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Scan Tool Does Not Communicate with Low Speed GMLAN Device

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.

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Low Speed GMLAN Serial Data

1

U0100-U02FF*

1

--

Ground (DLC, terminal 5)

--

1*

--

--

* An open between the data link connector serial data circuit terminal 1 or ground circuit terminal 5 and the BCM circuit will only effect the communication with the scan tool.

1. No communication on any low speed GMLAN module

Circuit/System Description

The serial data is transmitted over a single wire to the appropriate control modules. The transmission speed for GMLAN low-speed is up to 83.33 Kb/s. Under normal vehicle operating conditions, the speed of the bus is 33.33 Kb/s. The control modules toggle the serial data circuit between 0-5 V during normal communications. To wake the control modules connected to the GMLAN low speed serial data circuit, a voltage wake up pulse of 10 V is sent out. If serial data is lost, control modules will set a no communication code against the non-communicating control module. A loss of serial data communications DTC does not represent a failure of the module that set it.

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-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 body control module (BCM) and the scan tool with the low speed GMLAN serial data system inoperative. This condition is due to the BCM using both the high and low speed GMLAN systems.
    • An open in the low speed GMLAN serial data circuit between the splice pack and a module will only affect that specific module. This type of failure will set a loss of communication DTC for each module effected, and the other modules will still communicate.
    • An open-circuit in the data link connector (DLC) ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle.
    • The engine may not start when there is a total malfunction of the low speed GMLAN serial data circuit.
    • Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed LAN communications with the scan tool.
        These conditions may be caused by the installation of an aftermarket navigation radio module (see bulletins). Some customers may comment of one or more of the following concerns:
       - Vehicle will not crank
       - Vehicle cranks but will not start
       - Vehicle stability enhancement system warning lights and messages
       - PRNDL gear indicator position errors

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

Circuit/System Testing

Note: Use the schematic to identify the following:

   • Control modules the vehicle is equipped with
   • Control module and splice pack locations on the low speed GMLAN serial data circuit
   • The low speed GMLAN serial data circuit terminals for each control module, component, or splice pack

  1. Attempt to communicate with all control modules on the low speed GMLAN serial data circuit. Refer to Data Link References . Communications should not be available with two or more control modules on the low speed GMLAN serial data circuit.
    2. ⇒If only one control module is not communicating, diagnose that control module only. Refer to DTC U0100-U02FF .
    ⇒If one or more control modules are communicating but not all, refer to Testing the Serial Data Circuit for an Open/High Resistance.
  2. Vehicle OFF, disconnect the scan tool from the X84 data link connector (DLC). The following tests will be done at the DLC connector.
  3. 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.
    4. ⇒If greater than the specified range, test the ground circuit for an open/high resistance.
  4. Vehicle in Service Mode, test for less than 4.5 V between the serial data circuit terminal 1 and earth.
    5. ⇒If greater than the specified range, test the serial data circuit for a short to voltage. Refer to Testing the Serial Data Circuit for a Short to Voltage.
  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 100 Ω between the serial data circuit terminal 1 and earth.
    6. ⇒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. Disconnect the harness connector at the first splice pack closest in the circuit to the DLC.
  7. Test for less than 2 Ω between the DLC serial data circuit terminal 1 and the splice pack connector serial data input terminal.
    8. ⇒If greater than the specified range, test the serial data circuit for an open/high resistance. If the circuit tests normal, replace the splice pack.

Testing the Serial Data Circuits for a Short to Voltage

  1. Vehicle OFF, disconnect the appropriate harness connectors at the following components:
    2. • All low speed serial data splice packs
    • Any control module or component that may be between the splice packs
  2. Vehicle in Service Mode, test for less than 4.5 V between the DLC serial data circuit terminal 1 and earth.
    3. ⇒If greater than the specified range, repair the short to voltage.
  3. Test for less than 4.5 V between each serial data circuit at each component connector and ground.
    4. ⇒If greater than the specified range, perform the appropriate test listed below:
    • Serial data circuits with one control module, test the serial data circuit for a short to voltage. If the circuit tests normal, replace the control module.
    • Serial data circuits with 2 or more control modules, test each section of the serial data circuit for a short to voltage. If the circuits test normal, replace the control module that causes the short to voltage when connected.

Testing the Serial Data Circuits for a Short to Ground

  1. Vehicle OFF, disconnect the appropriate harness connectors at the following components:
    2. • All low speed serial data splice packs
    • Any control module or component that may be between the splice packs
  2. Test for infinite resistance between the DLC serial data circuit terminal 1 and ground.
    3. ⇒If not the specified value, repair the short to ground.
  3. 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 each serial data circuit at the splice pack and earth.
    4. ⇒If less than the specified range, perform the appropriate test listed below:
    • Serial data circuits with one control module, test the serial data circuit for a short to ground. If the circuit tests normal, replace the control module.
    • Serial data circuits with 2 or more control modules, test each section of the serial data circuit for a short to ground. If the circuits test normal, replace the control module that causes the short to ground when connected.

Testing the Serial Data Circuit for an Open/High Resistance

  1. If equipped with a second splice pack, Vehicle in Service Mode, verify the scan tool communicates with one or more control modules connected to the second splice pack.
    2. ⇒If all control modules on the second splice pack do not communicate, test the serial data circuit between the first and second splice packs for an open/high resistance. If the circuit tests normal, test the second splice pack for an open/high resistance. If the splice pack tests normal, replace any control module or component that may be between those two splice packs.
  2. If equipped with a third splice pack, Vehicle in Service Mode, verify the scan tool communicates with one or more control modules connected to the third splice pack.
    3. ⇒If all control modules on the third splice pack do not communicate, test the serial data circuit between the second and third splice packs for an open/high resistance. If the circuit tests normal, test the third splice pack for an open/high resistance. If the splice pack tests normal, replace any control module or component that may be between the those two splice packs.
  3. Vehicle OFF, disconnect the splice pack containing the serial data circuits to the control modules that are not communicating.
  4. Install a 3 A fused jumper wire between the splice pack connector serial data input terminal and a low speed GMLAN serial data circuit that is not communicating.
  5. Vehicle in Service Mode, verify that the scan tool communicates with the control modules connected to the low-speed GMLAN serial data circuit.
    6. ⇒If any control module does not communicate, test each section of the serial data circuit for an open/high resistance. If the circuits test normal, replace the control module that does not communicate when connected.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

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


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