For 1990-2009 cars
Data Link Communications Components
Data Link Connector Schematics
Handling ESD Sensitive Parts Notice
Handling ESD Sensitive Parts Notice

### 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. When a module receives a message for a critical operating parameter, the module records the ID number of the module which sent the message for State of Health monitoring (Node Alive messages). A critcal operating parameter is one which, when not received, requires that the module use a default value for that parameter. Once an ID number is learned by a module, it will monitor for that modules's "Node Alive" message. Each module on the class 2 serial data circuit which is powered and performing functions that require detection of a communications malfunction is required to send a "Node Alive" message every two seconds. When no message is detected from the electronic brake control module (EBCM) for five seconds, DTC U1040 is set.

### Conditions for Setting the DTC

The ignition is turned to the ON position, and a module on the class 2 serial data circuit does not receive the EBCM ID for 5 seconds.

### Conditions for Clearing the DTC

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

• | A history DTC can be cleared with the scan tool.

### Diagnostic Aids

When multiple Loss of Communication DTCs are set concurrently, the cause is likely to be two opens in the class 2 serial data circuit. Use the Data Link Schematic in order to determine the location of the opens.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
Important: A short to ground will cause an open fuse(s). Before performing this diagnostic procedure, inspect the fuse(s) for an open.
1 | Test the battery positive voltage circuits to the EBCM for an open or a short to ground. Did you find and correct the condition? | -- | Go to Step 5 | Go to Step 2
2 |

1. Turn OFF the ignition.
2. Test the ground circuits to the EBCM for an open.

Did you find and correct the condition?

--

Go to Step 5

Go to Step 4

3 | Inspect for poor connections at the battery positive voltage circuits, the ground circuits and the class 2 serial data circuit to the EBCM. Did you find and correct the condition? | -- | Go to Step 5 | Go to Step 4
4 | Replace the EBCM. Refer to Electronic Brake Control Module Replacement in ABS. Is the repair complete? | -- | Go to Step 5 | --
5 | Select the DISPLAY DTCs function for the EBCM. Does the scan tool display any DTCs which do not begin with "U"? | -- | Go to the applicable DTC table | Go to Step 6
6 |

1. Use the scan tool in order to clear the DTCs.
2. Continue diagnosing or clearing the DTCs until the EBCM has been diagnosed and all DTCs have been cleared.

Is the action complete?

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Go to Diagnostic System Check - Data Link Communications

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