SYMPTOM CODE B) (FLASH CODE 51)
FUEL INJECTION QUANTITY CIRCUIT
MALFUNCTION ....................................... 6E-340
Circuit Description ..................................... 6E-340
Diagnostic Aids .......................................... 6E-341
Diagnostic Trouble Code (DTC) P1630
(Symptom Code A) (Flash Code 51) Fuel
Injection Quantity Circuit Malfunction ...... 6E-341
Diagnostic Trouble Code (DTC) P1630
(Symptom Code B) (Flash Code 51) Fuel
Injection Quantity Circuit Malfunction ...... 6E-341
DIAGNOSTIC TROUBLE CODE (DTC) P1650
(SYMPTOM CODE A)
(FLASH CODE 44) CAN DEVICE OFFLINE 6E-342
DIAGNOSTIC TROUBLE CODE (DTC) P1650
(SYMPTOM CODE B) (FLASH CODE 44)
CAN DEVICE HANG-UP ......................... 6E-342
Circuit Description ..................................... 6E-342
Diagnostic Aids .......................................... 6E-342
Diagnostic Trouble Code (DTC) P1650
(Symptom Code A) (Flash Code 44)
CAN Device Offline .................................. 6E-343
Diagnostic Trouble Code (DTC) P1650
(Symptom Code B) (Flash Code 44)
CAN Device Hang-up .............................. 6E-348
DIAGNOSTIC TROUBLE CODE (DTC) P1651
(SYMPTOM CODE A) (FLASH CODE 45)
CAN MALFUNCTION .............................. 6E-349
DIAGNOSTIC TROUBLE CODE (DTC) P1651
(SYMPTOM CODE B) (FLASH CODE 45)
CAN RECEIVES ERROR ........................ 6E-349
Circuit Description ..................................... 6E-349
Diagnostic Aids .......................................... 6E-349
Diagnostic Trouble Code (DTC) P1651
(Symptom Code A) (Flash Code 45)
CAN Malfunction ...................................... 6E-350
Diagnostic Trouble Code (DTC) P1651
(Symptom Code B) (Flash Code 45) CAN Receives Error ................................ 6E-351
DIAGNOSTIC TROUBLE CODE (DTC) P1690
(SYMPTOM CODE 4) (FLASH CODE 77)
CHECK ENGINE LAMP (MIL) CIRCUIT
VOLTAGE LOW ...................................... 6E-356
DIAGNOSTIC TROUBLE CODE (DTC) P1690
(SYMPTOM CODE 8) (FLASH CODE 77)
CHECK ENGINE LAMP (MIL) CIRCUIT
VOLTAGE HIGH ...................................... 6E-356
Circuit Description ...................................... 6E-356
Diagnostic Aids .......................................... 6E-356
Diagnostic Trouble Code (DTC) P1690
(Symptom Code 4) (Flash Code 77) Check
Engine Lamp (MIL) Circuit Voltage Low .. 6E-357
Diagnostic Trouble Code (DTC) P1690
(Symptom Code 8) (Flash Code 77) Check
Engine Lamp (MIL) Circuit Voltage High . 6E-360
SYMPTOM DIAGNOSIS ............................ 6E-361
PRELIMINARY CHECKS .......................... 6E-361
VISUAL/PHYSICAL CHECK ...................... 6E-361
INTERMITTENT ........................................ 6E-361
ENGINE CRANKS BUT WILL NOT RUN .. 6E-362
HARD START SYMPTOM ......................... 6E-366
ROUGH, UNSTABLE, OR INCORRECT
IDLE, STALLING SYMPTOM .................. 6E-370
SURGES AND/OR CHUGS SYMPTOM ... 6E-377
HESITATION, SAG, STUMBLE SYMPTOM 6E-383
CUTS OUT, MISSES SYMPTOM .............. 6E-389
LACK OF POWER, SLUGGISH OR
SPONGY SYMPTOM .............................. 6E-396
POOR FUEL ECONOMY SYMPTOM ....... 6E-402
EXCESSIVE WHITE SMOKE .................... 6E-407
EXCESSIVE BLACK SMOKE .................... 6E-413
ON-VEHICLE SERVICE PROCEDURE .... 6E-418
ENGINE CONTROL MODULE (ECM) ... 6E-418
CRANKSHAFT POSITION (CKP) SENSOR 6E-419
ENGINE COOLANT TEMPERATURE
(ECT) SENSOR ....................................... 6E-419
MASS AIR FLOW (MAF) & INTAKE AIR
TEMPERATURE (IAT) SENSOR ............ 6E-420
THROTTLE POSITION SENSOR (TPS) ... 6E-420
EGR EVRV
