For 1990-2009 cars

### 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 DLC allows communication with the modules for diagnostic purposes.

### 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.

### 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. DTCs U1300 and/or U1301 may be retrieved with a history status. These DTCs are not the cause of the present condition.

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

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. A poor connection at connector C1 terminal D2 would cause this condition but may not set a DTC.

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

7. This test isolates the serial data circuit within the left IP junction block.

8. This test isolates the BCM class 2 serial data circuit.

9. This test isolates the SDM class 2 serial data circuit.

10. This test isolates the EBCM class 2 serial data circuit.

11. This test isolates the PCM class 2 serial data circuit.

12. This test isolates the instrument panel cluster class 2 serial data circuit.

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

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

Step | Action | Yes | No
--- | --- | --- | ---
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 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 Testing for Intermittent Conditions and Poor Connections

in Wiring Systems

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 in Wiring Systems:

• | Testing for Intermittent Conditions and Poor Connections

• | Circuit Testing

• | Connector Repairs

• | Wiring Repairs

Did you find and correct the condition?

Go to Step 19

Go to Step 6

6 | Is U1000 the only DTC displayed in the previously specified range? | Go to DTC U1000 and DTC U1255 Class 2 Communication Malfunction | 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.

Did you find and correct the condition?

Go to Step 19

Go to Step 8

8 | Test the DLC ground circuit terminal 5 for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 19 | Go to Step 9
9 |

1. Disconnect the left IP junction block connector C1.
2. Inspect for poor connections/terminal tension at the left IP junction block connector. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 19

Go to Step 10

10 | Test the class 2 serial data circuit for an open or a short between the DLC terminal 2 and the junction block connector terminal D2. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 19 | Go to Step 11
11 | Test for an open or short between the following terminals of the left IP junction block connector C1:

• | D2

• | A3

• | E3

Refer to Circuit Testing in Wiring Systems.

Did you find and correct the condition?

Go to Step 19

Go to Step 12

12 |

1. Turn off the ignition.
2. Disconnect the BCM.
3. Test the class 2 serial data circuit between the left IP junction block connector and the BCM 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 in Wiring Systems.

Did you find and correct the condition?

Go to Step 19

Go to Step 13

13 |

1. Turn off the ignition.
2. Disconnect the SDM.
3. Test the class 2 serial data circuit between the left IP junction block connector and the SDM 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 in Wiring Systems.

Did you find and correct the condition?

Go to Step 19

Go to Step 14

14 |

1. Turn off the ignition.
2. Disconnect the EBCM.
3. Test the class 2 serial data circuit between the left IP junction block connector and the EBCM 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 in Wiring Systems.

Did you find and correct the condition?

Go to Step 19

Go to Step 15

15 |

1. Turn off the ignition.
2. Disconnect the PCM.
3. Test the class 2 serial data circuit between the left IP junction block connector and the PCM 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 in Wiring Systems.

Did you find and correct the condition?

Go to Step 19

Go to Step 16

16 |

1. Turn off the ignition.
2. Disconnect the instrument panel cluster.
3. Test the class 2 serial data circuit between the left IP junction block connector and the instrument cluster 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 in Wiring Systems.

Did you find and correct the condition?

Go to Step 19

Go to Step 17

17 | Inspect for poor connections/terminal tension at the class 2 serial data circuit of each module that is not communicating. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 19 | Go to Step 18
18 | Replace the module which is not communicating. Refer to Control Module References for the appropriate repair instructions for module replacement. Did you complete the replacement? | Go to Step 19 | --
19 |

1. Reconnect all of the disconnected modules.
2. Reconnect any other disconnected connectors.
3. Install a scan tool.
4. Turn on the ignition leaving the engine off.
5. 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.
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 21

Go to Step 20

20 | Did you record any DTCs which do not begin with a U? | Go to Step 21 | Go to Step 23
21 | Diagnose the DTCs as directed by the diagnostic procedures for the particular module or malfunction. Did you complete the action? | Go to Step 22 | --
22 | Did you diagnose all of the DTCs? | Go to Step 23 | Go to Step 21
23 | Clear the DTCs using the scan tool. Did you complete the action? | System OK | --