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For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2008 🢒 Corvette 🢒 Diagnostic Navigation 🢒 Vehicle Diagnostic Information 🢒 Scan Tool Does Not Communicate with High 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

High Speed GMLAN Serial Data (+)

1

2*

1

--

High Speed GMLAN Serial Data (-)

1

2*

1

--

Ground (DLC, terminal 5)

--

1

--

--

* An open between the data link connector (DLC) and the first splice will only effect the communication with the scan tool. The vehicle modules will still communicate.

1. No communication with any high speed GMLAN module

2. No communications with one or more high speed GMLAN modules. An open in only one high speed GMLAN serial data circuit may allow degraded communication between the modules

Circuit/System Description

Modules connected to the high speed GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules when the ignition switch is in any position other than OFF. The high speed GMLAN serial data circuits must be operational for the vehicle to start due to body control module (BCM) and engine control module (ECM) communications.

Diagnostic Aids
    • A control module may have a U code stored in history that does not require any repairs. Issues with late or corrupted messages between control modules can be temporary with no apparent symptom or complaint; this does not mean the control module is faulty. Do not replace a control module based only on a history U code.
    • 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 BCM and the scan tool with the high 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 DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle. When the scan tool attempts to communicate with the vehicle a message "no CANdi module detected" will be displayed.
    • The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus.
    • Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed GMLAN communications with the scan tool.
    • The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus.
        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.
       - Tire Pressure Monitor (TPM) system warning lights.

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: 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
   • High speed GMLAN serial data circuit terminating resistors
   • Control module locations on the high speed GMLAN serial data circuits
   • Each control module's high speed GMLAN serial data circuit terminals

  1. Attempt to communicate with all control modules on the high speed GMLAN serial data circuit, refer to Data Link References. Communications should not be available with two or more control modules on the high speed GMLAN serial data circuit.
    2. ⇒If only one control module is not communicating, diagnose that control module only; refer to DTC U0100-U0299 or U2105-U2199.
  2. Disconnect the scan tool from the DLC. The following tests will be done at the DLC connector.
  3. All access doors closed, ignition OFF for 60 seconds, 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 3V between the serial data circuits listed below and ground:
    5. • Terminal 6
    • Terminal 14
    ⇒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 for 60 seconds, test for greater than 100 Ω between the serial data circuits listed below and ground:
    6. • Terminal 6
    • Terminal 14
    ⇒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.
  6. Test for 50-70 Ω between the serial data circuits terminal 6 and terminal 14.
    7. ⇒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.

Testing the Serial Data Circuits for a Short to Voltage

  1. Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module.
  2. Ignition ON, test for greater than 3V 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 greater than 3V.
    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 high speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to voltage.
  4. Ignition ON, test for greater than 3V 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 greater than 3V.
    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, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module.
  2. Ignition OFF for 60 seconds, test for less than 100 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 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 high speed GMLAN serial data circuits at another control module, in the direction of the circuit shorted to ground.
  4. Ignition OFF for 60 seconds, test for less than 100 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 100 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
    • 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, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module, that is not communicating.
  2. Ignition OFF for 60 seconds, 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 high speed GMLAN serial data circuits at another control module, in the direction of the circuits shorted together.
  5. Ignition OFF for 60 seconds, 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 within 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
  1. Ignition OFF, disconnect the harness connectors with the high speed GMLAN serial data circuits at an easily accessible control module, that is not communicating.
  2. Ignition OFF for 60 seconds, 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 high speed GMLAN serial data circuits at another control module, in the direction of the open circuit.
  5. Ignition OFF for 60 seconds, 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.
    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

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

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