(Electrical Vacuum Regulating Valve) ..... 6E-421

4JA1/4JH1 ENGINE DRIVEABILITY AND EMISSIONS 6E–45
FUSE A ND RELAY LOCATION (LHD & RHD)
FUSE
SLOW BLOW FUSE
RELAYNo. Capacity Indication on label No. Capacity Indication on label
1——12 15A CIGER
2 10A ABS 13 15A AUDIO (+B)
3——14 20A DOOR LOCK
4 15A BACK UP 15 10A METER (+B)
5 15A METER 16 10A ROOM
6 10A TURN 17 10A ANTI THEFT
7 15A ELEC.IG 18 15A STOP
8 15A ENGINE 19 15A ACC SOCKET
9 20A FRT WIPER 20 10A STARTER
10 15A EGR 21 10A SRS
11 10A AUDIO
No. Capacity Indication on label
SBF-10 20A RR DEF
SBF-11 30A POWER WINDOW
Connector No. B-7 B-8 B-40
4JA1-TC, 4JH1-TC REAR
DEFOGGERPOWER
WINDOWACC
SOCKET
FUSE BOX

6E–54 4JA1/4JH1 ENGINE DRIVEABILITY AND EMISSIONS
41 41 A/C Compresso r
Re la yWH T/
GRNLess than
1V10-14V A/C comp. is operated:
L e s s t h a n 1 V
A/C comp. is not oper-
ated: 10 - 14VCo nnect DC V 41 GND
42 42 Che ck Engine La mp GRN/
YELLess than
1VLamp is turned on: Less than 1V
Lamp is turned off: 10-14VCo nnect DC V 42 GND
43 43 Glow La mp ORG/
BLULess than
1VLamp is turned on: Less than 1V
Lamp is turned off: 10-14VCo nnect DC V 43 GND
44 44 No Conne ctio n - - - - - - - - -
45 45 To Da ta Link Co nnec-
to r No . 6BLU Less than
1V10-14V Co nnect DC V 45 GND
46 46 No Conne ctio n - - - - - - - - -
47 47 No Conne ctio n - - - - - - - - -
48 48 No Conne ctio n - - - - - - - - -
49 49 Throttle Po sitio n Sen-
so r (TPS) Gro undBLK/
GRNIdle :
Approx.
0.4K ohm /
WOT:
Approx.
4.0K ohm---Discon-
ne ctOhm 38 49
50 50 No Conne ctio n - - - - - - - - -
51 51 No Conne ctio n - - - - - - - - -
52 52 No Conne ctio n - - - - - - - - -
53 53 No Conne ctio n - - - - - - - - -
54 54 No Conne ctio n - - - - - - - - -
55 55 No Conne ctio n - - - - - - - - -
56 56 No Conne ctio n - - - - - - - - -
57 57 Throttle Po sitio n Sen-
sor (TPS) Po wer Sup-
plyRED/
GRNLess than
1VAppro x . 5V Co nnect DC V 57 49
58 58 ECM Re lay BLU/
BLK10-14V Less than 1V Connect DC V 58 GND
59 59 No Conne ctio n - - - - - - - - -
60 60 No Conne ctio n - - - - - - - - -
61 61 No Conne ctio n - - - - - - - - -
62 62 No Conne ctio n - - - - - - - - -
63 63 ECM Power Supply Less than
1V10-14V - DC V 63 GND
64 64 No Conne ctio n - - - - - - - - -
65 65 Bra ke Switch 2 Signa l WHT/
BLKLess than
1VPedal is not stepped on: 10-14V
Pedal is stepped on: Less than 1VCo nnect DC V 65 GND
66 66 No Conne ctio n - - - - - - - - -
67 67 No Conne ctio n - - - - - - - - -
68 68 Vehicle Spe ed Se n-
so r (VSS)YEL/
GRN- Approx . 14.5Hz by wav e fo rm or
approx. 6.0V at vehicle speed 20km/hCo nnect AC V 68 GND
69 69 Idle Switch GRN/
BLKLess than
1VPedal is not stepped on: Less than 1V
Pedal is stepped on: Approx. 5VCo nnect DC V 69 GND
70 70 No Conne ctio n - - - - - - - - -
71 71 No Conne ctio n - - - - - - - - -
72 72 No Conne ctio n - - - - - - - - -
73 73 No Conne ctio n - - - - - - - - -
74 74 No Conne ctio n - - - - - - - - -Signal or Continuity Tester Position
Pin
No. B/Box
No.Pin Function Wire
ColorKey SW
OffKey SW
OnEngine
IdleEngine
2000rpmECM
Connec-
tionRange (+) (-)

4JA1/4JH1 ENGINE DRIVEABILITY AND EMISSIONS 6E–77
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 of understanding
about system operation is needed:
–Owner’s manual level
–Service manual level
–In-depth (engineering) level
–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 of 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 ex isted (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 ex perience
Identical vehicle for comparisonCircuit 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 on
ISUZU Web site.
