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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 Communication Bus B Off

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Battery Positive Voltage (B+)

1

1

--

--

Ground (DLC, terminal 5)

--

1

--

--

Mid Speed GMLAN Serial Data (+)

1

U0074 00, 3*

1

--

Mid Speed GMLAN Serial Data (-)

1

U0074 00, 3*

1

--

Chassis High Speed GMLAN Serial Data (+)

2

U0074 00, 3*

U0074 00, 2

-

Chassis High Speed GMLAN Serial Data (-)

2

U0074 00, 3*

U0074 00, 2

--

* 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 GMLAN module.

2. No communication on any chassis high speed GMLAN module.

3. 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

    •  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 Verification
  1. Verify there are no other U code DTCs set.
    2. ⇒If other U code DTCs are set, diagnose them first. Refer to Control Module U Code List .
  2. 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.

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. 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.
    2. ⇒If only one control module is not communicating, diagnose that control module only. Refer to DTC U0100-U02FF .
  2. Disconnect the scan tool from the X84 DLC. The following tests will be done at the X84 DLC connector.
  3. Ignition 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.
    4. ⇒If greater than the specified range, test the ground circuit for an open/high resistance.
  4. Ignition ON, test for less than 4.5 V between each serial data circuit listed below and ground:
    5. • 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.
  5. Ignition 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 each serial data circuit listed below and ground:
    6. • 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.
  6. Test for 50-70 Ω between the serial data circuits listed below:
    7. • 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.
  7. If the circuits test normal, refer to Testing the Ignition Circuit.

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.
    3. ⇒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.
    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
    • 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 metres 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.
    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 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.
    5. ⇒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:
    6. • 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 metres 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.
    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 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.
    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

Testing the Serial Data Circuits for a Open/High Resistance

  1. Ignition 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. 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.
    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 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.
    6. ⇒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:
    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 Ignition Circuit

  1. Ignition OFF, disconnect the harness connectors with the ignition circuits at an easily accessible control module that is not communicating.
  2. Ignition OFF, verify that a test lamp does not illuminate between each ignition circuit terminal and ground, at the control module connector that was just disconnected.
    3. ⇒If the test lamp does illuminate, test the ignition circuit for a short to voltage. If the circuits test normal, replace the control module that was just disconnected.
  3. Ignition ON, verify that a test lamp illuminates between each ignition circuit terminal and ground.
    4. ⇒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 control module that was just disconnected.
  4. Ignition OFF, connect the harness connectors at the control module that was just disconnected.
  5. 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 ignition circuits.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

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


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