Isuzu D-Max / Isuzu Rodeo (TFR/TFS). Manual - part 112

6E–52

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. Next, 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 next 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)

• Experience

• 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.
CAUTION: 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.
CAUTION: 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 expectations 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

• Examining 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 expectation and system
operation, you should explain 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:

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6E–53

• Vehicle service information (service manual, etc.)

• ISUZU field support

• Experience

• Identical vehicle or system for comparison

6. Re-examine the complaint

When you do not successfully find/isolate the problem
after executing a diagnostic path, you should re-
examine the complaint.

What you should do

In this case, you will need to backtrack and review
information accumulated from step 1 through 4 of
Strategy Based Diagnostics. You also should repeat any
procedures that require additional attention.
A previous path may be eliminated from consideration
only if you are certain that all steps were executed as
directed. You must then select another diagnostic path
(step 5a, 5b, 5c or 5d). If all possible options have been
explored, you may call or seek ISUZU field support.

What resources you should use

Whenever possible, you should use the following
resources to facilitate the diagnostic process:

• Service manual

• Accumulated information form a previous diagnostic

path

• Service information and publications

• ISUZU field support

7. Repair and Verify Fix

What you should do

After you have located the cause of the problem, you
must execute a repair by following recommended
service manual procedures.
When the repair is completed, you should verify the fix
by performing the system checks under the conditions
listed in the customer complaint.
If applicable, you should carry out preventive measures
to avoid a repeat complaint.

What resources you should use

Whenever possible, you should use the following
resources to facilitate the repair process:

• Electrical repair procedures

• Service manual information and publications

6E–54

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 executive.
When a diagnostic test reports a pass result, the
diagnostic executive 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 executive 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 Executive are listed as
follows:

• 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

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6E–55

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 expose 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 Tech2.  Some common uses of the
Tech2 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 Tech2.  When reading DTC(s),
follow instructions supplied by Tech2 manufacturer. 
For the 1998 model year, Isuzu dealer service
departments will continue to use Tech2.

Clearing Diagnostic Trouble Codes

To clear Diagnostic Trouble Codes (DTCs), use the
Tech2  “clear DTCs” or “clear information” function.
When clearing DTCs follow instructions supplied by the
Tech2 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.