The initial checks may vary depending on the
complex ity 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 or
mov ements
Investigate the vehicle service history (call other
dealerships, if appropriate)
What resources you should use
Whenever appropriate, you should use the following
resources for assistance in performing preliminary
checks:
Tech II 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

6E–78 4JA1/4JH1 ENGINE DRIVEABILITY AND EMISSIONS
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 relatedto 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 ex perience 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 or
normally closed?
–Is the power switched or is the ground
switched?
–Is it a variable resistance circuit (ECT sensor
or TP sensor, for ex ample)?
–Is it a signal generating device (MAF sensor of
VSS, for example)?
–Does it rely on some mechanical/vacuum
device to operate?
Physical:
–Where are the circuit components (component
locators and wire harness routing diagrams):
–Are there areas where wires could be chafed
or pinched (brackets or frames)?
–Are there areas subjected to ex treme
temperatures?

4JA1/4JH1 ENGINE DRIVEABILITY AND EMISSIONS 6E–79
–Are there areas subjected to vibration or
movement (engine, transmission or
suspension)?
–Are there areas ex posed to moisture, road salt
or other corrosives (battery acid, oil or 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 further investigation)
For two or more circuits that do not share a common
power or ground, concentrate on areas where
harnesses are routed together or 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 testingat the component
If a number of components do no operate, begin tests
at the area of commonality (such as power sources,
ground circuits, switches or major 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)
Ex perience
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
ex ternal and internal system conditions to duplicate the
problem.
What you should do
Step 1: A cquire information
A thorough and comprehensive customer check sheet
is critical to intermittent problem diagnosis. You should
require this, since it will dictate the diagnostic starting
point. The vehicle service history file is another
source for accumulating information about the
complaint.
Step 2: A nalyze the intermittent problem
Analyze the customer check sheet and service history
file to determine conditions relevant to the suspect
system(s).
Using service manual information, you must identify,
trace and locate all electrical circuits related to the
malfunctioning system(s). If there is more than one
system failure, you should identify, trace and locate
areas of commonality shared by the suspect circuits.

6E–80 4JA1/4JH1 ENGINE DRIVEABILITY AND EMISSIONS
Step 3: Simulate the symptom and isolate the
problem
Simulate the symptom and isolate the system by
reproducing all possible conditions suggested in Step 1
while monitoring suspected circuits/components/
systems to isolate the problem symptom. Begin with the
most logical circuit/component.
Isolate the circuit by dividing the suspect system into
simpler circuits. Nex t, confine the problem into a smaller
area of the system. Begin at the most logical point (or
point of easiest access) and thoroughly check the
isolated circuit for the fault, using basic circuit tests.
Hints
You can isolate a circuit by:
Unplugging connectors or removing a fuse to
separate one part of the circuit from another
If only component fails to operate, begin testing the
component
If a number of components do not operate, begin test
at areas of commonality (such as power sources,
ground circuits, switches, main connectors or major
components)
Substitute a known good part from the parts
department or the vehicle system
Try the suspect part in a known good vehicle
See Symptom Simulation Tests on the nex t page for
problem simulation procedures. Refer to service manual
sections 6E and 8A for information about intermittent
diagnosis. Follow procedures for basic circuit testing in
service manual section 8A.
What resources you should use
Whenever appropriate, you should use the following
resources to assist in the diagnostic process:
Service manual
Bulletins
Digital multimeter (with a MIN/MAX feature)
Tech II and Tech II upload function
Circuit testing tools (including connector kits/
harnesses and jumper wires)
Ex perience
Intermittent problem solving simulation methods
Customer complaint check sheet
Symptom Simulation Tests
1. Vibration
This method is useful when the customer complaint
analysis indicates that the problem occurs when the
vehicle/system undergoes some form of vibration.
