DTC U0074
Diagnostic Instructions
DTC Descriptor
| DTC U0074 00: Control Module
Communication Bus B Off |
Diagnostic Fault Information
Circuit
|
Short to Ground
|
Open/High Resistance
|
Short to Voltage
|
Signal Performance
|
Ground (DLC, terminal 5)
|
-
|
1
|
-
|
-
|
Mid Speed GMLAN Serial Data (+)
|
U0074
|
U0074, 2*
|
U0074
|
-
|
Mid Speed GMLAN Serial Data (-)
|
U0074
|
U0074, 2*
|
U0074
|
-
|
Chassis High Speed GMLAN Serial Data (+)
|
U0074
|
U0074, 2*
|
U0074
|
-
|
Chassis High Speed GMLAN Serial Data (-)
|
U0074
|
U0074, 2*
|
U0074
|
-
|
* An open between the data link connector (DLC) and the first
splice will only effect the communication with the scan tool. The
vehicle control modules will still communicate.
1. No communication on any mid speed or chassis high speed GMLAN
module.
2. An open in only one mid speed or chassis high speed GMLAN
serial data circuit may allow degraded communication between the
controls modules.
|
|
| |
Circuit/System Description
Control modules connected to the mid speed or chassis high speed
GMLAN serial data circuits monitor for serial data communications
during normal vehicle operation. Operating information and commands
are exchanged among the control modules when the ignition switch is
in any position other than OFF. The mid speed or chassis high speed
GMLAN serial data bus uses a terminating resistor that is in
parallel with the mid speed or chassis high speed GMLAN (+) and (-)
circuits.
Conditions for Running the DTC
The system voltage is between 9-16 V.
Conditions for Setting the DTC
A supervised periodic message that includes the transmitter
module availability has not been received.
Action Taken When the DTC Sets
Specific subsystems will not function.
Conditions for Clearing the DTC
| • |
A current DTC clears when the
malfunction is no longer present. |
| • |
A history DTC clears when the
module ignition cycle counter reaches the reset threshold of 50,
without a repeat of the malfunction. |
Diagnostic Aids
| • |
Use the
Data Link References to identify the mid speed or chassis high
speed GMLAN serial data modules. |
| • |
Some control modules may not
have internal protection for specific voltage outputs and may open
a battery positive voltage or ignition voltage source fuse. If a
voltage input fuse is open and no short is found in that circuit,
ensure that no control module output voltage circuit is shorted to
ground before replacing the control module. |
| • |
An open in the DLC ground
circuit terminal 5 will allow the scan tool to operate to set up
the vehicle on the tool and then not communicate with the
vehicle. |
Reference Information
Schematic Reference
Data Communication Schematics
Connector End View Reference
Component Connector End Views
Description and Operation
Data Link Communications Description and Operation
Electrical Information Reference
Scan Tool Reference
Control Module References for scan tool information
Circuit/System Verification
Verify if DTC U0074 is set by the K73 Telematics Communication
Interface Control Module.
If DTC U0074 is set by the K73 Telematics Communication Interface
Control Module, refer to
Scan Tool Does Not Communicate with High Speed GMLAN Device .
Circuit/System Testing
Note: Each control module
may need to be disconnected to isolate a circuit fault. Use the
schematic to identify the following:
| • |
Control modules
the vehicle is equipped with |
| • |
Mid speed or
chassis high speed serial data circuit terminating resistors
|
| • |
Control module
locations on the serial data circuits |
| • |
Each control
module's mid speed or chassis high speed serial data circuit
terminals |
| 1. |
Refer to
Data Link References to determine if the control module setting
DTC U0074 is on the mid speed or chassis high speed serial data
circuits. Only diagnose the affected serial data circuits.
