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Isuzu N-Series. Service manual - part 792

Engine Control System (4JH1) 6E-219

Turn OFF the ignition.

Replace the glow relay with the starter relay or
replace with a known good relay.

Turn ON the ignition, with the engine OFF.

Connect a test lamp between the metal bus
bar (glow plug power supply J-122 connector)
and a known good ground.

Perform the Glow Relay test with the Tech 2.

Command the Glow Relay ON with the Tech 2
while observing the test lamp.

Does the test lamp turn ON only when commanded
ON with the Tech 2?

—

Go to Step 9

Go to Step 6

Inspect the Glow (60A) slow blow fuse (SBF-3) in
the fuse & relay box.

Is the Glow (60A) slow blow fuse (SBF-3) open?

—

Go to Step 10

Go to Step 7

Turn OFF the ignition.

Remove the glow relay in the fuse & relay box.

Connect a test lamp between the voltage feed
circuit of the glow relay terminal (pin 3 of X-20
connector) and a known good ground.

Turn ON the ignition, with the engine OFF.

Does the test lamp illuminate?

—

Go to Step 8

Go to Step 11

Turn ON the ignition, with the engine OFF.

Connect a test lamp between the power
supply circuit of glow plugs (pin 2 of X-20
connector) and a known good ground.

Does the test lamp illuminate?

—

Go to Step 13

Go to Step 12

Turn OFF the ignition.

Inspect for an intermittent and for poor
connections at the glow relay terminals (pins 2
and 3 of X-20 connector).

Repair the connection(s) as necessary.

Did you find and correct the condition?

—

Go to Step 16

Go to Step 14

Replace the Glow (60A) slow blow fuse (SBF-
3). If the slow blow fuse continues to open,
repair the short to ground on a circuit fed by
the slow blow fuse or check for a shorted
attached component.

Repair the short to ground or replace the
component as necessary.

Did you complete the repair?

—

Go to Step 16

—

Repair the open circuit in the battery voltage supply
circuit between the Glow (60A) slow blow fuse
(SBF-3) and the glow relay (pin 3 of X-20
connector).

Did you complete the repair?

—

Go to Step 16

—

Repair the open circuit in the switched battery
voltage supply circuit between the glow relay (pin 2
of X-20 connector) and the glow plugs (J-122
terminal).

Did you complete the repair?

—

Go to Step 16

—

Step

Action

Value(s)

Yes

6E-220 Engine Control System (4JH1)

Important:
The glow plugs may be burnt out if the battery
voltage supply circuit is shorted to a voltage
source.

Repair the short to battery or ignition voltage on the
switched battery voltage supply circuit between the
glow relay (pin 2 of X-20 connector) and the glow
plugs (J-122 terminal).

Did you complete the repair?

—

Go to Step 16

—

Replace the glow relay.

Did you complete the replacement?

—

Go to Step 16

—

Replace the appropriate glow plug.

Did you complete the replacement?

—

Go to Step 16

—

Reconnect all previously disconnected
components, relay, fuse or harness
connector(s).

Turn OFF the ignition for 30 seconds.

Turn ON the ignition, with the engine OFF.

Connect a test lamp between the metal bus
bar (glow plug power supply J-122 connector)
and a known good ground.

Perform the Glow Relay test with the Tech 2.

Command the Glow Relay ON with the Tech 2
while observing the test lamp.

Does the test lamp turn ON only when commanded
ON with the Tech 2?

—

Go to Step 4

Go to Step 2

Step

Action

Value(s)

Yes

Engine Control System (4JH1) 6E-221

Symptoms – Engine Controls

Important Preliminary Inspections Before Starting
Perform Diagnostic System Check – Engine Controls
before using the symptom tables, and verify that all of
the following are true:

• The engine control module (ECM) and malfunction

indicator lamp (MIL) are operating correctly.

• There are no diagnostic trouble codes (DTCs)

stored, or a DTC exists but without the MIL.

• The Tech 2 data is within the normal operating

range. Refer to the Tech 2 Data List in this section.

• Verify the customer concern and locate the correct

symptom in the table of contents. Inspect the
items indicated under that symptom.

Visual and Physical Inspection
Several of the symptom procedures ask for careful
visual and physical inspection. This step is extremely
important. The visual and physical inspection can lead
to correcting a problem without further inspections, and
can save valuable time. Ensure that:

• The ECM grounds are clean, tight, and in their

proper location.

• The vacuum hoses are not split or kinked, and

properly connected. Inspect thoroughly for any
type of leak or restriction.

• The mass air flow (MAF) sensor is properly

installed. The arrows on the plastic portion of the
sensor must point toward the engine.

• The air intake ducts are not collapsed or damaged.
• There are no leaks at the MAF sensor, any

connections or intake manifold sealing surfaces.

• The engine harness wiring and terminals are

properly connected and are not pinched or cut.

Intermittent
Important:
Inspect for improper installation of electrical
components if an intermittent condition exists. Inspect
for aftermarket add-on electrical equipment devices,
lights, and cellular phones. Verify that no aftermarket
equipment is connected to the keyword 2000 serial
data circuit. If you cannot locate an intermittent
condition, a cellular phone communication signal may
cause the condition.

