Isuzu N-Series. Manual - part 713

6E-274    Engine Control System (4HK1) 

U2104, U2106 (Flash Code 84, 85)

Circuit Description
The engine control module (ECM), the transmission
control module (TCM), the electric hydraulic control
module (EHCU) (ABS module) and the data recording
module (DRM) communicate control and diagnostic
information via a controller area network (CAN)
communication bus. The ECM monitors CAN
operational status by expecting a constant flow of
messages from the TCM, EHCU and DRM. If the ECM
fails to receive an expected message from the TCM,
EHCU and DRM, DTC U2104 or U2106 will set
depending on what communication is lost.

Condition for Running the DTC

• The ignition switch is ON.

Condition for Setting the DTC
U2104

• The ECM detects that the CAN Bus OFF status.

U2106

• The ECM detects that the CAN Bus messages

from the TCM are not being received.

Action Taken When the DTC Sets

• The ECM illuminates the malfunction indicator

lamp (MIL) when the diagnostic runs and fails.

• The ECM blinks the exhaust brake indicator lamp

when the diagnostic runs and fails.

• The ECM inhibits exhaust brake control.

Condition for Clearing the MIL / DTC

• The ECM turns OFF the MIL when the key is

cycled after a current DTC clears.

• The ECM turns OFF the exhaust brake indicator

lamp when the diagnostic runs and does not fail.

• A current DTC clears when the diagnostic runs and

does not fail.

• A history DTC clears after 20 consecutive driving

cycles without a fault. Or clear with the Tech 2 or
accelerator pedal operation.

Diagnostic Aid

• If an intermittent condition is suspected, refer to

Intermittent Conditions in this section.

Notice:

• Diagnostic charts are different depending on the

specification of the vehicle.

DTC U2104, U2106 (Flash Code 84, 85)
Schematic Reference: Engine Controls Schematics
Connector End View Reference: Engine Controls
Connector End Views or Engine Control Module (ECM)
Connector End Views

Chart for without Smoother and ABS

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.

Monitor the engine control module (ECM)
Diagnostic Trouble Code (DTC) Information
with the Tech 2.

4.

Start the engine.

Does the DTC fail this ignition?

—

Go to Step 3

Go to Diagnostic 

Aids

3

1.

Turn OFF the ignition.

2.

Measure the resistance across the controller
area network (CAN) Low and High circuits by
back probing the resistor 1 (pins 1 and 2 of B-
387 connector) (resistor 1 is located near the
driver’s side dash panel. It is blue connector).

Is the resistance within the specified value (parallel
resistance of the 120 

Ω resister 1 and the 120 Ω

resistor 2 should be 60 

Ω)?

50 – 70 

Ω

Go to Step 9

Go to Step 4

4

Is the resistance less than the specified value at
Step 3?

—

Go to Step 5

Go to Step 7

 Engine Control System (4HK1)    6E-275

5

1.

Keep the DMM with connected.

2.

Disconnect the engine control module (ECM)
harness connectors.

Is the resistance within the specified value?

50 – 70 

Ω

Go to Step 23

Go to Step 6

6

1.

Keep the DMM with connected.

2.

Disconnect the data recording module (DRM)
harness connector (DRM is located behind the
glove box).

Is the resistance within the specified value?

50 – 70 

Ω

Go to Step 22

Go to Step 16

7

1.

Disconnect the resistor 1 harness connector.

2.

Measure the resistance of the resistor 1.

Is the resistance within the specified value?

110 – 130 

Ω

Go to Step 8

Go to Step 20

8

1.

Disconnect the resister 2 harness connector
(resistor 2 is located behind the radio and
above the heater unit. It is blue connector).

2.

Measure the resistance of the resistor 2.

Is the resistance within the specified value?

110 – 130 

Ω

Go to Step 17

Go to Step 21

9

1.

Disconnect the resistor 1 and resistor 2
harness connector.

2.

Turn ON the ignition, with the engine OFF.

3.

Connect a DMM between the CAN Low circuit
(pin 1 of B-387 connector) and a known good
ground at the resistor 1 harness connector.
Record the voltage reading.

4.

Connect a DMM between the CAN High circuit
(pin 2 of B-387 connector) and a known good
ground at the resistor 1 harness connector.
Record the voltage reading.

Are both voltage readings within the specified
value?

1.3 – 3.7 volts

Go to Step 10

Go to Step 12

10

1.

Turn OFF the ignition.

2.

Disconnect the ECM harness connectors.

3.

Turn ON the ignition, with the engine OFF.

4.

