Data Communications (Powertrain Expansion Bus)
DTC U0074 (Powertrain Expansion Bus)
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 Instructions provides an overview of each diagnostic category.
DTC Descriptor
DTC U0074 00
-
Control Module Communication Powertrain Expansion CAN Bus Off
Diagnostic Fault Information
Circuit/System Description
The serial data is transmitted on two twisted wires that allow speeds up to 500 Kb/s. The twisted pair is terminated with two 120 ohms resistors, which can be a separate resistor in a connector assembly or internal to a control module. The resistors are used as the load for the Powertrain high speed GMLAN bus during normal vehicle operation. The Powertrain high speed GMLAN is a differential bus. The Powertrain high speed GMLAN serial data bus (+) and Powertrain high speed GMLAN serial data (-) are driven to opposite extremes from a rest or idle level of approximately 2.5 V. Driving the lines to their extremes, adds 1 V to the Powertrain high speed GMLAN serial data bus (+) circuit and subtracts 1 V from the Powertrain high speed GMLAN serial data bus (-) circuit. If serial data is lost, control modules will set a no communication code against the non-communicating control module. Note that a loss of serial data DTC does not represent a failure of the control module that set it.
Conditions for Running the DTC
* The system voltage is greater than 9.5 V.
* The vehicle power mode requires serial data communications.
Conditions for Setting the DTC
A supervised periodic message that includes the transmitter module availability has not been received for more than 450 ms.
Action Taken When the DTC Sets
* DTC U0074 is a type B DTC.
* DTC U0074 in the hybrid powertrain control module will cause the malfunction indicator lamp (MIL) to illuminate and result in the engine operating in conventional engine mode and will maintain 14 V module operation.
Conditions for Clearing the DTC
* DTC U0074 is a type B DTC.
* The hybrid powertrain control module turns off the MIL after the diagnostic runs and does not fail during subsequent ignition cycles. Normal operation will resume 5 seconds after subsequent ignition cycle.
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 are 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.
* Use the Data Link References Initial Inspection and Diagnostic Overview to identify control modules connected to the powertrain high speed GMLAN serial data circuits.
* 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.
Reference Information
Schematic Reference
* Data Communication Schematics [1][2]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
Scan Tool Reference
Control Module References Programming and Relearning for scan tool information
Circuit/System Verification
Verify that DTC P0606 is not set.
If the DTC is set, refer to DTC P0601-P0604, P0606, or P062F Data Communications.
Circuit/System Testing
Danger:
Always perform the High Voltage Disabling procedure prior to servicing any High Voltage component or connection. Personal Protection Equipment (PPE) and proper procedures must be followed.
The High Voltage Disabling procedure will perform the following tasks:
* Identify how to disable high voltage.
* Identify how to test for the presence of high voltage.
* Identify condition under which high voltage is always present and personal protection equipment (PPE) and proper procedures must be followed.
Before working on any high voltage system, be sure to wear the following Personal Protection Equipment:
* Safety glasses with appropriate side shields when within 15 meters (50 feet) of the vehicle, either indoors or outdoors.
* Certified and up-to-date Class "0" Insulation gloves rated at 1000V with leather protectors.
- Visually and functionally inspect the gloves before use.
- Wear the Insulation gloves with leather protectors at all times when working with the high voltage battery assembly, whether the system is energized or not.
Failure to follow the procedures exactly as written may result in serious injury or death.
Danger:
The eAssist battery pack assembly will utilize an exchange program. Please consult the most recent revision of bulletin/PI# PIC5520, available in Service Information♦(SI), for a list of approved eAssist battery pack assembly service procedures. Components that may be removed and serviced without exchanging the complete battery pack assembly are identified in the bulletin/PI. Please contact the GM Technical Assistance Center (1-877-446-8227) if you have any questions.
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 ground, B+, ignition, and Powertrain high speed GMLAN serial data circuit terminals
1. Perform the High Voltage Disabling procedure. Refer to High Voltage Disabling .
2. Reconnect the 12 V battery.
3. 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.
4. Ignition ON, test for less than 4.5 V between each Powertrain high speed GMLAN serial data circuit at the control module that was just disconnected and ground.
• If one or more Powertrain high speed GMLAN serial data circuits are greater than the specified range, test the Powertrain high speed GMLAN serial data circuit for a short to voltage. Refer to Testing the Serial Data Circuits for a Short to Voltage.
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 greater than 100 ohms between each Powertrain high speed GMLAN serial data circuit at the control module connector that was just disconnected and ground.
