Circuit Description
The serial data circuit is used to communicate information between each system connected to the serial data line. Each system is assigned a specific recognition code (source ID). This code is used to identify which system is communicating on the serial data line. Systems communicate with others by periodically sending messages on the serial data line. These messages contain specific information (or commands) a system requires to function properly. When a system receives a message it also learns the source ID of the system that sent it. This source ID is used to determine which system actually sent the message and for State of Health (SOH) monitoring. SOH monitoring determines if a system is unable to communicate properly. Systems periodically send a SOH message to each other containing their source ID. If a system fails to receive a SOH message back from a system that it communicates with, then a DTC identifying what system did not respond will set. Any system that also cannot communicate properly, will also store the appropriate communication DTC that was assigned to that system. Each system also monitors the serial data line for a short to ground or short to voltage condition, if a system detects a short in the serial data line, a DTC U1255 will set.
Conditions for Setting the DTC
| • | A system detects an open or short (to ground or voltage) in the serial data line circuit. |
| • | An open occurs in the serial data line before the ignition is turned ON. |
| • | Condition must be present for 1 second. |
Action Taken When the DTC Sets
| • | Stores a DTC U1255 in the system that detected the malfunction on the serial data line circuit. |
| • | The system that detected a malfunction on the serial data line may set false DTCs and substitute default values for certain functions. |
| • | No driver warning message will be displayed for this DTC. |
Conditions for Clearing the DTC
| • | This DTC requires an ignition cycle in order to change from current to history. |
| • | A system no longer detects an open or short (to ground or voltage) in the serial data line circuit. |
| • | A history DTC will clear after 50 consecutive ignition cycles if the condition for the malfunction is no longer present. |
| • | Use the IPC clearing DTCs feature. |
| • | Use a scan tool. |
Diagnostic Aids
| • | If the problem is an intermittent loss of communications. Carefully inspect the serial data line and related components for the following intermittent conditions: |
| - | There is an intermittent short (to ground or voltage) on the serial data line. |
| - | An intermittent open in the serial data line that occured before the ignition is turned ON. |
| - | Damaged or loose star connector #1 or star connector #2 terminals. |
| • | If the serial data line is shorted to ground or shorted to voltage, all systems connected to the same serial data line will not be able to communicate properly. Systems capable of storing loss of communication DTCs (DTCs with the letter U as a prefix) will have these codes stored in their memory. If a DTC U1255 is stored in a system, check the serial data line circuit for an intermittent open or short (to ground or voltage). After repairing a DTC U1255, clear all DTCs from all systems capable of storing communication DTCs. |
| • | The conditions that set a DTC U1255 may also set other communication DTCs. Always diagnose a DTC U1255 first before attempting to diagnose other communication DTCs. |
| • | A DTC U1255 is displayed as a history code. If the condition that set a DTC U1255 were current, the scan tool will not be able to communicate with any system due to the short on the serial data line circuit. |
| • | If the DTC is a history DTC, the problem may be intermittent. Perform the tests shown while moving related wiring and connectors. This can often cause the malfunction to occur. |
Test Description
The numbers below refer to the step numbers on the diagnostic table:
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This test checks if communications can be established between the scan tool and the other systems connected to the same serial data line.
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This test will check for any opens or shorts (to ground or voltage) in the serial data line. It also checks for any intermittent malfunctions associated with the serial data line.
Step | Action | Value(s) | Yes | No | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Were you sent here from the Data Link Communications Diagnostic System Check? | -- | ||||||||||||
Does the scan tool communicate with other systems? | -- | |||||||||||||
Check the serial data line for the following intermittent conditions:
Was a problem found and repaired? | -- | |||||||||||||
4 |
Does DTC U1255 reset? | -- | Go to Diagnostic Aids | |||||||||||
5 | Replace the applicable module that reset a DTC U1255. Refer to the applicable Removal and Installation procedure. Is the replacement complete? | -- | -- | |||||||||||
6 |
Are all repairs complete? | -- | -- |