Connect a DMM between the CAN Low circuit
(pin 1 of B-387 connector) and a known good
ground at the resistor 1 harness connector.
Record the voltage reading.

5.

Connect a DMM between the CAN High circuit
(pin 2 of B-387 connector) and a known good
ground at the resistor 1 harness connector.
Record the voltage reading.

Are both voltage readings within the specified
value?

1.3 – 3.7 volts

Go to Step 11

Go to Step 14

Step

Action

Value(s)

Yes

No

6E-276    Engine Control System (4HK1) 

11

1.

Turn OFF the ignition.

2.

Reconnect the ECM harness connectors.

3.

Disconnect the DRM harness connector.

4.

Turn ON the ignition, with the engine OFF.

5.

Connect a DMM between the CAN Low circuit
(pin 1 of B-387 connector) and a known good
ground at the resistor 1 harness connector.
Record the voltage reading.

6.

Connect a DMM between the CAN High circuit
(pin 2 of B-387 connector) and a known good
ground at the resistor 1 harness connector.
Record the voltage reading.

Are both voltage readings within the specified
value?

1.3 – 3.7 volts

Go to Diagnostic 

Aids

Go to Step 15

12

1.

Turn OFF the ignition.

2.

Disconnect the ECM harness connectors.

3.

Disconnect the DRM harness connector.

4.

Turn ON the ignition, with the engine OFF.

5.

Test the CAN Low and CAN High circuits
among the resistor 1, resistor 2, ECM and
DRM for a short to ground and short to
voltage.

Did you find and correct the condition?

—

Go to Step 24

Go to Step 13

13

1.

Turn OFF the ignition.

2.

Reconnect the ECM harness connectors.

3.

Turn ON the ignition, with the engine OFF.

4.

Connect a DMM between the CAN Low circuit
(pin 1 of B-387 connector) and a known good
ground at the resistor 1 harness connector.
Record the voltage reading.

5.

Connect a DMM between the CAN High circuit
(pin 2 of B-387 connector) and a known good
ground at the resistor 1 harness connector.
Record the voltage reading.

Are both voltage readings within the specified
value?

1.3 – 3.7 volts

Go to Step 22

Go to Step 23

14

1.

Test for an open or high resistance in the CAN
Low or High circuit (which ever voltage
reading did not read between 1.3 – 3.7 volts in
Step 10) between the DRM and joint
connection.

2.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 24

Go to Step 18

15

1.

Test for an open or high resistance in the CAN
Low or High circuit (which ever voltage
reading did not read between 1.3 – 3.7 volts in
Step 11) between the ECM and joint
connection.

2.

Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 24

Go to Step 19

Step

Action

Value(s)

Yes

No

 Engine Control System (4HK1)    6E-277

16

Repair the short circuit each other across the CAN
Low and High circuits among the resistor 1, resistor
2, ECM and DRM.

Did you complete the repair?

—

Go to Step 24

—

17

1.

Repair the open circuit or high resistance on
the CAN Low and High circuits between the
resistor 1 (pins 1 and 2 of B-387 connector)
and the resistor 2 (pins 1 and 2 of B-431
connector).

2.

Inspect for an intermittent, for poor
connections and corrosion on the CAN Low
and High circuits at the harness connector of
the each resistor.

Did you complete the repair?

—

Go to Step 24

—

18

1.

Inspect for an intermittent, for poor
connections and corrosion on the CAN Low
and High circuits at the harness connector of
the DRM (pins 2 and 8 of B-429 connector).

2.

Repair the connection(s) as necessary.

Did you find and correct the condition?

—

Go to Step 24

Go to Step 22

19

1.

Inspect for an intermittent, for poor
connections and corrosion on the CAN Low
and High circuits at the harness connector of
the ECM (pins 18 and 37 of J-191 connector).

2.

Repair the connection(s) as necessary.

Did you find and correct the condition?

—

Go to Step 24

Go to Step 23

20

Replace the resistor 1.

Did you complete the replacement?

—

Go to Step 24

—

21

Replace the resistor 2.

Did you complete the replacement?

—

Go to Step 24

—

22

Replace the DRM. Refer to Data Recording Module
(DRM) Replacement.

Did you complete the replacement?

—

Go to Step 24

—

23

Important:
Replacement ECM must be programmed.

Replace the ECM. Refer to Engine Control Module
(ECM) Replacement / Fuel Injector ID Code Data
Programming in this section.

Did you complete the replacement?

—

Go to Step 24

—

24

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.

Start the engine.

Did the DTC fail this ignition?

—

Go to Step 3

Go to Step 25

25

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