Isuzu N-Series. Service manual - part 929

Transmission Control System (Smoother) 7A-171

U0401 (Flash Code 24)

Circuit Description
The accelerator pedal position (APP) signal to the
transmission control module (TCM) is sent from the
engine control module (ECM) via a controller area
network (CAN) communication bus based on inputs
from the APP sensor 1 and APP sensor 2. If the TCM
detects no APP signal from the ECM, this DTC will set.
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

• The TCM detects that the accelerator pedal

position CAN Bus messages from the ECM are not
being received.

Action Taken When the DTC Sets

• The TCM illuminates the transmission warning

lamp when the diagnostic runs and fails.

• The TCM inhibits auto shift control.

• 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.

• 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 this DTC is present, poor shift quality (feel) will

appear.

DTC U0401 (Flash Code 24)

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

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. Start the engine.
4. Monitor the Diagnostic Trouble Code (DTC)

Information with the Tech 2.

Is DTC U0100 or U0121 set?

—

Go to Applicable

DTC

Go to Step 3

1. Select the “4HK1” in the system selection

menu.

2. Monitor the DTC Information with the Tech 2.

Is the engine control module (ECM) DTC P1271,
P1277, P1278, P1282 or P1283 set?

—

Go to Applicable

DTC in Engine

Control Section

Go to Diagnostic

Aids

7A-172 Transmission Control System (Smoother)

U0406 (Flash Code 25)

Circuit Description
The vehicle speed signal to the transmission control
module (TCM) is sent from the engine control module
(ECM) via a controller area network (CAN)
communication bus based on input from the vehicle
speed sensor (VSS). If the TCM detects no vehicle
speed signal from the ECM, this DTC will set.
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

• The TCM detects that the vehicle speed CAN Bus

messages from the ECM are not being received.

Action Taken When the DTC Sets

• The TCM illuminates the transmission warning

lamp when the diagnostic runs and fails.

• The TCM inhibits auto shift control.
• The TCM selects highest gear when shifted from N

to D.

• 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.

• 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.

Notice:

• If this DTC is present, the Output Shaft Speed and

Vehicle Speed parameter on the Tech 2 will
indicate estimated value based on other inputs.

DTC U0406 (Flash Code 25)

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

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. Start the engine.
4. Monitor the Diagnostic Trouble Code (DTC)

Information with the Tech 2.

Is DTC U0100 or U0121 set?

—

Go to Applicable

DTC

Go to Step 3

1. Select the “4HK1” in the system selection

menu.

2. Monitor the DTC Information with the Tech 2.

Is the engine control module (ECM) DTC P0500 or
P0501 set?

—

Go to Applicable

DTC in Engine

Control Section

Go to Diagnostic

Aids

Transmission Control System (Smoother) 7A-173

Transmission Warning Lamp Inoperative

Circuit Description
Ignition voltage is supplied to the transmission warning
lamp via meter fuse. The transmission control module
(TCM) turns the transmission warning lamp ON by
grounding the lamp control circuit.
Transmission Warning Lamp Operation
The transmission warning lamp is located on the
instrument panel cluster (IPC).
Transmission Warning Lamp Function

• The transmission warning lamp informs the driver

that a malfunction has occurred and the vehicle
should be taken in for service as soon as possible.

• The transmission warning lamp illuminates during

a bulb test.

• A DTC will be stored if the transmission warning

lamp is requested by the TCM.

Transmission Waning Lamp Illumination

• The transmission warning lamp illuminates for

approximately 3 seconds when the ignition switch
is turned ON, with the engine OFF.

• The transmission warning lamp remains

illuminated if the transmission control module
(TCM) detects a fault.

Transmission Warning Lamp Inoperative

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

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. Verify whether the instrument panel cluster

(IPC) is operational.

2. Turn OFF the ignition for 30 seconds.
3. Turn OFF the emergency switch.
4. Turn ON the ignition, with the engine OFF.

Does the transmission warning lamp illuminate?

—

Go to Intermittent

Conditions

Go to Step 3

Inspect the Meter (10 A) fuse (F-16) in the glove
box fuse block that supplies ignition voltage to the
transmission warning lamp.

Is the fuse open?

—

Go to Step 10

Go to Step 4

1. Turn OFF the ignition.
2. Remove the Meter (10 A) fuse (F-16) that

supplies voltage to the transmission warning
lamp.

3. Disconnect the transmission control module

(TCM) harness connector (B-425 connector).

4. Turn ON the ignition with the engine OFF.
5. Measure the voltage from the transmission

warning lamp control circuit at the TCM (pin 11
of B-425 connector) to a known good ground.

Is the voltage less than the specified value?

1 volt

Go to Step 5

Go to Step 11

1. Turn OFF the ignition.
2. Install the Meter (10 A) fuse (F-16) that

supplies voltage to the transmission warning
lamp.

3. Turn ON the ignition, with the engine OFF.
4. Connect a 3-amp fused jumper wire between

the transmission warning lamp control circuit in
the TCM harness connector and a known good
ground.

Is the transmission warning lamp illuminated?

—

Go to Step 9

Go to Step 6

7A-174 Transmission Control System (Smoother)

1. Turn ON the ignition.
2. Operate the 1st gear start switch while

observing the 1st START indicator lamp in the
IPC.

Does the 1st START indicator lamp operate
correctly?

—

Go to Step 7

Go to Step 12

1. Turn OFF the ignition.
2. Disconnect the IPC harness connector (B-53

connector).

3. Test the transmission warning lamp control

circuit between the TCM (pin 11 of B-425
connector) and the IPC (pin 8 of B-53
connector) for an open or high resistance.

4. Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 15

Go to Step 8

1. Test for an intermittent and for a poor

connection at the IPC (pin 8 of B-53
connector).

2. Repair the circuit(s) as necessary.

Did you find and correct the condition?

—

Go to Step 15

Go to Step 13

1. Test for an intermittent and for a poor

connection at the TCM harness connector (pin
11 of B-425 connector).

2. Repair the connector(s) as necessary.

Did you find and correct the condition?

—

Go to Step 15

Go to Step 14

Replace the Meter (10 A) fuse (F-16). If the fuse
continues to open, repair the short to ground on the
ignition voltage circuit or check for a shorted
component.

Did you complete the repair?

—

Go to Step 15

—

Repair the short to voltage on the transmission
warning lamp control circuit.

Did you complete the repair?

—

Go to Step 15

—

Repair the open in the transmission warning lamp
ignition voltage circuit.

Did you complete the repair?

—

Go to Step 15

—

Replace the IPC. Refer to Instrument Panel Cluster
(IPC) Replacement in the Cab and Chassis
Electrical section.

Did you complete the replacement?

—

Go to Step 15

—

Important:
Replacement TCM must be programmed and
learned.
Replace the TCM. Refer to Transmission Control
Module (TCM) Replacement in this section.

Did you complete the replacement?

—

Go to Step 15

—

1. Remove all test equipment.
2. Connect any disconnected components or any

disconnected fuse.

3. Turn ON the ignition, with the engine OFF.

Does the transmission warning lamp illuminate?

—

Go to Step 16

Go to Step 2

Step

Action

Value(s)

Yes

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