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 Powertrain Control Module (PCM), and no SOH message response was sent back. |
| • | Condition must be present for 5 seconds. |
Action Taken When the DTC Sets
| • | Stores a DTC U1016 in the HVAC 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 PCM. |
| • | 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. |
| - | 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 U1016 is stored in the HVAC Control Assembly memory, check for the same DTC stored in Instrument Panel Cluster (IPC). The IPC also monitors the SOH message from the PCM. If the IPC has a DTC U1016 stored, check the PCM for an intermittent malfunction. If the IPC does not have a DTC U1016 stored, check for an open in the serial data line between the HVAC Control Assembly and the PCM. 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 U1016, 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 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 for a stored HVAC Control Assembly DTC U1255. Conditions that set a DTC U1255 may also set a DTC U1016, therefore always diagnose a DTC U1255 first.
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This test will determine if DTC U1016 was set due to a loss of communications with the HVAC control head or the PCM. The Instrument Panel Cluster (IPC) also communicates with the PCM. If a DTC U1016 is stored in the IPC memory, the PCM 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 PCM. 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 PCM? | -- | ||||||||||||
3 |
Is DTC U1255 stored in the HVAC memory? | -- | ||||||||||||
4 |
Is DTC U1016 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 U1016 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 | -- |
