Isuzu N-Series. Manual - part 785

Engine Control System (4JH1) 6E-191

P1520 (Symptom Code B) (Flash Code 47)

Circuit Description
The neutral switch is installed on the transmission gear
control box. When the gear position is neutral, the
switch is closed and the switch signal is provided to the
engine control module (ECM). If the ECM detects the
neutral switch signal is not changed from other than
neutral during the predetermined engine speed and
vehicle speed range, this DTC will set. This DTC runs
only manual transmission.

Condition for Running the DTC

• The ignition switch is ON.
• The engine speed is higher than 665 RPM.
• The vehicle speed is lower than 2 km/h (3 MPH).
• The clutch pedal is released after once depressed.

Condition for Setting the DTC

• The ECM detects that the neutral switch signal is

continuously OFF (other than neutral) during three
consecutive driving cycles.

Action Taken When the DTC Sets

• The ECM illuminates the malfunction indicator

lamp (MIL) when the diagnostic runs and fails.

Condition for Clearing the MIL/DTC
Important:

• Following conditions must be met two consecutive

driving cycles. (One driving cycle: Ignition switch
ON > Engine run > Ignition switch OFF for 10
seconds)

- The engine speed is higher than 665 RPM
- The vehicle speed is lower than 2 km/h (3

MPH)

- The clutch pedal is released after once

depressed

- Neutral position

• After the above procedure is done, 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.

DTC P1520 (Symptom Code B) (Flash Code 47)
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 ON the ignition, with the engine OFF.

Observe the Neutral Switch parameter with
the Tech 2 while shift to neutral and other
positions.

Does the Tech 2 indicate ON when the position is
neutral and OFF when other than the neutral?

—

Go to Diagnostic

Aids

Go to Step 3

Turn OFF the ignition.

Disconnect the neutral switch harness
connector.

Turn ON the ignition, with the engine OFF.

Observe the Neutral Switch parameter with
the Tech 2 while momentarily jumping across
the neutral switch harness connectors
between pins 1 and 2 of J-50 connector.

Does the Tech 2 indicate ON when the circuit is
jumped and OFF when the circuit is not jumped?

—

Go to Step 7

Go to Step 4

Connect a test lamp between the ignition
voltage feed circuit of the neutral switch
harness (pin 1 of J-50 connector) and a known
good ground.

Turn ON the ignition, with the engine OFF.

Does the test lamp illuminate?

—

Go to Step 6

Go to Step 5

6E-192 Engine Control System (4JH1)

Repair the open circuit or high resistance on the
ignition voltage feed circuit between the ECM main
relay (pin 5 of X-14 connector) and the neutral
switch (pin 1 of J-50 connector).

Did you complete the repair?

—

Go to Step 11

—

Test the neutral switch signal circuit between
the engine control module (ECM) (pin 87 of B-
233 connector) and the neutral switch (pin 2 of
J-50 connector) for an open circuit or high
resistance.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 11

Go to Step 8

Turn OFF the ignition.

Inspect for an intermittent and for poor
connections at the harness connectors of the
neutral switch (pins 1 and 2 of J-50
connector).

Repair the connection(s) as necessary.

Did you find and correct the condition?

—

Go to Step 11

Go to Step 9

Turn OFF the ignition.

Disconnect the ECM harness connector.

Inspect for an intermittent and for a poor
connection on the neutral switch circuit at the
harness connector of the ECM (pin 87 of B-
233 connector).

Repair the connection(s) as necessary.

Did you find and correct the condition?

—

Go to Step 11

Go to Step 10

Replace the neutral switch. Refer to Neutral Switch
Replacement in Transmission Section.

Did you complete the replacement?

—

Go to Step 11

—

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 11

—

Step

Action

Value(s)

Yes

Engine Control System (4JH1) 6E-193

Reconnect all previously disconnected
harness connector(s).

Clear the DTCs with the Tech 2.

Important:
Following conditions must be met two consecutive
driving cycles before clear with the Tech 2. (One
driving cycle: Ignition switch ON > Engine run >
Ignition switch OFF for 10 seconds)

• The engine speed is higher than 665 RPM
• The vehicle speed is lower than 2 km/h (3

MPH)

• The clutch pedal is released after once

depressed

• Neutral position

Turn ON the ignition, with the engine OFF.

Observe the Neutral Switch parameter with
the Tech 2 while shift to neutral and other
positions.

Does the Tech 2 indicate ON when the position is
neutral and OFF when other than the neutral?

—

Go to Step 12

Go to Step 3

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-194 Engine Control System (4JH1)

P1576 (Symptom Code 4) (Flash Code 71)

Circuit Description
The engine control module (ECM) controls the exhaust
brake which energizes the intake throttle solenoid valve
and exhaust brake solenoid valve based on vehicle
running condition. The ECM commands each solenoid
valve to close each throttle valve by applying vacuum.
If the ECM detects that an open circuit or short to
ground on the exhaust brake solenoid valve control
circuit, this DTC will set.

Condition for Running the DTC

• The ignition switch is ON.

Condition for Setting the DTC

• The ECM detects a low voltage condition on the

exhaust brake solenoid valve control circuit for
longer than 3 seconds when the solenoid valve is
commanded OFF.

Action Taken When the DTC Sets

• The ECM illuminates the malfunction indicator

lamp (MIL) when the diagnostic runs and fails.

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.

Diagnostic Aids

• If an intermittent condition is suspected, refer to

Intermittent Conditions in this section.

Test Description
The number below refers to the step number on the
diagnostic table.
5. If the exhaust brake solenoid valve control circuit
between the ECM and the solenoid valve is normal,
voltage level high DTC P1576 (Symptom Code 8) will
set.

DTC P1576 (Symptom Code 4) (Flash Code 71)
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.

Start the engine.

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

Does the DTC fail this ignition?

—

Go to Step 3

Go to Diagnostic

Aids

Turn OFF the ignition.

Disconnect the exhaust brake solenoid valve
harness connector.

Connect a test lamp between the voltage feed
circuit of the exhaust brake solenoid valve
harness (pin 1 of J-31 connector) and a known
good ground.

Turn ON the ignition, with the engine OFF.

Does the test lamp illuminate?

—

Go to Step 4

Go to Step 6

Turn OFF the ignition.

Connect a test lamp between the control
circuit of the exhaust brake solenoid valve
harness (pin 2 of J-31 connector) and battery
voltage.

Turn ON the ignition, with the engine OFF.

Does the test lamp illuminate?

—

Go to Step 8

Go to Step 5

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