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For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2004 🢒 Classic 🢒 Preface 🢒 Cautions and Notices 🢒 Scan Tool Does Not Communicate with Class 2 Device

Circuit Description

Each control module connected to the Class 2 serial data circuit monitors serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. Connecting a scan tool to the DLC allows communication with the modules for diagnostic purposes. Diagnostic trouble codes (DTCs) may 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

The engine will not start when there is a total malfunction of the Class 2 serial data circuit while the engine is not running.

The Class 2 serial data circuit for the EBCM and PCM are on a common terminal on the junction block-left I/P.

The Class 2 serial data circuit for the BCM and SDM are on a common terminal on the junction block-left I/P.

The Class 2 serial data circuit for the DLC and Radio are on a common terminal on the junction block-left I/P.

Test Description

  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.

    2.

  2. DTCs U1300, U1301 and 1305 may be retrieved with a history status. These DTCs are not the cause of the present condition.

    3.

  3. When testing for shorts to B+ the tests must be run with ignition switch in the RUN position.

    4.

  4. A state of health DTC U1XXX with a history status may be present along with a U1000 or U1255 having a current status. This indicates the malfunction occurred when the ignition was on.

    5.

  5. Data link connector (DLC) terminals 2 and 5 provide the connection from the vehicles Class 2 serial data circuit and the Tech 2. A poor connection at this connector could cause corrupted data transmissions between the two devices. Pin 2 is the signal terminal and pin 5 is the signal ground terminal.

    6.

  6. This step tests for an open from pin 2 of the DLC connector and the fuse block-left I/P harness connector, it also tests for a short to B+ and ground between the fuse block-left I/P stationary junction terminals.

  7. This is an active test of the Class 2 serial data circuit with the radio being isolated from the communication bus, from the procedures in step 9. If you are able to communicate with the Class 2 serial data circuit during this step, the radio should be replaced.

    8.

  8. This is an active test of the Class 2 serial data circuit with the BCM isolated from the communication bus. If you are able to communicate with the Class 2 serial data circuit during this step, the BCM should be replaced.

    9.

  9. This is an active test of the Class 2 serial data circuit with the SDM isolated from the communication bus. If you are able to communicate with the Class 2 serial data circuit during this step, the SDM should be replaced.

    10.

  10. This is an active test of the Class 2 serial data circuit with the EBCM isolated from the communication bus. If you are able to communicate with the Class 2 serial data circuit during this step, the EBCM should be replaced.

    11.

  11. This is an active test of the Class 2 serial data circuit with the PCM isolated from the communication bus. If you are able to communicate with the Class 2 serial data circuit during this step, the PCM should be replaced. The SDM is reconnected to allow at least one possible module for the Tech 2 to communicate with.

    12.

  12. This is an active test of the Class 2 serial data circuit with the RCDLR, if the vehicle is equipped, isolated from the communication bus. If you are able to communicate with the Class 2 serial data circuit during this step, the RCDLR should be replaced.

    13.

  13. This step isolates all the modules from the Class 2 serial data circuit and tests for a short to B+ or ground. The harness connector to the fuse block-left I/P must be installed and ignition switch placed in the RUN position in order to supply the B+ circuits routed throughout the vehicle.

    14.

  14. In this step instructions are given to analyze each branch of the Class 2 serial data circuit, in order to isolate the cause of the short.

    15.

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

    16.

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

Step

Action

Yes

No

Schematic Reference: Data Link Connector Schematics

Connector End View Reference: Master Electrical Component List and Inline Harness Connector End Views in Wiring Systems

1

Install a scan tool.

Does the scan tool power up?

Go to Step 2

Go to Scan Tool Does Not Power Up

2

  1. Turn on the ignition with 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 DTC's in the range of U1000 to U1305?

Go to Step 4

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

4

Are DTC's 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 in Wiring Systems for the proper procedures:

   •  Circuit Testing
   •  Connector Repairs
   •  Wiring Repairs

Did you complete the repair?

Go to Step 21

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6

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

Go to DTC U1000 and U1255

Go to Diagnostic Trouble Code (DTC) List

7

  1. Turn off the ignition.
  2. Disconnect the scan tool from the data link connector DLC.
  3. Inspect for poor connections/terminal tension, at the DLC terminals 2 and 5. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems for the proper procedure.

Did you find and correct the condition?

Go to Step 20

Go to Step 8

8

Test the DLC ground circuit terminal 5 for a high resistance or an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems for the proper procedure.

Did you find and correct the condition?

Go to Step 20

Go to Step 9

9

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.

