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 information and commands are exchanged among the modules. Connecting a scan tool to the DLC allows communication with the modules for diagnostic purposes.

The diagnostic procedure assumes that all modules are powered. When you are uncertain whether this condition is met, perform the Diagnostic System Check for the module in question. 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

• | Use a J 35616-A when probing or checking electrical connector terminals. The J 35616-A prevents terminal damage, plus checks for proper terminal contact tension.

• | If a short to B+ is present a history U1301 will set in the modules. Refer to DTC U1301 Class 2 Short to Battery .

• | If a short to ground is present a history U1300 will set in the modules. Refer to DTC U1300 Class 2 Short to Ground .

• | If the malfunction is intermittent refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.

• | Disconnecting modules and turning the ignition switch to ON may cause DTCs in these modules. Check for DTCs in each module upon completion of the required repair.

### 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 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 5

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 U1255?

Go to Step 4

Go to Diagnostic Aids

4 | Is U1255 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
5 |

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 Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to the Applicable Diagnostic System Check

Go to Step 6

6 | Test the DLC ground circuit terminal 5 for an open. Refer to Testing for Continuity in Wiring Systems. Did you find and correct the condition? | Go to the Applicable Diagnostic System Check | Go to Step 7
7 |

• | Disconnect each module 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 Testing for Short to Ground and Testing for a Short to Voltage in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 8

8 |

1. Reconnect the BCM.
2. Reconnect all disconnected in-line harness connectors.
3. Install a 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 9

Go to Step 10

9 | Continue connecting the disconnected modules and attempting to communicate after each connection until communication cannot be established or all the modules are reconnected. Did you find the condition? | Go to Step 11 | Go to Diagnostic Aids
10 | Inspect for poor connections/terminal tension at the Class 2 serial data circuit of the BCM. Did you find and correct the condition? | Go to Go to Step 13 | Go to Step 12
11 | Replace the most recently reconnected module. Refer to the appropriate repair instructions for module replacement. Did you complete the replacement? | Go to Step 13 | --
12 | Replace the BCM. Did you complete the replacement? | Go to Step 13 | --
13 |

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.
6. The scan tool may require the following:

- | A power up reset before communication will occur due to a short on the Class 2 serial data circuit.

- | Turning off the scan tool before you display DTCs.

7. 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.)
8. 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 15

Go to Step 14

14 | Did you record any DTCs which do not begin with a U? | Go to Step 15 | Go to Step 17
15 | Diagnose the DTCs as directed by the diagnostic procedures for the particular module or malfunction. Did you complete the action? | Go to Step 16 | --
16 | Did you diagnose all of the DTCs? | Go to Step 17 | Go to Step 15
17 | Clear the DTCs using the scan tool. Did you complete the action? | System OK | --