Circuit Description
The serial data circuit is used to communicate information between each module connected to the serial data line. Each module is assigned a specific recognition code (source ID). This code is used to identify which module is communicating on the serial data line. Modules communicate with others by periodically sending messages on the serial data line. These messages contains specific information (or commands) a system requires to function properly. When a module receives a message it also learns the source ID of the system that sent it. This source ID is used to determine which module actually sent the message and for State of Health (SOH) monitoring. SOH monitoring determines if a module is unable to communicate properly. Modules periodically send a SOH message to each other containing their source ID. If a system fails to receive a SOH message from a module that it communicates with, then a DTC identifying what module did not respond will set. Any module that also cannot communicate properly, will also store the appropriate communication DTC that was assigned to that module. Each module also monitors the serial data line for an open or short (to ground or voltage) condition. If the applicable module detects any of these malfunctions as current, then that module will store a DTC U1255.
Conditions for Setting the DTC
| • | The applicable DCM does not receive a State of Health (SOH) message back from the Powertrain Control Module (PCM), Body Control Module (BCM) or the Instrument Panel Cluster (IPC) when the ignition is first turned ON. |
| • | The applicable DCM detects an open or short (to ground or voltage) on the serial data line circuit. |
| • | Condition must be present for 1 seconds. |
Action Taken When the DTC Sets
| • | Stores a history DTC U1255 in the DCM memory. |
| • | The LDCM RDCM may substitute default values for certain systems. |
| • | This DTC can only be set as a history code even if the malfunction is current. |
| • | No driver warning message will be displayed for this DTC. |
Conditions for Clearing the DTC
| • | The DCM receives a SOH message back from the PCM, BCM and IPC. |
| • | The DCM no longer detects an open or short (to ground or voltage) on the serial data line circuit. |
| • | 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 open or short (to ground or voltage) in the serial data line. |
| - | Intermittent loss of communication with the PCM, BCM or IPC. |
| - | Damaged or loose star connector 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 the DCM memory, check for the same DTC stored in other systems capable of storing this DTC. If other systems have a DTC U1255 stored, check the serial data line circuit for a short (to ground or voltage). If other systems do not have a DTC U1255 stored, check for an open in the serial data line circuit between the DCM and the PCM, BCM or IPC. After repairing a DTC U1255, clear all DTCs from all systems capable of storing this DTC. |
| • | Communication DTCs do not necessarily indicate a malfunction. These DTCs are only intended to aid in diagnosing an intermittent communication malfunction that is accompanied by a symptom. If the vehicle does not experience a symptom related to a communication malfunction, these DTCs can be disregarded. |
| • | The DCM will set a DTC U1255 if the DCM is unable to communicate with the PCM, BCM or IPC when the ignition is first cycled ON, |
| • | 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 PCM. If the scan tool cannot communicate with the PCM. Refer to Powertrain On Board Diagnostic (OBD) System Check in Engine Controls.
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This test checks if communications can be established between the scan tool and the BCM. If the scan tool cannot communicate with the BCM. Refer to Diagnostic System Check - Body Control System in Body Control System.
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This test checks if communications can be established between the scan tool and the IPC. If the scan tool cannot communicate with the IPC. Refer to Diagnostic System Check - Instrument Cluster in Instrument Panel, Gauges and Console.
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This test will check for any opens in the serial data line between the DCM and the PCM, BCM or IPC. 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 DCM Diagnostic System Check? | -- | ||||||||||||
Does the scan tool communicate with the PCM? | -- | |||||||||||||
Attempt to establish communications with the Body Control Module (BCM). Does the scan tool communicate with the BCM? | -- | |||||||||||||
Attempt to establish communications with the Instrument Panel Cluster (IPC). Does the scan tool communicate with the IPC? | -- | |||||||||||||
Check the serial data line for the following intermittent conditions:
Was a problem found and repaired? | -- | |||||||||||||
6 |
Does DTC U1255 reset? | -- | System OK | |||||||||||
7 | Replace the applicable DCM. Refer to Door Control Module Replacement . Is the replacement complete? | -- | -- | |||||||||||
8 |
Is the repair complete? | -- | -- |
