U0074
DTC U0074
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructionsprovides an overview of each diagnostic category.
DTC Descriptor
DTC U0074 00
- Control Module Communication Bus B Off
Diagnostic Fault Information
Circuit/System Description
The PTLAN serial data circuits are serial data buses used to communicate information between the control modules. The PTLAN serial data circuits also connect directly to the data link connector (DLC) for programming purposes. If this serial data system is inoperative the modules will still communicate with the scan tool due to other serial data systems but not with each other. Communication with the scan tool for the modules on the PTLAN bus is through the serial data gateway module (SDGM) on the HSGMLAN serial data system.
Conditions for Running the DTCs
* Supply voltage to the modules are in the normal operating range.
* The ignition switch in any position other than START.
Conditions for Setting the DTC
The module setting the DTC has attempted to establish communications on the PTLAN serial data circuits more than 3 times.
Action Taken When the DTCs Sets
* The module suspends all message transmission.
* The MIL and battery indicators will illuminate, along with several DIC messages.
* The vehicle may continue to run for several minutes, then will stall and not start.
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, without a repeat of the malfunction.
Diagnostic Aids
* Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no associated "current" or "active" status. Loss-of- communication U-codes such as these can set for a variety of reasons. Many times, they're transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:
- A control module on the data communication circuit was disconnected while the communication circuit is awake.
- Power to one or more modules was interrupted during diagnosis
- A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.
- Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialize at the same time.
- If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.
* Do not replace a control module reporting a U code. The U code identifies which control module needs to be diagnosed for a communication issue.
* Communication may be available between the BCM and the scan tool with the Powertrain High Speed GMLAN serial data system inoperative. This condition is due to the BCM using both the high and low speed GMLAN systems.
* The engine will not start when there is a total malfunction of the Powertrain High Speed GMLAN serial data bus.
Reference Information
Schematic Reference
* Data Communication Schematics Electrical Diagrams
* Control Module References Programming and Relearning
Connector End View Reference
Component Connector End Views Connector End Views By Name
Description and Operation
Data Link Communications Description and Operation Description and Operation
Electrical Information Reference
* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs
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
* Powertrain High Speed GMLAN serial data circuit terminating resistors
* Control module locations on the Powertrain High Speed GMLAN serial data circuits
* Each control module's Powertrain High Speed GMLAN serial data circuit terminals
1. Attempt to communicate with all control modules on the Powertrain High Speed GMLAN serial data circuit, refer to. Communications should not be available with two or more control modules on the Powertrain High Speed GMLAN serial data circuit.
• If only one control module is not communicating, diagnose that control module only; refer to.
2. Disconnect the scan tool from the DLC. The following tests will be done at the DLC connector.
3. Ignition ON, test for less than 4.5 V between the serial data circuits listed below and ground:
* Terminal 3
* Terminal 11
• 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.
4. Ignition OFF for 60 seconds, test for greater than 100 ohm between the serial data circuit listed below and ground:
* Terminal 3
* Terminal 11
• If not the specified value, test the serial data circuit for a short to ground; refer to Testing the Serial Data Circuits for a Short to ground.
5. Test for 50-70 ohm between the serial data circuit terminal 3 and terminal 11.
• If less than 35 ohm, 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 ohm 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 ohm 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.
Testing the Serial Data Circuits for a Short to Voltage
1. Ignition OFF, disconnect the harness connectors with the Powertrain High Speed GMLAN serial data circuits at an easily accessible control module.
2. Ignition ON, test for greater than 4.5 V between each serial data circuit at the control module connector that was just disconnected 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 Powertrain High Speed GMLAN 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 the serial data circuits at the control module connector that was just disconnected 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, disconnect the harness connectors with the Powertrain High Speed GMLAN serial data circuits at an easily accessible control module.
2. Ignition OFF for 60 seconds, test for less than 1k ohm between each serial data circuit at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are less than 1k ohm.
• 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 Powertrain High Speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to ground.
4. Ignition OFF for 60 seconds, test for less than 1k ohm between the serial data circuits at the control module connector that was just disconnected and ground. Verify that one or more serial data circuits are less than 1k ohm.
• If greater than the specified range for all serial data circuits, 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, disconnect the harness connectors with the Powertrain High Speed GMLAN serial data circuits at an easily accessible control module that is not communicating.
2. Ignition OFF for 60 seconds, test for less than 35 ohm between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are less than 35 ohm.
• 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 Powertrain High Speed GMLAN serial data circuits at another control module, in the direction of the circuits shorted together.
5. Ignition OFF for 60 seconds, test for less than 35 ohm between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are less than 35 ohm.
• If each pair of serial data circuits is within 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 an Open/High Resistance
1. Ignition OFF, disconnect the harness connectors with the Powertrain High Speed GMLAN serial data circuits at an easily accessible control module that is not communicating.
2. Ignition OFF for 60 seconds, test for greater than 100k ohm between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are greater than 100k ohm.
• 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 Powertrain High Speed GMLAN serial data circuits at another control module, in the direction of the open circuit.
5. Ignition OFF for 60 seconds, test for greater than 100k ohm between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits are greater than 100k ohm.
• If each pair of serial data circuits is less than the specified range, replace the control module that was just 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
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
* GMLAN Wiring Repairs GMLAN Wiring Repairs
* Control Module References Programming and Relearningfor module replacement, setup, and programming