For 1990-2009 cars

### Circuit Description

Modules connected to the Class 2 data link monitor the link for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module receives a message for a critical operating parameter, the module records the identification number of the module which sent the message for State of Health monitoring (Node Alive messages). A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. Once an identification number is learned by a module, it will monitor for that module's "Node Alive" message. Each module on the Class 2 serial data circuit which is powered and performing functions that require detection of communications malfunction is required to send a "Node Alive" message every two seconds. When no message is detected from a learned identification number for five seconds, a DTC U1128 is set.

### Conditions for Running the DTC

• | The voltage supplied to the module is in the normal operating voltage range (approximately 9-16 volts).

• | DTC U1300 and DTC U1301 do not have a current status.

• | The vehicle power mode (ignition switch position) requires serial data communication to occur.

### Conditions for Setting the DTC

A message from a learned identification number has not been detected for the past five seconds.

### Conditions for Clearing the DTC

• | A current DTC U1128 will clear when a "Node Alive" message from the failed identification number is detected on the Class 2 serial data circuit or at the end of the current ignition cycle.

• | A history U1128 will clear upon receipt of a scan tool CLEAR DTCs command.

### Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

1. A module which loses power during an ignition cycle will cause other module(s) to set Lost Communication DTCs.

2. This step inspects for a poor connection to the radio receiver preventing serial communication.

3. The module which was not communicating due to an open in the Class 2 serial data circuit may have Lost Communication DTCs for those modules that it was monitoring.

4. The module which can communicate, BCM and RCDLR (if equipped) indicates the module (the Radio) which cannot communicate. It is necessary to clear the DTCs from these modules to avoid future misdiagnosis.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 |

1. Perform the Diagnostic System Check for the radio receiver, refer to Radio/Audio System Diagnostic System Check in Entertainment.
2. Ensure that the module is properly powered.

Was a repair made when the Radio Diagnostic System Check was performed?

--

Go to Step 3

Go to Step 2

2 |

1. Remove the radio and disconnect the radio receiver connector.
2. Inspect for poor connections/terminal tension at radio connector terminal F6. Refer to Intermittents and Poor Connections Diagnosis in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 3

Go to Step 4

3 |

1. Install the scan tool.
2. Turn ON the ignition, with the engine OFF.
3. Select DTC Information for the radio.

Does the scan tool display any DTCs which do not begin with a "U"?

--

Go to the Applicable DTC Table

Go to Step 5

4 | Measure the resistance of the serial data circuit from the radio receiver terminal F6 to the data link connector (DLC) terminal 2. Does the resistance measure less than the specified value? | .5 ohms | Go to Step 6 | Go to Step 7
5 | Clear the DTCs using the scan tool. Did you complete the action? | -- | Go to Step 8 | --
6 |

1. Replace the radio.
2. Perform the setup procedure for a new radio.
3. Install the scan tool.
4. Select RADIO THEFT from the SPECIAL FUNCTIONS menu.
5. Follow the instructions on the screen.

Did you complete the replacement?

--

Go to Step 8

--

7 | Locate and repair an open or high resistance in the serial data circuit between the radio and the DLC. Did you find and correct the condition? | -- | Go to Step 3 | --
8 | Select the Display DTCs function for the module(s) which had the Lost of Communications With IRC DTC set. Does the scan tool display any DTCs which do not begin with a "U"? | -- | Go to the Applicable DTC Table | Go to Step 9
9 |

1. Clear the DTCs using the scan tool.
2. Continue diagnosing or clearing the DTCs until all modules have been diagnosed and all DTCs have been cleared.

Did you complete the action?

--

System OK

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