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 data link connector (DLC) allows communication with the modules for diagnostic purposes.

### Diagnostic Aids

The engine will not start when there is a total loss of class 2 serial data communication while the ignition is OFF. 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 internal condition within a module or connector on the class 2 serial data circuit, that causes a short to voltage or ground to the class 2 serial data circuit.

### 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:

• | The body control module (BCM)

• | The electronic brake control module (EBCM)

• | The inflatable restraint sensing and diagnostic module (SDM)

• | The instrument panel cluster (IPC)

• | The powertrain control module (PCM)

• | The radio

• | The rear seat entertainment (RSE) assembly

• | The transfer case shift control module (TCSCM)

• | The vehicle communication interface module (VCIM)

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

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

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

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

Step | Action | Yes | No
--- | --- | --- | ---
Schematic Reference: Data Communication Schematics Connector End View Reference: Master Electrical Component List
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle
2 | Does the scan tool power up? | Go to Step 3 | Go to Scan Tool Does Not Power Up
3 |

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 4

Go to Step 9

4 |

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 5

Go to Step 8

5 | Are the history DTCs U1300, U1301 or U1305 retrieved from any module? | Go to Step 6 | Go to Step 7
6 | 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:

• | Testing for Intermittent Conditions and Poor Connections

• | Circuit Testing

• | Connector Repairs

• | Wiring Repairs

Did you find and correct the condition?

Go to Step 18

Go to Step 7

7 | Are U1000 or U1255 the only DTCs displayed in the previously specified range? | Go to DTC U1000 and U1255 | Go to DTC U1001-U1254
8 | Diagnose the non-communicating module by using the DTC U1001-U1254 procedure for the module which is not communicating. Refer to DTC U1001-U1254 . Did you complete the action? | Go to Diagnostic System Check - Vehicle | --
9 |

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 10

10 | 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 11
11 | Repeat the following steps on the class 2 serial data circuits until the communication with body control module (BCM) is established or all modules listed below were disconnected.

1. Disconnect the modules one by one, in the following order:

• | Vehicle communication interface module (VCIM)

• | Rear seat entertainment (RSE) assembly

• | Transfer case shift control module (TCSCM)

• | Electronic brake control module (EBCM)

• | Inflatable restraint sensing and diagnostic module (SDM)

• | Radio

• | Instrument panel cluster (IPC)

• | Powertrain control module (PCM)

2. Turn ON the ignition, with the engine OFF.
3. Attempt to communicate with the BCM, after each disconnected module.

Does the scan tool communicate with the BCM?

Go to Step 12

Go to Step 15

12 | Important: Perform this step only if the suspected module is IPC or Radio.

1. Turn OFF the ignition.
2. If the IPC is suspected module, test the class 2 serial data circuit between IPC and TCCM 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 .
3. If the Radio is suspected module, test the class 2 serial data circuit from Radio to RSE and VCIM 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 18

Go to Step 13

13 | 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 18 | Go to Step 14
14 | Replace the last disconnected module. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 18 | --
15 |

1. Turn OFF the ignition.
2. Disconnect the BCM.
3. Test all of the BCM class 2 serial data circuits for an open, short to voltage, or short to ground. 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 18

Go to Step 16

16 | Inspect for poor connections at the class 2 serial data circuit of the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs . Did you find and correct the condition? | Go to Step 18 | Go to Step 17
17 | Replace the BCM. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 18 | --
18 |

1. Reconnect all of the disconnected modules.
2. Install a scan tool.
3. Turn ON the ignition, leaving the engine OFF.
4. 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 the scan tool prior to performing this test.

5. 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.
6. 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 Diagnostic Trouble Code (DTC) List - Vehicle

Go to Step 19

19 | Did you record any DTCs which do not begin with a "U"? | Go to Diagnostic Trouble Code (DTC) List - Vehicle | Go to Step 20
20 | Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | --