GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2008 🢒 Cobalt 🢒 Power and Signal Distribution 🢒 Data Communications 🢒 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

--

--

Battery Positive Voltage (B+)

1

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 on any high speed GMLAN module
  2. 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
    • 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.
    • Use the Data Link References to identify the high speed GMLAN serial data modules.
    •  This test is used for a total high speed GMLAN communication failure. If only one module is not communicating and sets no DTC, ensure that the vehicle is equipped with the module, then use DTC U0100-U0299 for diagnostics.
    • An open in the DLC ground circuit terminal 5 will allow the scan tool to operate and set up the vehicle on the tool and then not communicate with the vehicle. When the scan tool is to the point of communicating with the vehicle, a message on the scan tool will indicate "no CANdi module detected" and will not communicate.
    • The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus.
        The following conditions may cause a total loss of high speed GMLAN data communication:
       - A short between high speed GMLAN (+) and high speed GMLAN (-) circuits
       - Any of the high speed GMLAN serial data circuits shorted to ground or voltage
       - A module internal malfunction that causes a short to voltage or ground on the high speed GMLAN circuits

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
  1. Test for less than 1.0 ohm between the ground circuit terminal 5 of the DLC and ground.
    2. ⇒If greater than the specified range, test the ground circuit for open/high resistance.
  2. Ignition OFF, disconnect the X2 harness connector at the BCM.
  3. Test for less than 2 ohms on the serial data circuit between terminal 6 of the DLC and terminal 1 of the BCM.
    4. ⇒If greater than the specified range, test the serial data circuit for open/high resistance.
  4. Ignition OFF, disconnect the X4 harness connector at the BCM.
  5. Test for less than 1 ohm between ground circuit terminal E12 and ground.
    6. ⇒If greater than the specified range, test the ground circuit for an open/high resistance.
  6. Ignition OFF, disconnect the X3 harness connector at the BCM.
  7. Ignition ON, verify a test lamp illuminates between each B+ circuit terminal D12 of the BCM and ground.
    8. ⇒If the test lamp does not illuminate, test the B+ circuit for an open/high resistance or short to ground.
  8. Ignition ON, test for less than 1 volt between the serial data circuit terminal 6 of the DLC and ground, and terminal 14 of the DLC and ground.
    9. ⇒If greater than the specified range, test the serial data circuit for a short to voltage.
  9. Test for infinite resistance between the serial data circuit terminal 6 of the DLC and ground, and terminal 14 of the DLC and ground.
    10. ⇒If less than the specified value, test the serial data circuit for a short to ground.
  10. Disconnect the X2 harness connector at the electronic power steering control module (EPS).
  11. Test for infinite resistance between the serial data circuits terminal 6 and terminal 14 of the DLC.
    12. ⇒If less than the specified value, test the serial data circuits for a short between them.
  12. Connect the X2 X3 and X4 harness connectors at the BCM and the X2 harness connector at the EPS.
  13. Disconnect the appropriate harness connector at the ECM.
    14. • RPO L61/LE5 -- X1
    • RPO LNF -- X2
  14. Attempt to communicate with the vehicle control interface module (VCIM). Communication should not be available.
    15. ⇒If communication is available, replace the ECM.
  15. Ignition OFF, disconnect the harness connector at the TCM.
  16. Attempt to communicate with the VCIM. Communication should not be available.
    17. ⇒If communication is available, test the following serial data circuits for a short between the circuits, short to voltage, or a short to ground between the TCM and the ECM. If the circuits test normal, replace the TCM.
    • RPO L61/LE5 -- terminal 43
    • RPO L61/LE5 -- terminal 44
    • RPO L61/LE5 -- terminals 43 and 44
    • RPO LNF --terminal 33
    • RPO LNF -- terminal 53
    • RPO LNF -- terminals 33 and 53
  17. Ignition OFF, disconnect the harness connector atthe EBCM.
  18. Attempt to communicate with the VCIM. Communication should not be available.
    19. ⇒If communication is available, test the folowing serial data circuits for a short between the circuits, short to voltage, or a short to ground between the EBCM and the TCM. If the circuits test normal, replace the EBCM.
    • RPO JM4 - terminal 21
    • RPO JM4 - terminal 23
    • RPO JM4 - terminals 21 and 23
    • RPO JL4 - terminal 14
    • RPO JL4 - terminal 11
    • RPO JL4 - terminals 14 and 11
  19. Ignition OFF, disconnect the X3 harness connector at the VCIM.
  20. Attempt to communicate with the EPS. Communication should not be available.
    21. ⇒If communication is available, test the following serial data circuits for a short between the circuits, short to voltage, or a short to ground between the VCIM and the TCM. If the circuits test normal, replace the VCIM.
    • terminal 9
    • terminal 8
  21. Ignition OFF, disconnect the X2 harness connector at the BCM. Attempt to communicate with the EPS. Communication should not be available.
    22. ⇒If communication is available, test the following serial data circuits for a short between the circuits, short to voltage, or a short to ground between the BCM and the VCIM. If the circuits test normal, replace the BCM.
    • terminal 2
    • terminal 20
  22. Ignition OFF, disconnect the X2 harness connector at the EPS.
  23. Ignition ON, test for less than 1 volt between the following serial data circuits of EPS and ground.
    24. • terminal 4
    • terminal 5
    ⇒If greater than the specified range, test the circuit for a short to voltage.
  24. Test for infinite resistance between the following serial data circuits of EPS and ground.
    25. • terminal 4
    • terminal 5
    ⇒If less than the specified value, test the circuit for a short to ground.
  25. Test for infinite resistance between the serial data circuits terminal 4 terminal 5 of the EPS.
    26. ⇒If less than the specified Value, test the serial data circuits for a short between them.
  26. If the circuits test normal, replace the EPS.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

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