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 Body Control Module (BCM), and no SOH message response was sent back. |
| • | Condition must be present for 5 seconds. |
Action Taken When the DTC Sets
| • | Stores a DTC U1064 in the HVAC Control Assembly memory. |
| • | No driver warning message will be displayed for this DTC. |
| • | The HVAC Control Assembly may substitute defaults values for certain systems. |
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 BCM. |
| • | 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 BCM. |
| - | 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. Use a scan tool to monitor communications with the BCM, if the HVAC control head loses communications with the BCM at any time, the HVAC Control Assembly will store a DTC U1064. Test driving the vehicle while monitoring communications with the BCM can assist in diagnosing an intermittent malfunction. Check for an open in the serial data line between the HVAC Control Assembly and the BCM. The HVAC Control Assembly and BCM must also be checked for intermittent operation due to a loss of power or ground. After repairing a DTC U1064, 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 BCM. If the scan tool cannot communicate with the BCM. Refer to Diagnostic System Check - Body Control System .
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This test checks for a stored HVAC Control Assembly DTC U1255. Conditions that set a DTC U1255 may also set a DTC U1064, therefore always diagnose a DTC U1255 first.
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This test will determine if DTC U1064 was set due to a loss of communication with the HVAC Control Assembly or the BCM. The Instrument Panel Cluster (IPC) also communicates with the BCM. If a DTC U1064 is stored in the IPC memory, the BCM must be diagnosed first.
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This test will check for any opens in the serial data line between the HVAC Control Assembly and the BCM. 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 BCM? | -- | ||||||||||||
3 |
Is DTC U1255 stored in the HVAC memory? | -- | ||||||||||||
4 |
Is DTC U1064 stored in the IPC memory? | -- | ||||||||||||
5 | Check the serial data line for the following intermittent conditions:
Was a concern found and repaired? | -- | ||||||||||||
6 |
Does DTC U1064 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 | -- |
