U1820
DTC U1820
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptor
DTC U1820 00
- Lost Communication with Brake System Control Module on Bus B
Diagnostic Fault Information
Circuit/System Description
Modules connected to the PTLAN 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.
Conditions for Running the DTC
* The system voltage is between 9-16 volts.
* The vehicle power mode master requires serial data communication to occur for this specific module.
Conditions for Setting the DTC
A supervised periodic message that includes the transmitter module availability has not been received.
Action Taken When the DTC Sets
The modules is never signaled, therefore the specific subsystems will not function.
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.
Reference Information
Schematic Reference
* Data Communication Schematics Electrical Diagrams
* Control Module References Programming and Relearning
Connector End View Reference
Component Connector End Views Connector End Views By Name
Description and Operation
Data Link Communications Description and Operation Description and Operation
Electrical Information Reference
* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs
Circuit/System Testing
1. Verify the following DTCs are not set in the modules as follows:
* SDGM - U1818
* HPCM - U189A
• If any listed DTC is present, refer to Diagnostic Trouble Code (DTC) List - Vehicle Diagnostic Trouble Code (DTC) List - Vehicle to diagnose that DTC prior to this diagnostic.
2. Ignition OFF, disconnect the harness connector of the yaw rate and lateral acceleration sensor.
3. Verify that a test lamp illuminates between the ground circuit terminal 6 and B+.
• If the test lamp does not illuminate, test the circuit for an open/high resistance or short to ground. If the circuit tests normal, replace the EBCM.
4. Ignition ON, verify that a test lamp illuminates between the ignition voltage circuit terminal 1 and ground.
• If the test lamp does not illuminate, test the circuit for an open/high resistance. If the circuit tests normal, replace the EBCM.
5. Ignition OFF, disconnect harness connector X1 of the SDGM.
6. Test for less than 1.0 ohm between the SDGM and the yaw rate and lateral acceleration sensor on the following circuit terminals.
* PTLAN + SDGM terminal 14 and yaw rate and lateral acceleration sensor terminal 2
* PTLAN - SDGM terminal 12 and yaw rate and lateral acceleration sensor terminal 3
• If greater than the specified range, test the circuit for an open/high resistance.
7. If the circuits test normal, replace the SDGM.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
* GMLAN Wiring Repairs GMLAN Wiring Repairs
* Control Module References Programming and Relearningfor module replacement, setup, and programming