Isuzu N-Series. Manual - part 779

 Engine Control System (4JH1)    6E-167

5

1.

Turn OFF the ignition.

2.

Connect a DMM between the 5 volts reference
circuit of the APP sensor harness (pin 3 of B-
80 connector) and a known good ground.

3.

Turn ON the ignition, with the engine OFF.

Is the DMM voltage more than the specified value?

4.7 volts

Go to Step 6

Go to Step 9

6

Is the DMM voltage less than the specified value at
step 5?

5.3 volts

Go to Step 7

Go to Step 10

7

Important:
The APP sensor may be damaged if the sensor
signal circuit is shorted to a voltage source.

1.

Test the signal circuit between the engine
control module (ECM) (pin 38 of E-234
connector) and the APP sensor (pin 2 of B- 80
connector) for the following conditions:

• A short to battery or ignition voltage
• A short to any 5 volts reference

2.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 15

Go to Step 14

8

1.

Test the low reference circuit between the
ECM (pin 49 of B-234 connector) and the APP
sensor (pin 1 of B-80 connector) for an open
circuit or high resistance.

2.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 15

Go to Step 12

9

1.

Test the 5 volts reference circuit between the
ECM (pin 57 of B-234 connector) and the APP
sensor (pin 3 of B-80 connector) for the
following conditions:

• A short to ground
• A short to the low reference circuit

2.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 15

Go to Step 14

10

1.

Test the 5 volts reference circuit between the
ECM (pin 57 of B-234 connector) and the APP
sensor (pin 3 of B-80 connector) for a short to
battery or ignition voltage.

2.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 15

Go to Step 14

11

1.

Turn OFF the ignition.

2.

Inspect for an intermittent and for a poor
connection at the harness connector of the
APP sensor (pin 1 of B-80 connector).

3.

Repair the connection(s) as necessary.

Did you find and correct the condition?

—

Go to Step 15

Go to Step 13

Step

Action

Value(s)

Yes

No

6E-168    Engine Control System (4JH1) 

12

1.

Turn OFF the ignition.

2.

Disconnect the ECM harness connector.

3.

Inspect for an intermittent and for a poor
connection on the APP sensor circuit at the
harness connector of the ECM (pin 49 of B-
234 connector).

4.

Repair the connection(s) as necessary.

Did you find and correct the condition?

—

Go to Step 15

Go to Step 14

13

Replace APP sensor.  Refer to Accelerator Pedal
Position (APP) Sensor Replacement in this section.

Did you complete the replacement?

—

Go to Step 15

—

14

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 15

—

15

1.

Reconnect all previously disconnected
harness connector(s).

2.

Clear the DTCs with the Tech 2.

3.

Turn OFF the ignition for 30 seconds.

4.

Turn ON the ignition, with the engine OFF.

5.

Fully depress and release the accelerator
pedal many times.

6.

Monitor the DTC Information with the Tech 2.

Did the DTC fail this ignition?

—

Go to Step 3

Go to Step 16

16

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

No

 Engine Control System (4JH1)    6E-169

P1120 (Symptom Code 2) (Flash Code 21)

Circuit Description
The accelerator pedal position (APP) sensor is
mounted on the accelerator pedal bracket.  The engine
control module (ECM) uses the APP sensors to
determine the amount of acceleration or deceleration
desired by the person driving the vehicle via the fuel
injection control.  The APP sensor has the following
circuits.

• 5 volts reference circuit
• Low reference circuit
• APP sensor signal circuit

The ECM supplies 5 volts to the APP sensor on the 5
volts reference circuit.  The ECM also provides a
ground on the low reference circuit.  The APP sensor
provides a signal to the ECM on the APP sensor signal
circuit, which is relative to the position changes of the
accelerator pedal angle.  The ECM monitors the APP
sensor signal for voltages outside the normal range of
the APP sensor.  If the ECM detects an excessively low
APP sensor signal voltage, this DTC will set.

Condition for Running the DTC

• The ignition switch is ON.

Condition for Setting the DTC

• The ECM detects that the APP sensor signal

voltage is less than 0.3 volts for 0.5 seconds.

Action Taken When the DTC Sets

• The ECM illuminates the malfunction indicator

lamp (MIL) when the diagnostic runs and fails.

• The ECM uses an engine speed substitution of

1400 RPM.

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.
4. If the APP sensor signal circuit between the ECM
and the sensor is normal, the sensor signal voltage
high DTC P1120 (Symptom Code 1) will set.

DTC P1120 (Symptom Code 2) (Flash Code 21)
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

No

1

Did you perform the Diagnostic System Check –
Engine Controls?

—

Go to Step 2

Go to Diagnostic 

System Check  

Engine Controls

2

1.

Install the Tech 2.

2.

Turn OFF the ignition for 30 seconds.

3.

Start the engine.

4.

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

3

1.

Turn OFF the ignition.

2.

Disconnect the accelerator pedal position
(APP) sensor harness connector.

3.

Connect a DMM between the 5 volts reference
circuit of the APP sensor harness (pin 3 of B-
80 connector) and a known good ground.

4.

Turn ON the ignition, with the engine OFF.

Is the DMM voltage more than the specified value?

4.7 volts

Go to Step 4

Go to Step 5

6E-170    Engine Control System (4JH1) 

4

1.

Turn OFF the ignition.

2.

Connect a 3-amp fused jumper wire across
the APP sensor connector between pins 2 and
3 of the B-80 connector.

3.

Turn ON the ignition, with the engine OFF.

Does DTC P1120 (Symptom Code 1) set, but not
P1120 (Symptom Code 2)?

—

Go to Step 7

Go to Step 6

5

1.

Test the 5 volts reference circuit between the
engine control module (ECM) (pin 57 of B-234
connector) and the APP sensor (pin 3 of B-80
connector) for the following conditions:

• An open circuit
• A short to ground
• A short to the low reference circuit
• High resistance

2.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 11

Go to Step 8

6

1.

Test the signal circuit between the ECM (pin
38 of E-234 connector) and the APP sensor
(pin 2 of B-80 connector) for the following
conditions:

• An open circuit
• A short to ground
• A short to the low reference circuit
• High resistance.

2.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 11

Go to Step 8

7

1.

Turn OFF the ignition.

2.

Inspect for an intermittent and for poor
connections at the harness connector of the
APP sensor (pins 2 and 3 of B-80 connector).

3.

Repair the connection(s) as necessary.

Did you find and correct the condition?

—

Go to Step 11

Go to Step 9

8

1.

Turn OFF the ignition.

2.

Disconnect the ECM harness connector.

3.

Inspect for an intermittent and for poor
connection on the APP sensor circuit at the
harness connector of the ECM (pins 38 and
57 of B-234 connector).

4.

Repair the connection(s) as necessary.

Did you find and correct the condition?

—

Go to Step 11

Go to Step 10

9

Replace APP sensor.  Refer to Accelerator Pedal
Position (APP) Sensor Replacement in this section.

Did you complete the replacement?

—

Go to Step 11

—

10

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

No