|
| 2. |
Attempt to communicate with
all control modules on the mid speed or chassis high speed serial
data circuit. Refer to
Data Link References . Communications should not be available
with two or more control modules on the mid speed or chassis high
speed serial data circuit. |
| ⇒ |
If only one control module is not communicating,
diagnose that control module only. Refer to
DTC U0100-U02FF . |
| 3. |
Disconnect the scan tool from
the X84 DLC. The following tests will be done at the X84 DLC
connector. |
| 4. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Test for less than 10 Ω between the
ground circuit terminal 5 and ground. |
| ⇒ |
If greater than the specified range, test the
ground circuit for an open/high resistance. |
| 5. |
Ignition ON, test for less
than 4.5 V between each serial data circuit listed below and
ground: |
| |
• |
Mid speed GMLAN serial data
terminal 3 |
| |
• |
Mid speed GMLAN serial data
terminal 11 |
| |
• |
Chassis high speed GMLAN
serial data terminal 12 |
| |
• |
Chassis high speed GMLAN
serial data terminal 13 |
| ⇒ |
If greater than the specified range, test the
serial data circuit for a short to voltage. Refer to Testing the
Serial Data Circuits for a Short to Voltage. |
| 6. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Test for greater than 100 Ω between
each serial data circuit listed below and ground: |
| |
• |
Mid speed GMLAN serial data
terminal 3 |
| |
• |
Mid speed GMLAN serial data
terminal 11 |
| |
• |
Chassis high speed GMLAN
serial data terminal 12 |
| |
• |
Chassis high speed GMLAN
serial data terminal 13 |
| ⇒ |
If less than the specified range, test the serial
data circuit for a short to ground. Refer to Testing the Serial
Data Circuits for a Short to Ground. |
| 7. |
Test for 50-70 Ω between
the serial data circuits listed below: |
| |
• |
Mid speed GMLAN serial data
circuits terminal 3 and terminal 11 |
| |
• |
Chassis high speed GMLAN
serial data circuits terminal 12 and terminal 13 |
| ⇒ |
If less than 35 Ω, test for a short between
the serial data circuits. Refer to Testing the Serial Data Circuits
for a Short between the Circuits. |
| ⇒ |
If 35-50 Ω there may be a third terminating
resistor between the serial data circuits. This can happen if the
incorrect control module is installed. Some control modules are
available with and without the terminating resistors installed to
reduce the need of terminating resistors in the wiring
harness. |
| ⇒ |
If greater than 70 Ω but less than infinite,
test the serial data circuit for an open/high resistance. Refer to
Testing the Serial Data Circuits for an Open/High Resistance. |
| ⇒ |
If infinite, test the serial data circuits between
the DLC and the first connection to the serial data circuit for an
open/high resistance. |
| 8. |
If the circuits test normal,
refer to Testing the Control Module Circuits. |
Testing the Serial Data Circuits for a Short to Voltage
| 1. |
Ignition OFF, disconnect the
harness connectors with the mid speed or chassis high speed serial
data circuits at an easily accessible control module. |
| 2. |
Ignition ON, test for greater
than 4.5 V between each serial data circuit and ground. Verify that
one or more serial data circuits are greater than 4.5 V.
|
| ⇒ |
If all serial data circuits are less than the
specified range, replace the control module that was
disconnected. |
| 3. |
Ignition OFF, disconnect the
harness connectors with the mid speed or chassis high speed serial
data circuits at another control module, in the direction of the
circuit shorted to voltage. |
| 4. |
Ignition ON, test for greater
than 4.5 V between each serial data circuit and ground. Verify that
one or more serial data circuits are greater than 4.5 V.
