LEMON Manuals: Even more car manuals for everyone: 1960-2025
Home >> Buick >> 2012 >> Regal Base >> Repair and Diagnosis >> Engine Performance >> System >> Data Communications System >> Diagnostic Information and Procedures >> DTC U0074 (Powertrain Expansion Bus) >> Circuit/System Testing
April 5, 2026: LEMON Manuals is launched! Read the announcement.

Circuit/System Testing

WARNING:

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.

WARNING:

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 Ω 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 Ω between the Powertrain high speed GMLAN serial data circuits at the control module that was just disconnected.
    • If less than 110 Ω, 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 Ω 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 Ω 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 Ω.
    • 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 Ω 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 Ω.
    • 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 Ω 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 Ω.
    • 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 Ω 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.
scientia non olet · About LEMON Manuals