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DTC U0074

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    • Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptor

DTC U0074 00: Control Module Powertrain Expansion Communication Bus Off

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Powertrain High-Speed GMLAN Serial Data (+)

U0074

U0074, U0100-U02FF*

U0074

-

Powertrain High-Speed GMLAN Serial Data (-)

U0074

U0074, U0100-U02FF*

U0074

-

Earth (auxiliary data connector terminal 5)

-

U0074

--

--

* An open-circuit in only one powertrain high-speed GMLAN serial data circuit may allow degraded communication between the control modules.

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 Ω resistors, one is internal to the engine control module (ECM) and the other can be a separate resistor in a connector assembly or in another 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 module that set it.

The powertrain high-speed GMLAN serial data circuits connect directly to the auxiliary data connector. The scan tool does not communicate with powertrain high-speed GMLAN bus.

Conditions for Running the DTC
    • Supply voltage to the control modules are in the normal operating range.
    • The vehicle power mode requires serial data communications.

Conditions for Setting the DTC

The control module setting the DTC has attempted to establish communication on the powertrain high-speed GMLAN serial data circuits more than 3 times.

Action Taken When the DTC Sets
    • The module suspends all message transmission.
    • The MIL and battery indicators will illuminate, along with several DIC messages.
    • DTC U0074 is a type A DTC.

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.
    • DTC U0074 is a type A DTC.

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 control 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-initialise 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 control module and the scan tool with one or more GMLAN serial data systems inoperative. This condition is due to the control module using multiple GMLAN busses.
    • The engine will not start when there is a total malfunction of the powertrain high-speed GMLAN serial data bus.
    • Use Data Link References to determine which control modules connect to the powertrain high-speed GMLAN serial data bus.

Reference Information

Schematic Reference

    •  Data Communication Schematics
    •  Control Module References

Connector End View Reference

Component Connector End Views

Description and Operation

Data Link Communications Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Testing

Note: Some control modules with an internal terminating resistor have a loop in the harness that connects the internal terminating resistor to the serial data circuit. When wired this way, test these loop circuits for the appropriate failure mode short to voltage, short to ground, or open/high resistance prior to replacing the control module for each of the following tests.

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. Disconnect the scan tool from the X84 data link connector (DLC). The following tests will be done at the X84B auxiliary data connector.
  2. Vehicle OFF, and all vehicle systems OFF, all keys at least 3 metres 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.
    3. ⇒If greater than the specified range, test the ground circuit for an open/high resistance.
  3. Vehicle in Service Mode, test for less than 4.5 V between the serial data circuits listed below and earth:
    4. • Terminal 12
    • 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.
  4. Vehicle OFF, and all vehicle systems OFF, all keys at least 3 metres 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 the serial data circuits listed below and ground:
    5. • Terminal 12
    • Terminal 13
    ⇒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 Ω between the serial data circuits terminal 3 and terminal 11.
    6. ⇒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 X84B auxiliary data connector and the first connection to the serial data circuit for an open-circuit/high resistance.
  6. If the serial data circuits test normal, refer to Testing the Control Module Circuits.

Testing the Serial Data Circuits for a Short to Voltage

  1. Vehicle OFF, disconnect the harness connectors with the powertrain high-speed GMLAN serial data circuits at an easily accessible control module.
  2. Vehicle in Service Mode, test for greater than 4.5 V between each serial data circuit at the control module connector that was just disconnected and earth. Verify that one or more serial data circuits are greater than 4.5 V.
    3. ⇒If all serial data circuits are less than the specified range, replace the control module that was disconnected.
  3. Vehicle 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. Vehicle in Service Mode, test for greater than 4.5 V between the serial data circuits at the control module connector that was just disconnected and earth. Verify that one or more serial data circuits are greater than 4.5 V.
    5. ⇒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:
    6. • A short to voltage on the serial data circuit between 2 control modules or splice packs, if equipped
    • 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. Vehicle OFF, disconnect the harness connectors with the powertrain high-speed GMLAN serial data circuits at an easily accessible control module.
  2. Vehicle OFF, and all vehicle systems OFF, all keys at least 3 metres 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 Ω 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 Ω.
    3. ⇒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 earth.
  4. Vehicle OFF, and all vehicle systems OFF, all keys at least 3 metres 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 Ω 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 Ω.
    5. ⇒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:
    6. • A short to ground on the serial data circuit between 2 control modules or splice packs, if equipped
    • A short to ground on the serial data circuit between a control module and a terminating resistor
    • A short-circuit to earth on the serial data circuit between the X84B auxiliary data connector and the first control module or splice pack, if equipped

Testing the Serial Data Circuits for a Short between the Circuits

  1. Vehicle 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. Vehicle OFF, and all vehicle systems OFF, all keys at least 3 metres 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 Ω 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 110 Ω.
    3. ⇒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. Vehicle OFF, and all vehicle systems OFF, all keys at least 3 metres 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 Ω 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 110 Ω.
    6. ⇒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:
    7. • 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
    • Serial data circuits shorted together between the X84B auxiliary data connector and the first control module or splice pack, if equipped

Testing the Serial Data Circuits for an Open/High Resistance

  1. Vehicle 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. Vehicle OFF, and all vehicle systems OFF, all keys at least 3 metres 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 Ω 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 130 Ω.
    3. ⇒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. Vehicle OFF, and all vehicle systems OFF, all keys at least 3 metres 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 Ω between each pair of serial data circuits at the control module connector that was just disconnected. Verify that one pair of serial data circuits is greater than 130 Ω.
    6. ⇒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:
    7. • 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. Vehicle OFF, disconnect the harness connectors at an easily accessible control module that is not communicating.
  2. Vehicle OFF, and all vehicle systems OFF, all keys at least 3 metres 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 each earth circuit terminal and earth, and at the control module connector that was just disconnected.
    3. ⇒If greater than the specified range, test the earth circuit and earth connection for an open-circuit/high resistance.
  3. Vehicle OFF, verify that a test lamp does not illuminate between each ignition circuit terminal and earth.
    4. ⇒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. Vehicle in Service Mode, verify that a test lamp illuminates between each ignition circuit terminal and earth.
    5. ⇒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. Vehicle OFF, connect the harness connectors at the control module that was disconnected.
  6. Repeat step 1 at another control module that is not communicating until one of the following conditions are isolated:
    7. • An open-circuit/high resistance on the earth circuit or earth connection
    • A short-circuit to voltage, a short-circuit to earth or an open-circuit/high resistance on the ignition circuit

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  GMLAN Wiring Repairs
    • Control Module References for control module replacement, programming and setup
   


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