Isuzu D-Max / Isuzu Rodeo (TFR/TFS). Manual - part 15

4JA1-TC/4JH1-TC ENGINE DRIVEABILITY AND EMISSIONS

6E–55

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 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 on
ISUZU Web site.

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 or

movements

• 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

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4JA1-TC/4JH1-TC 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, https://www.einet.isuzu.

co.jp//

• 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 or

normally closed?

– Is the power switched or is the ground

switched?

– Is it a variable resistance circuit (ECT sensor or

TP sensor, for example)?

– 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 extreme

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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 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 testing

at 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)

• 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.

What you should do

Step 1: Acquire 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: Analyze 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.

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4JA1-TC/4JH1-TC 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: