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. The BCM also monitors the serial data line for an open or short (to ground or voltage) condition. If the BCM detects any of these malfunctions as current, the BCM will store a DTC.
Conditions for Setting the DTC
| • | The BCM does not receive a State of Health (SOH) message back from the Powertrain Control Module (PCM) or the Instrument Panel Cluster (IPC) when the ignition is first turned ON. |
| • | The BCM 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 DTC U1255 in the BCM memory. |
| • | The BCM may substitute default values for certain systems. |
| • | The PCM will not allow fuel enable processing to occur (vehicle may experience an engine cranks but does not run condition). |
| • | 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. |
| • | The BCM receives a SOH message back from the PCM and IPC. |
| • | The BCM no longer detects an open or short (to ground or voltage) on 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 open or short (to ground or voltage) in the serial data line. |
| - | Intermittent loss of communication with the PCM 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 BCM 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 BCM and the PCM 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 BCM will set a DTC U1255 if the BCM is unable to communicate with the PCM 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:
-
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.
-
This test checks if communications can be established between the scan tool and the IPC.
-
This test will check for any opens in the serial data line between the BCM and the PCM 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 BCM Diagnostic System Check? | -- | ||||||||||||
Does the scan tool communicate with the PCM? | -- | |||||||||||||
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? | -- | |||||||||||||
5 |
Does DTC U1255 reset? | -- | System OK | |||||||||||
6 |
Is the replacement complete? | -- | -- | |||||||||||
7 |
Is the repair complete? | -- | -- |
