GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Isuzu 🢒 2008 🢒 Ascender - 2WD 🢒 Suspension 🢒 Air Suspension 🢒 Scan Tool Does Not Communicate with Class 2 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

Class 2 Serial Data

1

2*

1

--

Ground (DLC, Terminal 5)

--

1*

--

--

* An open between the data link connector (DLC) and JX 205, or DLC terminal 5 ground will only effect the communication with the scan tool. Refer to Diagnostic Aids.

1. No communication on any class 2 serial data module

2. No communication on a specific class 2 serial data module

Circuit/System Description

Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. Connecting a scan tool to the data link connector (DLC) allows communication with the modules for diagnostic purposes. DTCs may be set due to this symptom and during this diagnostic procedure. Complete the diagnostic procedure in order to ensure all the DTCs are diagnosed and cleared from memory.

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-initialize 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 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
    •  Splice Pack 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

Class 2 Communication Initial Test

  1. Test for less than 1 ohm between the DLC ground circuit terminal 5 and ground.
    2. ⇒If greater than the specified range, test the ground circuit for open/high resistance.
  2. Ignition OFF, disconnect the harness connector of JX 306 and attempt to communicate with the BCM. Communication should not be available.
    3. ⇒If communication is available, refer to JX 306 Test.
  3. Ignition OFF, disconnect the harness connector of JX 205.
  4. Ignition ON, test for less than 1 volt between the serial data circuit terminal H of JX 205 and ground.
    5. ⇒If greater than the specified range, test the serial data circuit for a short to voltage.
  5. Test for infinite resistance between the serial data circuit terminal H of JX 205 and ground.
    6. ⇒If less than the specified value, test the serial data circuit for a short to ground.
  6. Ignition ON, test for less than 1 volt between the serial data circuit terminal A of JX 205 and ground.
    7. ⇒If greater than the specified range, test the serial data circuit for a short to voltage.
  7. Test for infinite resistance between the serial data circuit terminal A of JX 205 and ground.
    8. ⇒If less than the specified value, test the serial data circuit for a short to ground.
  8. Test for less than 1 ohm on the serial data circuit between terminal A of JX 205 and terminal 2 of the DLC.
    9. ⇒If greater than the specified range, test the serial data circuit for open/high resistance.
  9. Ignition OFF, disconnect the harness connectors X1 and X3 of the BCM
  10. Verify a test lamp illuminates between each ground circuit terminal of the BCM listed below and B+.
    11. • X3 - A11
    • X3 - B11
    ⇒If the test lamp does not illuminate, test the ground circuit for an open/high resistance.
  11. Ignition ON, verify a test lamp illuminates between each voltage circuit terminal of the BCM listed below and ground.
    12. • X1 - A3
    • X1 - A19
    • X3 - A1
    • X3 - A2
    • X3 - A20
    • X3 - B4
    • X3 - B13
    • X3 - B15
    • X3 - B17
    • X3 - B18
    ⇒If the test lamp does not illuminate, test the voltage circuit for an open/high resistance or short to ground.
  12. Ignition OFF, connect the harness connectors of the BCM.
  13. Ignition OFF, connect a 3A fused jumper wire between terminal A and terminal M of JX 205. Attempt to communicate with the BCM. Communication should be available.
    14. ⇒If communication is not available, test the serial data circuit for a short to voltage, or short to ground between the BCM and JX 205. If the circuits test normal, replace the BCM.

    Important: After each connection, wait for 5 seconds to determine that communication remains available.

  14. Connect another 3A fused jumper wire to terminal A of JX 205. Individually connect the other end of the jumper wire to each other terminal of JX 205 and verify that communication remains available to the BCM.
    15. ⇒If communication is interrupted, test the serial data circuit between JX 205 and the last module connected for a short to ground or a short to voltage. If the circuit tests normal, replace the module that caused no communication.

JX 306 Test

  1. Ignition OFF, connect a 3A fused jumper wire to terminal M of JX 306.

    2. Important: After each connection, wait for 5 seconds to determine that communication remains available.

  2. While communicating with the BCM, individually move the other end of the jumper wire to each of the terminals of JX 306 and verify that communication remains available to the BCM.
    3. ⇒If communication is interrupted, test the serial data circuit between JX 306 and the last module connected for a short to ground or a short to voltage. If the circuit tests normal, replace the module that caused no communication.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the repair.

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