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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+ | U1818 | U1818 | -- | -- |
Ignition | U1818 | U1818 | -- | -- |
Accessory Wake Up Serial Data 2 | P06E4* | U1818 | -- | -- |
Powertrain High-Speed GMLAN Serial Data (+) | U0074* | U1818* | U0074* | - |
Powertrain High-Speed GMLAN Serial Data (-) | U0074* | U1818* | U0074* | -- |
Ground | -- | U1818 | -- | -- |
* Other DTCs may set with this fault. | ||||
Control modules connected to the serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the control modules. The control modules have prerecorded information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are supervised and also, some periodic messages are used by the receiver module as an availability indication of the transmitter module. Each message contains the identification number of the transmitter module.
| • | The system voltage is at least 10 V. |
| • | The vehicle power mode master requires serial data communication to occur from this specific module. |
A supervised periodic message that includes the transmitter module availability has not been received.
DTC U1818 is a type A DTC.
DTC U1818 is a type A DTC.
| • | 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 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 . |
| ⇒ | If all the DTCs are set, refer to DTC U0074 . |
Note: Use the schematic to identify the following: • The control module location on the powertrain high-speed GMLAN serial data circuits • The control module B+, ignition, earth, accessory wake-up serial data 2 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 K20 ECM for a short-circuit to earth. If the circuits test normal, replace the K20 ECM. |
| ⇒ | 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 K20 ECM for a short-circuit to earth. If the circuits test normal, replace the K20 ECM. |
| ⇒ | If the test lamp does not illuminate, test the entire circuit, including the B+ circuit to the K114B hybrid powertrain control module 2, for an open-circuit/high resistance or short-circuit to earth. If the circuits test normal, replace the K114B hybrid powertrain control module 2. |
| ⇒ | If greater than the specified range, test the serial data circuit for an open-circuit/high resistance between the K20 ECM 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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