Important:
The problem may or may not turn ON the MIL or store a
DTC.

Faulty electrical connections or wiring cause most
intermittent problems. Perform a careful visual and
physical inspection of the suspect connectors for the
following conditions:

• Improperly mated connector halves
• Terminals that are not seated
• Terminals that are damaged or improperly formed

Reform or replace connector terminals in the problem
circuit in order to ensure proper contact tension.
Remove the terminal from the connector body in order
to inspect for poor terminal wire connection.
Road test the vehicle with the DMM connected to the
suspected circuit. An abnormal reading that occurs
when the malfunction occurs is a good indication that
there is a malfunction in the circuit being monitored.
Use the Tech 2 in order to help detect intermittent
conditions. Useful features of the Tech 2 include the
following:

• Trigger the Snapshot feature in order to capture

and store engine parameters when the malfunction
occurs. Review this stored information in order to
see the specific running conditions that caused the
malfunction.

• Use the Plot Function on the Tech 2 in order to plot

selected data parameters. Review this stored
information to aid in locating an intermittent
problem. Refer to the Tech 2 Users Guide for
more information.

Important:
If the intermittent condition exists as a start and then
stall, test for DTCs relating to the vehicle theft deterrent
system. Test for improper installation of electrical
options such as lights, cellular phones, etc.

Any of the following may cause an intermittent MIL with
no stored DTC:

• The ECM grounds are loose or dirty. Refer to

Engine Controls Schematics.

• The MIL circuit intermittently shorted to ground.
• Electrical system interference caused by a

malfunctioning relay, ECM driven solenoid, or
switch. The electrical component can cause a
sharp electrical surge. Normally, the problem will
occur when the malfunctioning component is
operating.

• There is an open diode across the A/C compressor

clutch or any other open diodes.

Important:
The following symptom tables contain groups of
possible causes for each symptom. The order of these
procedures is not important. If the Tech 2 readings do
not indicate the problems, then proceed in a logical
order, easiest to check or most likely to cause first. In
order to determine if a specific vehicle is using a
particular system or component, refer to Engine
Controls Schematics for an application.
Use the following tables when diagnosing a symptom
complaint:

6E-222 Engine Control System (4JH1)

• Intermittent Conditions
• Hard Start
• Rough, Unstable, or Incorrect Idle and Stalling
• Cuts Out, Misses
• Surge/Chuggles
• Lack of Power, Sluggishness, or Sponginess

• Hesitation, Sag, Stumble
• Fuel Knock/Combustion Noise
• Poor Fuel Economy
• Excessive Smoke (Black Smoke)
• Excessive Smoke (White Smoke)

Intermittent Conditions

Checks

Action

DEFINITION: The problem is not currently present but is indicated in DTC History.
OR
There is a customer complaint, but the symptom cannot currently be duplicated, if the problem is not DTC related.

Preliminary Checks

• Refer to Symptoms – Engine Controls before starting.

Harness/Connector

Many intermittent open or shorted circuits are affected by harness/connector
movement that is caused by vibration, engine torque, bumps/rough pavement, etc.
Test for this type of condition by performing the applicable procedure from the following
list:

• Move related connectors and wiring while monitoring the appropriate Tech 2 data.
• Move related connectors and wiring with the component commanded ON, and

OFF, with the Tech 2. Observe the component operation.

• With the engine running, move related connectors and wiring while monitoring

engine operation.

If harness or connector movement affects the data displayed, component/system
operation, or engine operation, inspect and repair the harness/connections as
necessary.
Refer to Electrical Connections or Wiring.

Electrical Connections or Wiring

Poor electrical connections, terminal tension or wiring problems cause most
intermittent. To perform the following inspections:

• Inspect for poor mating of the connector halves, or terminals improperly seated in

the connector body.

• Inspect for improperly formed or damaged terminals. Test for poor terminal tension.
• Inspect for poor terminal to wire connections including terminals crimped over

insulation. This requires removing the terminal from the connector body.

• Inspect for corrosion/water intrusion. Pierced or damaged insulation can allow

moisture to enter the wiring. The conductor can corrode inside the insulation, with
little visible evidence. Look for swollen and stiff sections of wire in the suspect
circuits.

• Inspect for wires that are broken inside the insulation.
• Inspect the harness for pinched, cut or rubbed through wiring.
• Ensure that the wiring does not come in contact with hot exhaust components.

Control Module Power and Grounds
Component Power and Grounds

Poor power or ground connections can cause widely varying symptoms.

• Test all control module power supply circuits. Many vehicles have multiple circuits

supplying power to the control module. Other components in the system may have
separate power supply circuits that may also need to be tested. Inspect
connections at the module/component connectors, fuses, and any intermediate
connections between the power source and the module/component. A test lamp or
a DMM may indicate that voltage is present, but neither tests the ability of the circuit
to carry sufficient current. Ensure that the circuit can carry the current necessary to
operate the component.

• Test all control module ground and system ground circuits. The control module

may have multiple ground circuits. Other components in the system may have
separate grounds that may also need to be tested. Inspect grounds for clean and
tight connections at the grounding point. Inspect the connections at the component
and in splice packs, where applicable. Ensure that the circuit can carry the current
necessary to operate the component.

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