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. 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

When the class 2 serial data circuit:

• | Is shorted to ground

• | Is shorted to voltage

• | Opens

The following DTCs may set:

• | U1300

• | U1301

• | U1305

Refer to DTC U1300, U1301, or U1305 .

### Test Description

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

1. The powertrain control module (PCM) communicates on the class 2 serial data circuit with the L18/8.1 engine. The transmission control module (TCM) communicates of the class 2 serial data circuit with the LG5/7.2 engine.

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

• | U1300

• | U1301

• | U1305

3. DLC terminal 2 and terminal 5 provide the connection to the class 2 serial data circuit and the signal ground circuit respectively.

4. This test isolates either the PCM or TCM class 2 serial data circuit, depending on RPO.

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 Link Connector Schematics
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 with the engine OFF.
2. With the scan tool, attempt to communicate with the following class 2 control module:

• | TCM LG5/7.2 Diesel

• | PCM L18/8.1 Gas

Does the scan tool communicate with the TCM/PCM on the Class 2 circuit?

Go to Step 3

Go to Step 6

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 that apply to other modules?

Go to the applicable Diagnostic System Check

Go to Step 10

4 | Important: Turn ON the ignition, with the engine OFF, when testing for a short to voltage. Use 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 9

Go to Step 5

5 |

1. Turn OFF the ignition.
2. Remove the scan tool from the 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 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 class 2 ground circuit 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 7
7 |

1. Turn OFF the ignition.
2. Remove the connector of the class 2 module which contains the class 2 circuit.
3. Test the TCM/PCM class 2 circuit for a short to ground or a short to battery 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 9

Go to Step 8

8 | Replace the PCM/TCM. Refer to Powertrain Control Module Replacement in 8.1 Engine Controls and Transmission Control Module Replacement in Automatic Transmission - Allison. Did you complete the replacement? | Go to Step 9 | --
9 |

1. Connect all of the modules.
2. Connect any other connectors.
3. Connect 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 circuit. Turn OFF 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 11

Go to Step 10

10 | Did you record any DTCs which do not begin with a "U"? | Go to Step 11 | Go to Step 12
11 | Diagnose the DTCs as directed by the diagnostic procedures for the stsyem that will exhibit symptoms. Refer to the applicable Diagnostic System Check. Did you complete the action? | Go to Step 12 | --
12 | Did you diagnose all of the DTCs? | Go to Step 13 | Go to Step 11
13 | Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | --