Circuit Description
The class 2 serial data line communicates information between modules. Each module connected to the serial data line is assigned a recognition code, or address. This code is used in order to identify which modules are communicating. Modules communicate with each other by periodically sending state of health (SOH) messages. When the ignition switch is in RUN, each module communicating on the serial data line sends a SOH message every 2 seconds. This ensures that the system is operating properly. When the heater and A/C programmer stops communicating on the class 2 serial data line, the SOH that is normally sent on the serial data line every 2 seconds disappears. The module, which expects to receive that message, detects the absence of the message. The module in turn sets DTC U1152 associated with the loss of SOH of the heater and A/C programmer.
Conditions for Setting the DTC
| • | The module does not receive a state of health (SOH) message back from the heater and A/C programmer. |
| The module retries the request 3 times, and then sets DTC U1152. |
| • | This DTC only sets if U1300 or U1301 are not current. |
Action Taken When the DTC Sets
| • | DTC U1152 is stored in the modules memory. |
| • | The module uses the default values for the initialization data. |
| • | This DTC does not cause a driver warning message to appear. |
Conditions for Clearing the DTC
| • | This DTC clears (status changes from current to history) under any of the following conditions: |
| - | The module detects a state of health message from the heater and A/C programmer. |
| - | At each ignition cycle |
| • | A history DTC clears after 50 consecutive ignition cycles if the conditions for setting the DTC are no longer present. |
| • | Current and history DTCs may be cleared using a scan tool. |
Diagnostic Aids
| • | DTC U1152 does not normally represent a failure within the module. |
| • | When Loss of Class 2 Serial Data Communication DTCs (DTCs with the letter U as the prefix) are stored as history DTCs, know all possible serial data DTCs set in each module before diagnosing the serial data line. |
| • | If U1152 is stored in the modules memory, inspect for the same DTC stored in other modules capable of storing this DTC. |
| • | If the DTC is a history DTC, the condition may be intermittent. |
Test Description
The numbers below refer to the step numbers on the diagnostic table:
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Data Link Communications Diagnostic System Check? | -- | Go to Step 2 | |
Using a scan tool, attempt to establish communications with the heater and A/C programmer. Does the scan tool communicate with the heater and A/C programmer? | -- | Go to Step 3 | Go to Diagnostic System Check in HVAC Systems with A/C - Automatic | |
3 | Inspect and repair the class 2 serial data line CKT 1807 (PPL) for intermittent connections. Refer to Testing for Electrical Intermittents . Is the repair complete? | -- | Go to Step 4 | -- |
4 |
Is the repair complete? | -- | -- |
