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 following conditions will cause a class 2 serial data communication malfunction:

• | The body control module (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.

• | A class 2 serial data circuit shorted to ground. A history U1300 will set in all modules. Refer to DTC U1300, U1301, or U1305 .

• | A class 2 serial data circuit shorted to voltage. A history U1301 will set in all modules. Refer to DTC U1300, U1301, or U1305 .

• | A class 2 serial data circuit shorted to ground or voltage. A history U1305 will set in all modules. Refer to DTC U1300, U1301, or U1305 .

• | 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.

• | If the malfunction is intermittent check for poor connections at the suspect module. Wiggling the harness wiring may help in locating the fault.

### Test Description

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

1. A partial loss of communication in the class 2 serial data circuit uses a different procedure than a total loss of communication of the class 2 serial data circuit.

2. The following DTCs may be retrieved with a history status. These DTCs 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 with a current status. This indicates that the malfunction occurred when the ignition was ON.

4. This step will isolate a wiring problem or a module malfunction.

5. This step analyzes the specific branch circuit for a malfunction.

6. An open circuit at this point will cause a loss of communication with the vehicle.

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

8. The communication concern 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 in Wiring Systems
1 | Did you perform the Diagnostic System Check for the system exhibiting symptom? | Go to Step 2 | Go to Control Module References in Body Control System for the applicable Diagnostic System Check
2 | Install a scan tool. Does the scan tool power up? | Go to Step 3 | Go to Scan Tool Does Not Power Up
3 |

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 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 U1255?

Go to Step 5

Go to Step 8

5 | Are 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 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 18

Go to Step 7

7 | Is U1000 the only DTC displayed in the previously specified range? | Go to DTC U1000 | Go to DTC U1001-U1254
8 | Diagnose the non communicating module by using the DTC U1001-U1254 Lost Communications procedure for the module not communicating. The DTC U1001-U1254 Lost Communications procedure will determine which module is not communicating. Refer to DTC U1001-U1254 .

Did you complete the action?

Go to Control Module References in Body Control Systems for the applicable Diagnostic System Check

--

9 |

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

Did you find and correct the condition?

Go to Control Module References in Body Control Systems for the applicable Diagnostic System Check

Go to Step 10

10 | Test the ground circuits of the data link connector (DLC) for an open or high resistance. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Control Module References in Body Control Systems for the applicable Diagnostic System Check | Go to Step 11
11 |

1. Disconnect all modules on the class 2 serial data circuit.

Important: Turn ON the ignition, leaving the engine OFF, when testing for a short to voltage.

2. 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.

Did you find and correct the condition?

Go to Step 18

Go to Step 12

12 |

1. Turn OFF the ignition.
2. Reconnect the body control module (BCM).
3. Install the scan tool.
4. Turn ON the ignition, with the engine OFF.
5. Attempt to communicate with the BCM.

Does the scan tool communicate with the BCM?

Go to Step 13

Go to Step 14

13 | Continue connecting the disconnected modules and attempting to communicate after each connection until communication can not be established or all modules are reconnected. When a module is connected, is communication lost with the class 2 serial data devices? | Go to Step 15 | Refer to Diagnostic Aids
14 | Inspect for poor connections and terminal tension at the class 2 serial data circuit of the BCM. Did you find and correct the condition? | Go to Step 18 | Go to Step 16
15 | Inspect for poor connections and terminal tension at the class 2 serial data circuit of the most recently reconnected module. Did you find and correct the condition? | Go to Step 18 | Go to Step 17
16 | Important: Perform the programming or setup procedure for the replaced control module if required. Replace the BCM. Refer to Body Control Module Replacement in Body Control System for the applicable replacement procedure. Did you complete the replacement? | Go to Step 18 | --
17 | Important: Perform the programming or setup procedure for the replaced control module if required. Replace the most recently reconnected module. Refer to Control Module References in Body Control System for the appropriate replacement procedure.

Did you complete the replacement?

Go to Step 18

--

18 |

1. Connect all of the modules.
2. Connect all the connectors.
3. Install a scan tool.
4. Turn ON the ignition, leaving the engine OFF.

Important: 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 you display DTCs.

5. Wait for 10 seconds.
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 your record any DTCs which begin with a letter U and with a current status?

Go to Step 20

Go to Step 19

19 | Did you record any DTCs which do not begin with a letter U? | Go to Step 20 | Go to Step 22
20 | Diagnose the DTCs as directed by the diagnostic procedures for the particular module or concern. Did you complete the action? | Go to Step 21 | --
21 | Did you diagnose all of the DTCs? | Go to Step 22 | Go to Step 20
22 | Clear the DTCs using the scan tool. Did you complete the action? | Go to Control Module References in Body Control Systems for the applicable Diagnostic System Check | --