Diagnostic Instructions
| • | 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. |
DTC Descriptors
DTC U1812 00: Battery Energy Control Module Bus B High Speed Communication Enable Circuit LowDiagnostic Fault Information
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
|---|---|---|---|---|
Wake Up Signal | U1814* in BCM | U1812* in HPCM | U209A* in SDGM | -- |
Wake Up Signal | B137A* in SDGM | U1812** | 1 | -- |
SDGM B+ | U1862* and U1812 in HPCM | U1862* and U1812 in HPCM | -- | -- |
SDGM ground | -- | U1862* and U1812 in HPCM | -- | -- |
* Other modules will set other DTC with this fault. ** Open at SDGM or BECM will set DTC U1812 in HPCM. Open at PIM will set DTC U189A in the HPCM. 1. Parasitic drain on battery, and may set DTC U1875, U1876, U1878, U1879 as history in the HPCM with the key ON, or as current with the key OFF. | ||||
Circuit/System Description
Modules connected to the high and low speed GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. The 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.
A wake up signal circuit is energized by the body control module (BCM) to activate the serial data gateway module (SDGM) and other modules immediately when serial data is required. Additionally after receiving this signal the SDGM energizes another wake up signal to activate the drive motor generator battery control module (BECM) among others. If the wake up signal circuit from the BCM to the SDGM is open then the SDGM will not energize the wake up signal circuit to the BECM.
Conditions for Running the DTC
| • | The system voltage is between 9-16 volts. |
| • | The vehicle power mode master requires serial data communication to occur. |
Conditions for Setting the DTC
The BECM does not receive voltage from the SDGM on the wake up signal circuit.
Action Taken When the DTC Sets
The modules is never signaled, therefore the specific subsystems may not function. Depending on the fault, the MIL may be illuminated.
Conditions for Clearing the DTC
| • | 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. |
Diagnostic Aids
| • | When testing circuits to the SDGM access can be accomplished by removing the instrument panel center trim panel. Note that this procedure does not allow removal of the SDGM, only access to the connector. Refer to Instrument Panel Center Trim Panel Replacement. |
| • | If the SDGM is not activated due to an open wake up circuit from the BCM, the SDGM will not energize the wake up signal circuit to the BECM when it should and the HPCM will set DTC U1812 |
| • | Different DTCs may set in different modules along with this DTC depending on where the fault occurs and the vehicle operating conditions when the fault occurs. |
Reference Information
Schematic Reference
Connector End View Reference
Description and Operation
Data Link Communications Description and Operation
Electrical Information Reference
Circuit/System Verification
Note: DTC U1812 will falsely set if the key is held in the start position but the engine does not crank due to another underlying condition.
With a scan tool, ensure the following DTCs are not present in the modules as follows:
| • | BCM - U1814 |
| • | BECM - U1862 |
| • | SDGM - B137A |
| ⇒ | If any listed DTC is present, refer to Diagnostic Trouble Code (DTC) List - Vehicle to diagnose that DTC prior to this diagnostic. |
Circuit/System Testing
Danger: Always perform the High Voltage Disabling procedure prior to servicing any High Voltage component or connection. Personal Protection Equipment (PPE) and proper procedures must be followed.
The High Voltage Disabling procedure will perform the following tasks:| • | Identify how to disable high voltage. |
| • | Identify how to test for the presence of high voltage. |
| • | Identify condition under which high voltage is always present and personal protection equipment (PPE) and proper procedures must be followed. |
- Ignition OFF, disconnect harness connector X2 of the SDGM.
- Ignition ON, verify that a test lamp illuminates between the wake up signal circuit terminal 19 and ground.
- Ignition OFF, connect the harness connector of the SDGM.
- Perform the High Voltage Disabling procedure.
- Ignition OFF, disconnect harness connector X1 of the BECM.
- Reconnect the 12-volt battery.
- Ignition ON, verify that a test lamp illuminates between the wake up signal circuit terminal 12 and ground.
- If all circuits test normal, replace the BECM.
| ⇒ | If the test lamp does not illuminate, test the circuit for an open/high resistance. |
| ⇒ | If the test lamp does not illuminate, test the circuit for an open/high resistance. If the circuit tests normal, replace the SDGM. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
Control Module References for module replacement, setup, and programming
