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For 1990-2009 cars only
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Circuit 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.

Diagnostic Aids

The engine will not start when there is a malfunction of the class 2 serial data circuit between the vehicle theft deterrent (VTD) system and the powertrain control module (PCM) while the engine is not running. The following conditions will cause a total loss of class 2 serial data communication:

    • A class 2 serial data circuit shorted to ground
    • A class 2 serial data circuit shorted to voltage
    • An open in the class 2 serial data circuit between the VTD system and the PCM
    • An internal malfunction of a module on the class 2 serial data circuit that causes a short to voltage or ground

On some vehicles, if the PCM is unable to communicate with the VTD system after the vehicle has started, the PCM will consider the VTD system to be malfunctioning. The PCM will enter a fail enable state and will command the security indicator to illuminate. When the PCM is in a fail enable state the vehicle will NOT stall or stop running. If the PCM is in a fail enable state when the ignition is switched OFF, the PCM will remain fail enable until communications with the VTD system has been restored. When the PCM is in a fail enable state the VTD system is NOT active and the vehicle will start. This feature is NOT available on all GM vehicle lines.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. A partial malfunction in the class 2 serial data circuit uses a different procedure from a total malfunction of the class 2 serial data circuit. The following modules communicate on the class 2 serial data circuit:

    2. • Chime module
    • Electronic brake control module (EBCM)
    • Instrument panel cluster (IPC)
    • PCM
    • Radio
    • Serial data gateway module (SDGM)
    • Inflatable restraint sensing and diagnostic module (SDM)
    • Transmission control module (TCM)

  2. The following DTCs may be retrieved with a history status, but are not the cause of the present condition.

    3. • U1300
    • U1301
    • U1305

  3. A state of health DTC with a history status may be present along with a U1000 or U1255 code having a current status. This indicates that the malfunction occurred when the ignition was ON.

    4.

  4. Data link connector terminals 2 and 5 provide the connection to the class 2 serial data circuit and the signal ground circuit respectively.

    5.

  5. A poor connection at star connector terminal A would cause this condition but will not set a DTC.

    6.

  6. An open in the class 2 serial data circuit between the DLC and star connector will prevent the scan tool from communicating with any module. This condition will not set a DTC.

    7.

  7. The class 2 serial data circuit is shorted to voltage or ground. The condition may be due to the wiring or due to a malfunction in one of the modules. When testing the wire for a short, make sure there is not a module connected to the wire being tested. This test isolates the IPC class 2 serial data circuit.

    8.

  8. The IPC detects that the ignition is ON and sends the appropriate power mode message to the other modules. Therefore, the IPC must remain connected to the DLC for any other module to communicate with the scan tool. This test isolates the star connector serial data circuits.

    9.

  9. If there are no current DTCs that begin with a "U", the communication malfunction has been repaired.

    10.

  10. The communication malfunction may have prevented diagnosis of the customer complaint.

Step

Action

Yes

No

Schematic Reference: Data Communication Schematics

Connector End View Reference: Data Communication Connector End Views

1

Does the scan tool power up?

Go to Step 2

Go to Scan Tool Does Not Power Up

2

  1. Turn ON the ignition leaving the engine OFF.
  2. Attempt to communicate with each module on the class 2 serial data circuit. If using a Tech 2, obtain this information using the Class 2 Message Monitor feature.

Does the scan tool communicate with any module on the class 2 serial data circuit?

Go to Step 3

Go to Step 7

3

  1. Select the Display DTCs function for each module. If using a Tech 2, use the Class 2 DTC Check feature in order to determine which modules do have DTCs set.
  2. Record all of the displayed DTCs, the DTC status and the module which set the DTC.

Did you record any DTCs in the range of U1000 to U1305?

Go to Step 4

Go to Diagnostic Aids

4

Are the DTCs U1300, U1301 or U1305 retrieved from any module?

Go to Step 5

Go to Step 6

5

Important:: Turn ON the ignition, with the engine OFF, when testing for a short to voltage. Use the DMM MIN/MAX function to capture intermittent conditions.