For connectors and wire harness, slightly shake
vertically and horizontally. Inspect the connector joint
and body for damage. Also, tapping lightly along a
suspected circuit may be helpful.For parts and sensors, apply slight vibration to the part
with a light tap of the finger while monitoring the system
for a malfunction.
2. Heat
This method is important when the complaint suggests
that the problem occurs in a heated environment. Apply
moderate heat to the component with a hair drier or
similar tool while monitoring the system for a
malfunction.
CA UTION: Care must be take to avoid overheating
the component.
3. Water and Moisture
This method may be used when the complaint suggests
that the malfunction occurs on a rainy day or under
conditions of high humidity. In this case, apply water in a
light spray on the vehicle to duplicate the problem.
CA UTION: Care must be take to avoid directly
exposing electrical connections to water.
4. Electrical loads
This method involves turning systems ON (such as the
blower, lights or rear window defogger) to create a load
on the vehicle electrical system at the same time you
are monitoring the suspect circuit/component.
5e. Vehicle Operates as Designed
This condition refers to instances where a system
operating as designed is perceived to be unsatisfactory
or undesirable. In general, this is due to:
A lack of understanding by the customer
A conflict between customer ex pectations and
vehicle design intent
A system performance that is unacceptable to the
customer
What you should do
You can verify that a system is operating as designed
by:
Reviewing service manual functional/diagnostic
checks
Ex amining bulletins and other service information for
supplementary information
Compare system operation to an identical vehicle
If the condition is due to a customer misunderstanding
or a conflict between customer ex pectation and system
operation, you should ex plain the system operation to
the customer.
If the complaint is due to a case of unsatisfactory
system performance, you should contact Technical
Assistance for the latest information.
What resources you should use
Whenever possible, you should use the following
resources to facilitate the diagnostic process:

6E–82 4JA1/4JH1 ENGINE DRIVEABILITY AND EMISSIONS
GENERAL SERVICE INFORMATION
Serviceability Issues
Non-OEM Parts
All of the OBD diagnostics have been calibrated to run
with OEM parts. Accordingly, if commercially sold
sensor or switch is installed, it makes a wrong diagnosis
and turns on the check engine lamp.
Aftermarket electronics, such as cellular phones,
stereos, and anti-theft devices, may radiate EMI into the
control system if they are improperly installed. This may
cause a false sensor reading and turn on the check
engine lamp.
Poor Vehicle Maintenance
The sensitivity of OBD diagnostics will cause the check
engine lamp to turn on if the vehicle is not maintained
properly. Restricted oil filters, fuel filters, and crankcase
deposits due to lack of oil changes or improper oil
viscosity can trigger actual vehicle faults that were not
previously monitored prior to OBD. Poor vehicle
maintenance can not be classified as a “non-vehicle
fault”, but with the sensitivity of OBD diagnostics,
vehicle maintenance schedules must be more closely
followed.
Related System Faults
Many of the OBD system diagnostics will not run if the
ECM detects a fault on a related system or component.
Visual/Physical Engine Compartment
Inspection
Perform a careful visual and physical engine
compartment inspection when performing any
diagnostic procedure or diagnosing the cause of an
emission test failure. This can often lead to repairing a
problem without further steps. Use the following
guidelines when performing a visual/physical
inspection:
Inspect all vacuum hoses for punches, cuts,
disconnects, and correct routing.
Inspect hoses that are difficult to see behind other
components.
Inspect all wires in the engine compartment for
proper connections, burned or chafed spots, pinched
wires, contact with sharp edges or contact with hot
exhaust manifolds or pipes.
Basic Knowledge of Tools Required
NOTE: Lack of basic knowledge of this powertrain
when performing diagnostic procedures could result in
an incorrect diagnosis or damage to powertrain
components. Do not attempt to diagnose a powertrain
problem without this basic knowledge.
A basic understanding of hand tools is necessary to
effectively use this section of the Service Manual.
ON-BOARD DIAGNOSTIC (OBD)
On-Board Diagnostic (Self Diagnosis
System) Tests
A diagnostic test is a series of steps, the result of which
is a pass or fail reported to the diagnostic ex ecutive.
When a diagnostic test reports a pass result, the
diagnostic ex ecutive records the following data:
The diagnostic test has been completed since the
last ignition cycle.
The diagnostic test has passed during the current
ignition cycle.
The fault identified by the diagnostic test is not
currently active.
When a diagnostic test reports a fail result, the
diagnostic ex ecutive records the following data:
The diagnostic test has been completed since the
last ignition cycle.
