For 1990-2009 cars

### Circuit Operation

Modules connected to the Class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating 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 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

• | The BCM detects that the ignition switch is ON and sends the appropriate power mode message to the other modules connected to the class 2 serial data circuit. The BCM must be connected to the class 2 serial data circuit for many other modules to communicate with the scan tool.

• | If the class 2 serial data circuit was shorted to B+ a history U1301 will set in the modules. Refer to DTC U1301

• | If the class 2 serial data circuit was shorted to ground is present a history U1300 will set in the modules. Refer to DTC U1300

• | If the malfunction is intermittent refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

### 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. The following modules communicate on the class 2 serial data circuit:

• | PCM

• | RIM (w/W8Y)

• | Radio (wo/W8Y)

• | Cellular Telephone (wo/W8Y)

• | OnStar® VIU (w/W8Y).

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 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. 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 wiring for a short, make sure there is not a module connected to the wire being tested.

6. Since the short is not in the wiring harness the malfunction is due to one of the modules.

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

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

Step | Action | Yes | No
--- | --- | --- | ---
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 9

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 DTCs in the range of U1000 to U1305?

Go to Step 6

Go to Diagnostic Trouble Code (DTC) List

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. | Go to Step 16 | 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 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 the Applicable Diagnostic System Check

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 the Applicable Diagnostic System Check | Go to Step 9
9 |

• | Disconnect all modules on the Class 2 serial data circuit.

• | Test the Class 2 serial data circuit for a short to ground or short to voltage between DLC terminal 2 and each module on the Class 2 serial data circuit. (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 16

Go to Step 10

10 |

1. Reconnect the BCM.
2. Install a scan tool.
3. Turn the ignition ON with the engine off.
4. Attempt to communicate with the BCM.

Does the scan tool communicate with the BCM?

Go to Step 11

Go to Step 12

11 | Continue connecting the disconnected modules and attempting to communicate after each connection until communication cannot be established or all the modules are reconnected. Do you lose communications with the class 2 serial data devices when you connect a module? | Go to Step 13 | Go to Diagnostic Aids
12 | Inspect for poor connections/terminal tension at the Class 2 serial data circuit of the BCM. 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 16 | Go to Step 15
13 | Inspect for poor connections/terminal tension at the Class 2 serial data circuit of the most recently reconnected module. 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 16 | Go to Step 14
14 | Replace the most recently reconnected module. Refer to Control Module References in Body Control System. Did you complete the replacement? | Go to Step 16 | --
15 | Replace the BCM. Refer to Body Control Module Replacement in Body Control System. Did you complete the replacement? | Go to Step 16 | --
16 |

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 before performing this function.
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 18

Go to Step 17

17 | Did you record any DTCs which do not begin with a U? | Go to Step 18 | Go to Step 20
18 | Diagnose the DTCs as directed by the diagnostic procedures for the particular module or malfunction. Did you complete the action? | Go to Step 19 | --
19 | Did you diagnose all of the DTCs? | Go to Step 20 | Go to Step 18
20 | Clear the DTCs using the scan tool. Did you complete the action? | System OK | --