Test the class 2 serial data circuit for an intermittent short to ground or an intermittent short to voltage. Refer to the following:

    •  Circuit Testing
    •  Connector Repairs
    •  Wiring Repairs

Did you find and correct the condition?

Go to Step 17

Go to Step 6

6

Is U1000 or U1255 the only DTC displayed in the previously specified range?

Go to DTC U1000 and U1255

Go to DTC U1001-U1254

7

  1. Turn OFF the ignition.
  2. Disconnect the scan tool from the data link connector (DLC).
  3. Inspect for poor connections at the DLC terminals 2 and 5. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Diagnostic System Check - Vehicle

Go to Step 8

8

Test the DLC ground circuit at terminal 5 for an open. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Diagnostic System Check - Vehicle

Go to Step 9

9

  1. Disconnect the star connector.
  2. Inspect for poor connections at the star connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Diagnostic System Check - Vehicle

Go to Step 10

10

Test the class 2 serial data circuit between the DLC and the star connector, terminal A for an open or a short. Turn ON the ignition leaving the engine OFF when testing for a short to voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Diagnostic System Check - Vehicle

Go to Step 11

11

  1. Using the correct terminal adapter, connect a jumper wire to the DLC terminal of the star connector.
  2. Connect the other end of the jumper wire, using correct terminal adapter, to the instrument panel cluster (IPC) terminal of the star connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Attempt to communicate with the IPC.

Does the scan tool communicate with the IPC?

Go to Step 13

Go to Step 12

12
  1. Turn OFF the ignition.
  2. Disconnect the IPC.
  3. Test the class 2 serial data circuit between the star connector and the IPC for an open, a short to ground or a short to voltage. Turn ON the ignition leaving the engine OFF when testing for a short to voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 17

Go to Step 15

13

Repeat the following steps on the class 2 serial data circuits connected to the star connector until the concern is located:

  1. Leave the jumper wire connected to the DLC terminal and to the IPC terminal of the star connector.
  2. Connect another jumper wire from the jumper wire connected to the DLC terminal of the star connector.
  3. Connect the other end of the jumper wire, using correct terminal adapter, to the terminal for the devices connected to star connector.
  4. Turn ON the ignition, with the engine OFF.
  5. Attempt to communicate with the IPC.

Does the scan tool communicate with the IPC?

Go to Step 17

Go to Step 14

14
  1. Turn OFF the ignition.
  2. Disconnect the module that caused the no communication condition.
  3. Test the class 2 serial data circuit between the star connector and the respective module for a short to ground or a short to voltage. Turn ON the ignition leaving the engine OFF when testing for a short to voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 17

Go to Step 15

15

Inspect for poor connections at the class 2 serial data circuit of the last disconnected module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 17

Go to Step 16

16

Replace the last disconnected module. Refer to Control Module References for the appropriate repair instructions.

Did you complete the replacement?

Go to Step 17

--

17

  1. Reconnect all of the disconnected modules.
  2. Reconnect any other disconnected connectors.
  3. Install a scan tool.
  4. Turn ON the ignition, with the engine OFF.
  5. Wait for 10 seconds.

    6. The scan tool may require a power up reset before communication will occur due to a short on the class 2 serial data circuit. Turn OFF or disconnect the scan tool prior to performing this test.

  6. Select the Display DTCs function for each module. If using a Tech 2, use the Class 2 DTC Check feature in order to determine which modules do have DTCs set.
  7. Record all of the displayed DTCs and the DTC status.

Did you record any DTCs which begin with a "U" and with a current status?

Go to Step 19

Go to Step 18

18

Did you record any DTCs which do not begin with a "U"?

Go to Step 19

Go to Step 21

19

Diagnose the DTCs as directed by the diagnostic procedures for the particular module or malfunction. Refer to Control Module References for the applicable Diagnostic System Check.

Did you complete the action?

Go to Step 20

--

20

Did you diagnose all of the DTCs?

Go to Step 21

Go to Step 19

21

Use the scan tool in order to clear the DTCs.

Did you complete the action?

System OK

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