The fault identified by the diagnostic test is currently
active.
The fault has been active during this ignition cycle.
The operating conditions at the time of the failure.
The Diagnostic Executive
The Diagnostic Executive is a unique segment of
software which is designed to coordinate and prioritize
the diagnostic procedures as well as define the protocol
for recording and displaying their results. The main
responsibilities of the Diagnostic Ex ecutive are listed as
follow s:
Commanding the check engine lamp on and off
DTC logging and clearing
Current status information on each diagnostic
Diagnostic Information
The diagnostic charts and functional checks are
designed to locate a faulty circuit or component through
a process of logical decisions. The charts are prepared
with the requirement that the vehicle functioned
correctly at the time of assembly and that there are not
multiple faults present.
There is a continuous self-diagnosis on certain control
functions. This diagnostic capability is complemented
by the diagnostic procedures contained in this manual.
The language of communicating the source of the
malfunction is a system of diagnostic trouble codes.
When a malfunction is detected by the control module, a
diagnostic trouble code is set and the check engine
lamp is illuminated.
Check Engine Lamp
The check engine lamp looks the same as the check
engine lamp you are already familiar with, the “Check
Engine” lamp.
Basically, the check engine lamp is turned on when the

4JA1/4JH1 ENGINE DRIVEABILITY AND EMISSIONS 6E–83
ECM detects a DTC that will impact the vehicle
emissions.
When the check engine lamp remains “ON” while the
engine is running, or when a malfunction is
suspected due to a driveability or emissions problem,
a Powertrain On-Board Diagnostic (OBD) System
Check must be performed. The procedures for these
checks are given in On-Board Diagnostic (OBD)
System Check. These checks will ex pose faults
which may not be detected if other diagnostics are
performed first.
Data Link Connector (DLC)
The provision for communication with the contorl
module is the Data Link Connector (DLC). It is located
behind the lower front instrument panel. The DLC is
used to connect to a Tech 2. Some common uses of the
Tech 2 are listed below:
Identifying stored Diagnostic Trouble Codes (DTCs).
Clearing DTCs.
Reading serial data.
Verifying Vehicle Repair
Verification of vehicle repair will be more
comprehensive for vehicles with OBD system
diagnostic. Following a repair, the technician should
perform the following steps:
1. Review and record the Fail Records for the DTC
which has been diagnosed.
2. Clear DTC(s).
3. Operate the vehicle within conditions noted in the
Fail Records.
4. Monitor the DTC status information for the specific
DTC which has been diagnosed until the diagnostic
test associated with that DTC runs.
Following these steps is very important in verifying
repairs on OBD systems. Failure to follow these steps
could result in unnecessary repairs.
Reading Flash Diagnostic Trouble Codes
The provision for communicating with the Engine
Control Module (ECM) is the Data Link Connector
(DLC). The DLC is located in the front console box . It is
used in the assembly plant to receive information in
checking that the engine is operating properly before it
leaves the plant.
The diagnostic trouble code(s) (DTCs) stored in the
ECM’s memory can be read either through a hand-held
diagnostic scanner plugged into the DLC or by counting
the number of flashes of the check engine lamp when
the diagnostic test terminal of the DLC is grounded. The
DLC terminal “6” (diagnostic request) is pulled “Low”
(grounded) by jumping to DLC terminal “4 or 5”, which is
a ground wire.
This will signal the ECM that you want to “flash” DTC(s),
if any are present. Once terminals “4 or 5” and “6” have
been connected, the ignition switch must be moved to
the “ON” position, with the engine not running.
The check engine lamp will indicate a DTC three times if
a DTC is present. If more than one DTC has been
stored in the ECM’s memory, the DTC(s) will be output
from the lowest to the highest, with each DTC being
displayed three times.
The DTC display will continue as long as the DLC is
shorted.
Reading Diagnostic Trouble Codes Using a
Tech 2
The procedure for reading diagnostic trouble code(s) is
to used a diagnostic Tech 2. When reading DTC(s),
follow instructions supplied by Tech 2 manufacturer.
For the 1998 model year, Isuzu dealer service
departments will continue to use Tech 2.
Clearing Diagnostic Trouble Codes
To clear Diagnostic Trouble Codes (DTCs), use the
Te c h 2 “clear DTCs” or “clear information” function.
When clearing DTCs follow instructions supplied by the
Tech 2 manufacturer.
History DTC
History DTC can be candeled after 40 cycle driving with
no defect. Or history code can be deleted by Tech 2
“Clear DTCs” function.