Isuzu N-Series. Manual - part 787

Engine Control System (4JH1) 6E-199

P1605 (Symptom Code C, D, E) (Flash Code 55)

Circuit Description
This diagnostic applies to internal microprocessor
integrity conditions within the engine control module
(ECM). The electronically erasable programmable
read only memory (EEPROM) memorize some data for
engine control and communication with other control
module. If the ECM detects memory area malfunction
inside of the ECM, this DTC will set.

Condition for Setting the DTC

• The ECM detects internal seed or key file in

EEPROM data is destroyed (Symptom Code C).

• The ECM detects reading or writing from the

EEPROM data is failed during initialization
(Symptom Code D).

• The ECM detects total sum of EEPROM data is not

equal to registered value during initialization
(Symptom Code E).

Action Taken When the DTC Sets

• The ECM illuminates the malfunction indicator

lamp (MIL) when the diagnostic runs and fails.

• The ECM uses default EEPROM data substitution

of faulty EEPROM. (Symptom Code D & E).

Condition for Clearing the MIL/DTC

• The ECM turns OFF the MIL when the diagnostic

runs and does not fail.

• A history DTC clears after 40 consecutive driving

cycles without a fault. Or clear with the Tech 2.

DTC P1605 (Symptom Code C, D, E) (Flash Code 55)

Step

Action

Value(s)

Yes

Did you perform the Diagnostic System Check –
Engine Controls?

—

Go to Step 2

Go to Diagnostic

System Check –

Engine Controls

Install the Tech 2.

Turn OFF the ignition for 30 seconds.

Turn ON the ignition, with the engine OFF.

Monitor the Diagnostic Trouble Code (DTC)
Information with the Tech 2.

Does the DTC fail this ignition?

—

Go to Step 3

Go to Step 4

Important:
Replacement ECM must be programmed.

Replace the ECM. Refer to Engine Control Module
(ECM) Replacement in this section.

Did you complete the replacement?

—

Go to Step 4

—

Clear the DTCs with the Tech 2.

Turn OFF the ignition for 30 seconds.

Turn ON the ignition, with the engine OFF.

Monitor the DTC Information with the Tech 2.

Did the DTC fail this ignition?

—

Go to Step 3

Go to Step 5

Observe the DTC Information with the Tech 2.

Are there any DTCs that you have not diagnosed?

—

Go to Diagnostic

Trouble Code (DTC)

List

System OK

6E-200 Engine Control System (4JH1)

P1625 (Symptom Code A) (Flash Code 76)

Circuit Description
The engine control module (ECM) main relay is
energized when the ECM receives an ignition voltage
switch ON signal. When the ignition switch is OFF, the
ECM main relay is de-energized after a certain length
of time passed. If the ECM detects the ECM main relay
is turned OFF before ECM commanded OFF, this DTC
will set.

Condition for Setting the DTC

• The ECM detects that the ECM main relay is

turned OFF before commanded turned OFF.

Action Taken When the DTC Sets

• The ECM does not illuminate the malfunction

indicator lamp (MIL) when the diagnostic runs and
fails.

Condition for Clearing the DTC

• A history DTC clears after 40 consecutive driving

cycles without a fault. Or clear with the Tech 2.

Diagnostic Aids

• If an intermittent condition is suspected, refer to

Intermittent Conditions in this section.

• Faulty ECM main relay may set this DTC.

DTC P1625 (Symptom Code A) (Flash Code 76)
Schematic Reference: Engine Controls Schematics
Connector End View Reference: Engine Controls
Connector End Views or Engine Control Module (ECM)
Connector End Views

Step

Action

Value(s)

Yes

Did you perform the Diagnostic System Check –
Engine Controls?

—

Go to Step 2

Go to Diagnostic

System Check –

Engine Controls

Install the Tech 2.

Turn OFF the ignition for 30 seconds.

Turn ON the ignition, with the engine OFF.

Monitor the Diagnostic Trouble Code (DTC)
Information with the Tech 2.

Turn OFF the ignition.

Turn ON the ignition, with the engine OFF.

Does the DTC fail this ignition?

—

Go to Step 3

Go to Diagnostic

Aids

Turn OFF the ignition.

Disconnect the ECM main relay.

Inspect for an intermittent, for poor
connections and corrosion at the harness
connector of the ECM main relay (pins 1, 2, 4
and 5 of X-14 connector).

Disconnect the engine control module (ECM)
harness connector.

Inspect for an intermittent, for poor
connections and corrosion on ECM main relay
control and battery voltage feed circuit at the
harness connector of the ECM (pins 3 and 58
of B-234 connector).

Test for high resistance on each ECM main
relay circuit.

Repair the connection(s) or circuit(s) as
necessary.

Did you find and correct the condition?

—

Go to Step 5

Go to Step 4

Replace the ECM main relay.

Did you complete the replacement?

—

Go to Step 5

—

Engine Control System (4JH1) 6E-201

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

Clear the DTCs with the Tech 2.

Turn OFF the ignition for 30 seconds.

Start the engine.

Observe the DTC Information with the Tech 2.

Turn OFF the ignition.

Turn ON the ignition, with the engine OFF.

Does the DTC fail this ignition?

—

Go to Step 3

Go to Step 6

Observe the DTC Information with the Tech 2.

Are there any DTCs that you have not diagnosed?

—

Go to Diagnostic

Trouble Code (DTC)

List

System OK

Step

Action

Value(s)

Yes

6E-202 Engine Control System (4JH1)

P1625 (Symptom Code B) (Flash Code 76)

Circuit Description
The engine control module (ECM) main relay is
energized when the ECM receives an ignition voltage
switch ON signal. When the ignition switch is OFF, the
ECM main relay is de-energized after a certain length
of time passed. If the ECM detects the ECM main relay
has been ON since the ignition switch was turned OFF,
this DTC will set.

Condition for Running the DTC

• The ignition switch is OFF.

Condition for Setting the DTC

• The ECM detects that the ECM main relay is

turned ON for longer than 2 seconds.

Action Taken When the DTC Sets

• The ECM does not illuminate the malfunction

indicator lamp (MIL) when the diagnostic runs and
fails.

Condition for Clearing the DTC

• A history DTC clears after 40 consecutive driving

cycles without a fault. Or clear with the Tech 2.

Diagnostic Aids

• If an intermittent condition is suspected, refer to

Intermittent Conditions in this section.

• Faulty or sticking ECM main relay may set this

DTC.

Test Description
The numbers below refer to the step number on the
diagnostic table.
2. If this DTC is present, Tech 2 can communicate with
the ECM when the ignition switch is OFF.

DTC P1625 (Symptom Code B) (Flash Code 76)
Schematic Reference: Engine Controls Schematics
Connector End View Reference: Engine Controls
Connector End Views or Engine Control Module (ECM)
Connector End Views

Step

Action

Value(s)

Yes

Did you perform the Diagnostic System Check –
Engine Controls?

—

Go to Step 2

Go to Diagnostic

System Check –

Engine Controls

Install the Tech 2.

Turn OFF the ignition for 30 seconds.

Monitor the Diagnostic Trouble Code (DTC)
Information with the Tech 2.

Does the DTC fail with the ignition switch OFF?

—

Go to Step 3

Go to Diagnostic

Aids

Disconnect the ECM main relay.

Monitor the DTC Information with the Tech 2.

Does the DTC fail with the ignition switch OFF?

—

Go to Step 6

Go to Step 4

Turn OFF the ignition.

Disconnect the ECM main relay.

Connect a test lamp between the control
circuit of the ECM main relay harness (pin 4 of
X-14 connector) and battery voltage.

Keep the ignition switch OFF.

Does the test lamp illuminate?

—

Go to Step 5

Go to Step 7

Test the control circuit of the ECM main relay
between the engine control module (ECM)
(pin 58 of B-234 connector) and the ECM
main relay (pin 4 of X-14 connector) for a short
to ground.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 9

Go to Step 8

Repair the short to battery on the voltage feed
supply circuit to the ECM between the ECM (pin 3
of B-234 connector) and the ECM main relay (pin 5
of X-14 connector).

Did you complete the repair?

—

Go to Step 9

—

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