Isuzu N-Series. Manual - part 926

Transmission Control System (Smoother) 7A-159

U0100, U0121 (Flash Code 42, 68)

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 TCM monitors CAN
operational status by expecting a constant flow of
messages from the ECM, EHCU and DRM. If the TCM
fails to receive an expected message from the ECM,
EHCU and DRM, DTC U0100 or U0121 will set
depending on what communication is lost.
Condition for Running the DTC

• The ignition switch is ON.
• The ignition voltage is more than 20 volts.
• The emergency switch is OFF.

Condition for Setting the DTC
U0100

• The TCM detects that the CAN Bus OFF Status.

U0121

• The TCM detects that the CAN Bus.

Action Taken When the DTC Sets

• The TCM illuminates the transmission warning

lamp when the diagnostic runs and fails. (U0100)

• The TCM does not illuminate the transmission

warning lamp when the diagnostic runs and fails.
(U0121)

• The TCM inhibits gear select & shift position

learning and half-clutch learning.

Condition for Clearing the DTC

• The TCM turns OFF the transmission warning

lamp when the diagnostic runs and does not fail.
(U0100)

• A current DTC clears when the diagnostic runs and

does not fail.

• A history DTC clears when the diagnostic runs and

does not fail.

Diagnostic Aids

• If an intermittent condition is suspected, refer to

Intermittent Conditions in this section.

• If DTC U0100 is present, auto shift control is

inhibited and poor shift quality (feel) will appear.

• If DTC U0121 is present, lock up clutch may

engage/disengage during an ABS event.

Notice:

• Diagnostic charts are different depending on the

specification of the vehicle.

DTC U0100, U0121 (Flash Code 42, 68)

Schematic Reference: Transmission Controls
Schematics
Connector End View Reference: Transmission
Controls Connector End Views or Transmission Control
Module (TCM) Connector End Views

Chart for without ABS

Step

Action

Value(s)

Yes

Did you perform the Diagnostic System Check -
Transmission Controls?

—

Go to Step 2

Go to Diagnostic

System Check -

Transmission

Controls

1. Install the Tech 2.
2. Turn OFF the ignition for 30 seconds.
3. Turn ON the ignition, with the engine OFF.
4. Monitor the transmission control module

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

Does the DTC fail this ignition?

—

Go to Step 3

Go to Diagnostic

Aids

Attempt to communicate with the engine control
module (ECM) via the Engine Data table.

Does the Tech 2 communicate with the ECM?

—

Go to Step 4

Go to Diagnostic

System Check -
Engine Controls

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 120Ω

resistor 2 should be 60

Ω)?

50 - 70

Ω

Go to Step 11

Go to Step 5

7A-160 Transmission Control System (Smoother)

Is the resistance less than the specified value at
Step 4?

—

Go to Step 6

Go to Step 9

1. Keep the DMM with connected.
2. Disconnect the TCM harness connectors.

Is the resistance within the specified value?

50 - 70

Ω

Go to Step 29

Go to Step 7

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 30

Go to Step 8

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 28

Go to Step 21

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 10

Go to Step 26

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 22

Go to Step 27

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 12

Go to Step 15

1. Turn OFF the ignition.
2. Disconnect the ECM harness connectors.
3. Disconnect the TCM harness connectors.
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 Step 13

Go to Step 18

Step

Action

Value(s)

Yes

Transmission Control System (Smoother) 7A-161

1. Turn OFF the ignition.
2. Reconnect the TCM harness connectors.
3. Disconnect the DRM harness connectors.
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 Step 14

Go to Step 19

1. Turn OFF the ignition.
2. Reconnect the ECM harness connectors.
3. Disconnect the TCM harness connectors.
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 20

1. Turn OFF the ignition.
2. Disconnect the ECM harness connectors.
3. Disconnect the TCM harness connectors.
4. Disconnect the DRM harness connector.
5. Turn ON the ignition, with the engine OFF.
6. Test the CAN Low and CAN High circuits

among the resistor 1, resistor 2, ECM, TCM
and DRM for a short to ground and short to
voltage.

Did you find and correct the condition?

—

Go to Step 31

Go to Step 16

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 17

Go to Step 30

Step

Action

Value(s)

Yes

7A-162 Transmission Control System (Smoother)

1. Turn OFF the ignition.
2. Reconnect the TCM 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 28

Go to Step 29

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
12) between the DRM and joint connection.

2. Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 31

Go to Step 23

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
13) between the TCM and joint connection.

2. Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 31

Go to Step 24

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
14) between the ECM and joint connection.

2. Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 31

Go to Step 25

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

Did you complete the repair?

—

Go to Step 31

—

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 31

—

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 31

Go to Step 28

Step

Action

Value(s)

Yes

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