• If one or more Powertrain high speed GMLAN serial data circuits are less than the specified value, test the Powertrain high speed GMLAN serial data circuit for a short to ground. Refer to Testing the Serial Data Circuits for a Short to Ground.
6. Test for 110-130 ohms between the Powertrain high speed GMLAN serial data circuits at the control module that was just disconnected.
• If less than 110 ohms, test for a short between the Powertrain high speed GMLAN serial data circuits. Refer to Testing the Serial Data Circuits for a Short Between The Circuits.
• If greater than 130 ohms but less than infinite, test the Powertrain high speed GMLAN serial data circuit for an open/high resistance. Refer to Testing the Serial Data Circuits for an Open/High Resistance.
7. 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 Powertrain high speed GMLAN serial data circuits at another control module that is not communicating, in the direction of the circuit shorted to voltage.
2. Ignition ON, test for greater than 4.5 V between each Powertrain high speed GMLAN serial data circuit at the control module connector that was just disconnected and ground. Verify that one or more Powertrain high speed GMLAN serial data circuits are greater than 4.5 V.
• If all Powertrain high speed GMLAN serial data circuits are less than the specified range, replace the control module that was just disconnected.
3. Repeat step 1 until one of the follow conditions are isolated:
* A short to voltage on the Powertrain high speed GMLAN serial data circuit between 2 control modules or splice packs, if equipped.
* A short to voltage on the Powertrain high speed GMLAN 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 another control module that is not communicating, in the direction of the circuit shorted to ground.
2. 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 1k ohms between each Powertrain high speed GMLAN serial data circuit at the control module connector that was just disconnected and ground. Verify that one or more Powertrain high speed GMLAN serial data circuits are less than 1k ohms.
• If all Powertrain high speed GMLAN serial data circuits are greater than the specified range, replace the control module that was just disconnected.
3. Repeat step 1 until one of the follow conditions are isolated:
* A short to ground on the Powertrain high speed GMLAN serial data circuit between 2 control modules or splice packs, if equipped.
* A short to ground on the Powertrain high speed GMLAN serial data circuit between a control module and a terminating resistor.
Testing the Serial Data Circuits for a Short Between The Circuits
1. Connect the harness connectors at the control module that was previously disconnected.
2. Ignition OFF, disconnect the harness connectors with the Powertrain high speed GMLAN serial data circuits at another control module that is not communicating, in the direction of the circuits shorted together.
3. 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 110 ohms between each pair of Powertrain high speed GMLAN serial data circuits at the control module connector that was just disconnected. Verify that one pair of Powertrain high speed GMLAN serial data circuits are less than 110 ohms.
• If each pair of Powertrain high speed GMLAN serial data circuits is greater than the specified range, replace the control module that was just disconnected.
4. Repeat step 1 until one of the following conditions are isolated:
* Powertrain high speed GMLAN serial data circuits shorted together between 2 control modules or splice packs, if equipped
* Powertrain high speed GMLAN 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. Connect the harness connectors at the control module that was previously disconnected.
2. Ignition OFF, disconnect the harness connectors with the Powertrain high speed GMLAN serial data circuits at another control module that is not communicating, in the direction of the open circuit.
3. 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 130 ohms between each pair of Powertrain high speed GMLAN serial data circuits at the control module connector that was just disconnected. Verify that one pair of Powertrain high speed GMLAN serial data circuits are greater than 130 ohms.
• If each pair of Powertrain high speed GMLAN serial data circuits is less than the specified range, replace the control module that was just disconnected.
4. Repeat step 1 until one of the following conditions are isolated:
* An open/high resistance on the Powertrain high speed GMLAN serial data circuit between 2 control modules or splice packs, if equipped
* An open/high resistance on the Powertrain high speed GMLAN 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. Test for less than 10 ohms between each ground circuit terminal at the control module that was just disconnected and ground.
• If greater than the specified range, test the ground circuit for an open/high resistance.
2. If equipped, ignition ON, verify that a test lamp illuminates between each B+ circuit terminal and ground.
• If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the control module for a short to ground. If the circuits test normal, replace the appropriate control module.
3. If equipped, 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 circuit fuse is open, test the control circuits of the control module for a short to ground. If the circuits test normal, replace the appropriate control module.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the repair.
* GMLAN Wiring Repairs GMLAN Wiring Repairs
* Control Module References Programming and Relearning for control module replacement, programming and setup