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 contain specific information (or commands) which allow other systems 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 able 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, a DTC identifying which module that did not respond will be set. Any module that also cannot communicate properly, will also store the appropriate communication DTC that was assigned to that module.
Conditions for Setting the DTC
| • | The HVAC Control Assembly sends out a State of Health (SOH) message to the Instrument Panel Cluster (IPC), and no SOH message response was sent back. |
| • | Condition must be present for 5 seconds. |
Action Taken When the DTC Sets
| • | Stores a DTC U1096 in the HVAC Control Assembly memory. |
| • | The HVAC Control Assembly may substitute default values for certain systems. |
| • | 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 HVAC Control Assembly receives a SOH message back from the IPC. |
| • | 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 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 U1096 is stored in the HVAC Control Assembly memory, check for the same DTC stored in the Remote Function Actuation (RFA) system. The RFA system also monitors the SOH message from the IPC. If the RFA system has a DTC U1096 stored, check the IPC for an intermittent malfunction. If the RFA system does not have a DTC U1096 stored, check for an open in the serial data line between the HVAC control head and the IPC. The HVAC Control Assembly must also be checked for intermittent operation due to a loss of power or ground to the HVAC Control Assembly itself. After repairing a DTC U1096, 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. |
| • | 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 IPC.
-
This test checks for a stored HVAC Control Assembly DTC U1255. Conditions that set a DTC U1255 may also set a DTC U1096, therefore always diagnose a DTC U1255 first.
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This test will determine if DTC U1096 was set due to a loss of communications with the HVAC control head or the IPC. The Remote Function Actuation (RFA) system also communicates with the IPC. If a DTC U1096 is stored in the RFA system, the IPC must be diagnosed first.
-
This test will check for any opens in the serial data line between the HVAC Control Assembly and the 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 HVAC System Check (CJ2)? | -- | Go to HVAC System Check | |||||||||||
2 |
Does the scan tool communicate with the IPC? | -- | ||||||||||||
3 |
Is DTC U1255 stored in the HVAC memory? | -- | ||||||||||||
4 |
Is DTC U1096 stored in the RFA memory? | -- | ||||||||||||
5 | Check the serial data line for the following intermittent conditions:
Was a concern found and repaired? | -- | ||||||||||||
6 |
Does DTC U1096 reset? | -- | System OK | |||||||||||
7 | Replace the HVAC Control Assembly. Refer to Control Assembly Replacement . Is the repair complete? | -- | -- | |||||||||||
8 |
Is the repair complete? | -- | Go to HVAC System Check | -- |
