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| • | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. |
| • | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
| • | Diagnostic Procedure Instructions provides an overview of each diagnostic category. |
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
|---|---|---|---|---|
B+ | U184A | U184A | -- | -- |
Ignition | U184A | U184A | -- | -- |
Low Speed GMLAN Serial Data | U0078* | U0078, U184A | U0078* | -- |
Ground | -- | U184A | -- | -- |
* Scan Tool Does Not Communicate With Low-Speed GMLAN Device | ||||
The serial data circuit is the means by which the control modules in the vehicle communicate with each other. Once the scan tool is connected to the serial data circuit through the data link connector (DLC), the scan tool can be used to monitor each control module for diagnostic purposes and to check for diagnostic trouble codes (DTCs). When the vehicle is ON, each control module communicating on the serial data circuit sends a state of health message to ensure that the control module is operating properly. When a control module stops communicating on the serial data circuit, for example if the control module loses power or ground, the state of health message it normally sends on the serial data circuit disappears. Other control modules on the serial data circuit, which expect to receive that state of health message, detect its absence; those control modules in turn set a DTC associated with the loss of state of health of the non communicating control module. A loss of serial data communications DTC does not represent a failure of the control modules that contain the stored code.
The system voltage is between 9–16 V.
A supervised periodic message that includes the transmitter module availability has not been received.
The control module is never signalled. Therefore, the specific subsystems will not function.
| • | A current DTC clears when the malfunction is no longer present. |
| • | A history DTC clears when the module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction. |
| • | Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no associated "current" or "active" status. Loss-of- communication U-codes such as these can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios: |
| - | A control module on the data communication circuit was disconnected while the communication circuit is awake. |
| - | Power to one or more control modules was interrupted during diagnosis. |
| - | A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold. |
| - | Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialise at the same time. |
| - | If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part. |
| • | A control module may have a U-code stored in history that does not require any repairs. Issues with late or corrupted messages between control modules can be temporary with no apparent symptom or complaint; this does not mean the control module is faulty. Do not replace a control module based only on a history U-code. |
| • | Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication issue. |
| • | Communication may be available between the control module and the scan tool with one or more GMLAN serial data systems inoperative. This condition is due to the control module using multiple GMLAN busses. |
| • | Use Data Link References Data Link References to determine what GMLAN serial data communications the non communicating control module uses. |
| • | The control module may not have internal protection for specific control circuits and may open a B+ or ignition fuse. If a fuse is open and the B+ or ignition circuit is not shorted to earth, ensure none of the control circuits are shorted to earth before replacing the non communicating control module. |
Data Link Communications Description and Operation
Control Module References for scan tool information
| ⇒ | If the DTC is set, refer to DTC U0100-U02FF . |
| ⇒ | If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle . |
Note: Use the schematic to identify the following: • The control module location on low-speed GMLAN serial data circuits • The control module B+, ignition, earth and serial data circuit terminals
| ⇒ | If greater than the specified range, test the ground circuit for an open/high resistance. |
| ⇒ | If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the K9 BCM for a short-circuit to earth. If the circuits test normal, replace the K9 BCM. |
| ⇒ | If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the K9 BCM for a short-circuit to earth. If the circuits test normal, replace the K9 BCM. |
| ⇒ | If greater than the specified range, test the serial data circuit for an open-circuit/high resistance between the K9 BCM and the control module setting the DTC. |
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for control module replacement, programming and setup |
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