  1. Disconnect the fuse block-left I/P connectors that contain the Class 2 serial data circuits. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's.
  2. Disconnect the connector containing the Class 2 serial data circuit from the radio. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's.
  3. Inspect the fuse block-left I/P connectors at the terminals containing the Class 2 serial data circuits for poor connections/terminal tension. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems for the proper procedure.
  4. Test for an open circuit between pin 2 on the DLC connector and the harness connector of the fuse block-left I/P. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's and Circuit Testing in Wiring Systems for the proper procedure.
  5. Test for a short to B+ and ground at pin 2 on the DLC connector. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's and Circuit Testing in Wiring Systems for the proper procedure.
  6. Test the Class 2 serial data circuit for a short to B+ or a short to ground between the mating connectors terminals integrated into the fuse block-left I/P, that contain the Class 2 serial data circuits. Refer to:
    7. •  Circuit Testing in Wiring Systems.
    •  Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 20

Go to Step 10

10

  1. Turn the ignition OFF.
  2. Reconnect the connectors containing the Class 2 serial data circuits to the to the fuse block-left I/P. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's.
  3. Make sure the Class 2 serial data circuit connector is disconnected from the radio module. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's.
  4. Install a scan tool.
  5. Turn the ignition ON with the engine off.
  6. Attempt to communicate with any module on the Class 2 serial data circuit.

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

Go to Step 19

Go to Step 11

11

  1. Turn the ignition OFF.
  2. Disconnect the connector containing the Class  2 serial data circuit from the BCM module. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's.
  3. Install a scan tool.
  4. Turn the ignition ON with the engine off.
  5. Attempt to communicate with any module on the Class 2 serial data circuit.

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

Go to Step 19

Go to Step 12

12

  1. Turn the ignition OFF.
  2. Disconnect the connector containing the Class  2 serial data circuit from the SDM module. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's.
  3. Install a scan tool.
  4. Turn the ignition ON with the engine off.
  5. Attempt to communicate with any module on the Class 2 serial data circuit.

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

Go to Step 19

Go to Step 13

13

  1. Turn the ignition OFF.
  2. Disconnect the connector containing the Class  2 serial data circuit from the EBCM module. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's.
  3. Install a scan tool.
  4. Turn the ignition ON with the engine off.
  5. Attempt to communicate with any module on the Class 2 serial data circuit.

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

Go to Step 19

Go to Step 14

14

  1. Turn off the ignition.
  2. Reconnect the connector containing the Class 2 serial data circuit to the SDM module. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's.
  3. Disconnect the connector containing the Class  2 serial data circuit from the PCM module. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's.
  4. Install a scan tool.
  5. Turn the ignition ON with the engine off.
  6. Attempt to communicate with any module on the Class 2 serial data circuit.

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

Go to Step 19

Go to Step 15

15

Does the vehicle have a RCDLR module?

Go to Step 16

Go to Step 17

16

  1. Turn off the ignition.
  2. Make sure the SDM module Class 2 serial data circuit connector is connected.
  3. Disconnect the connector containing the Class  2 serial data circuit from the RCDLR module. Refer to Data Link Connector Schematics for the appropriate circuit and connector ID's.
  4. Install a scan tool.
  5. Turn the ignition ON with the engine off.
  6. Attempt to communicate with any module on the Class 2 serial data circuit.

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

Go to Step 19

Go to Step 17

17

Important: 

   • Make sure all of the control modules Class 2 serial data circuit connectors are disconnected from all modules.
   • Make sure that all of the fuse block-left I/P harness connectors are installed.

  1. Turn the ignition ON with the engine off.
  2. Test the Class 2 serial data circuit at pin 2 of the DLC connector for a short to B+ or ground. Refer to Data Link Connector Schematics for the circuit ID's and Circuit Testing in Wiring Systems for the proper procedure.

Did you find a short?

Go to Step 18

Go to Step 19

18

  1. Turn the ignition OFF.
  2. Remove the fuse block-left I/P harness connectors.
  3. Remove all the Class 2 serial data circuit terminals from the fuse block-left I/P harness connectors. Refer to Data Link Connector Schematics for the circuit ID's and Connector Repairs in Wiring Systems for the proper procedures.
  4. Reconnect the fuse block-left I/P harness connectors to the fuse block-left I/P.
  5. Turn on the ignition with the engine off.
  6. Test all of the extracted Class 2 serial data circuit terminals for a short to B+ or Ground. Refer to Circuit Testing in Wiring Systems.
  7. If the terminal that contains the short malfunction has multiple conductors connected to it, the circuits will have to be removed from the terminal and Item 6 rerun to determine in which discreet conductor the short is in. Refer to Connector Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 20

--

19

Important: Setup and reprogram the module.

Replace the last module which was disconnected. Refer to Control Module References in Body Control System for the appropriate repair instructions for module replacement.

Did you complete the replacement?

Go to Step 20

--

20
  1. Reconnect all of the disconnected modules.
  2. Reinstall all extracted terminals into the appropriate connectors.
  3. Repair any wires that were disconnected or opened during the procedure.
  4. Reconnect all other disconnected connectors.
  5. Install a scan tool.
  6. Turn on the ignition with the engine off.
  7. Wait for 10 seconds. 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.
  8. Attempt to communicate with each module on the Class 2 serial data circuit.

Does the scan tool communicate with each module on the Class 2 data circuit?

Go to Step 21

Go to Step 2

21

  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 and the DTC status.

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

Go to Step 23

Go to Step 22

22

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

Go to Step 23

Go to Step 25

23

Diagnose the DTCs as directed by the diagnostic procedures for the particular module or malfunction.

Did you complete the action?

Go to Step 24

--

24

Did you diagnose all of the DTCs?

Go to Step 25

Go to Step 23

25

Clear the DTCs using the scan tool.

Did you complete the action?

System OK

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