|
| ⇒ |
If all serial data circuits are less than the
specified range, replace the control module that was just
disconnected. |
| 5. |
Repeat step 3 until one of the
follow conditions are isolated: |
| |
• |
A short to voltage on the
serial data circuit between 2 control modules |
| |
• |
A short to voltage on the
serial data circuit between a control module and a terminating
resistor |
Testing the Serial Data Circuits for a Short to Ground
| 1. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Disconnect the harness connectors with the
mid speed or chassis high speed serial data circuits at an easily
accessible control module. |
| 2. |
Test for less than 100 Ω
between each serial data circuit and ground. Verify that one or
more serial data circuits are less than 100 Ω. |
| ⇒ |
If all serial data circuits are greater than the
specified range, replace the control module that was
disconnected. |
| 3. |
Disconnect the harness
connectors with the mid speed or chassis high speed serial data
circuits at another control module, in the direction of the circuit
shorted to ground. |
| 4. |
Test for less than 100 Ω
between each serial data circuit and ground. Verify that one or
more serial data circuits are less than 100 Ω. |
| ⇒ |
If all serial data circuits are greater than the
specified range, replace the control module that was just
disconnected. |
| 5. |
Repeat step 3 until one of the
follow conditions are isolated: |
| |
• |
A short to ground on the
serial data circuit between 2 control modules |
| |
• |
A short to ground on the
serial data circuit between a control module and a terminating
resistor |
Testing the Serial Data Circuits for a Short between the
Circuits
| 1. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Disconnect the harness connectors with the
mid speed or chassis high speed serial data circuits at an easily
accessible control module, that is not communicating. |
| 2. |
Test for less than 110 Ω
between each pair of serial data circuits. Verify that one pair of
serial data circuits are less than 110 Ω. |
| ⇒ |
If each pair of serial data circuits is greater
than the specified range, replace the control module that was
disconnected. |
| 3. |
Connect the harness connectors
at the control module that was disconnected. |
| 4. |
Disconnect the harness
connectors with the mid speed or chassis high speed serial data
circuits at another control module, in the direction of the
circuits shorted together. |
| 5. |
Test for less than 110 Ω
between each pair of serial data circuits. Verify that one pair of
serial data circuits are less than 110 Ω. |
| ⇒ |
If each pair of serial data circuits is greater
than the specified range, replace the control module that was just
disconnected. |
| 6. |
Repeat step 3 until one of the
following conditions are isolated: |
| |
• |
Serial data circuits shorted
together between 2 control modules |
| |
• |
Serial data circuits shorted
together between a control module and a terminating resistor
|
| |
• |
A shorted terminating
resistor |
Testing the Serial Data Circuits for a Open/High
Resistance
| 1. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Disconnect the harness connectors with the
mid speed or chassis high speed serial data circuits at an easily
accessible control module, that is not communicating. |
| 2. |
Test for greater than 130
Ω between each pair of serial data circuits. Verify that one
pair of serial data circuits are greater than 130 Ω.
|
| ⇒ |
If each pair of serial data circuits is less than
the specified range, replace the control module that was
disconnected. |
| 3. |
Connect the harness connectors
at the control module that was disconnected. |
| 4. |
Disconnect the harness
connectors with the mid speed or chassis high speed serial data
circuits at another control module, in the direction of the open
circuit. |
| 5. |
Test for greater than 130
Ω between each pair of serial data circuits. Verify that one
pair of serial data circuits are greater than 130 Ω.
|
| ⇒ |
If each pair of serial data circuits is less than
the specified range, replace the control module that was
disconnected. |
| 6. |
Repeat step 3 until one of the
following conditions are isolated: |
| |
• |
An open/high resistance on the
serial data circuit between 2 control modules |
| |
• |
An open/high resistance on the
serial data circuit between a control module and a terminating
resistor |
| |
• |
An open/high resistance
terminating resistor |
Testing the Control Module Circuits
| 1. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Disconnect the harness connectors at an
easily accessible control module that is not communicating.
|
| 2. |
Test for less than 10 Ω
between each ground circuit terminal and ground, at the control
module connector that was just disconnected. |
| ⇒ |
If greater than the specified range, test the
ground circuit and ground connection for an open/high
resistance. |
| 3. |
Ignition OFF, verify that a
test lamp does not illuminate between each ignition circuit
terminal and ground. |
| ⇒ |
If the test lamp does illuminate, test the
ignition circuit for a short to voltage. If the circuits test
normal, replace the appropriate control module. |
| 4. |
Ignition ON, verify that a
test lamp illuminates between each ignition circuit terminal and
ground. |
| ⇒ |
If the test lamp does not illuminate, test the
ignition circuit for a short to ground or an open/high resistance.
If the circuits test normal, replace the appropriate control
module. |
| 5. |
Ignition OFF and all vehicle
systems OFF, all keys at least 3 meters away from vehicle, all
access doors closed. It may take up to 2 minutes for all vehicle
systems to power down. Test for less than 130 Ω between each
pair of serial data circuits. |
| ⇒ |
If greater than the specified range, test the
serial data circuits for an open/high resistance. |
| 6. |
Connect the harness connectors
at the control module that was just disconnected. |
| 7. |
Repeat step 1 at another
control module until a short to voltage, a short to ground, or an
open/high resistance is found on the ground, ignition, or serial
data circuits. |
Repair Instructions
Perform the
Diagnostic Repair Verification after completing the repair.
|
