GM Service Manual Online
For 1990-2009 cars only

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 Descriptor

DTC U1820 00: Lost Communication with Brake System Control Module on Bus B

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Ignition Voltage (circuit 948)

U1820*, U189A in HPCM, U1818 in SDGM

U1820*, U189A in HPCM, U1818 in SDGM

--

 

Ground (circuit 950)

--

U1820*, U189A in HPCM, U1818 in SDGM

--

 

PTLAN + (circuit 6105)

U0074*

U1820*, U189A in HPCM, U1818 in SDGM

U0074*

--

PTLAN - (circuit 6106)

U0074*

U1820*, U189A in HPCM, U1818 in SDGM

U0074*

--

* Other modules/DTCs will set with this fault.

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
    •  Control Module References

Connector End View Reference

Component Connector End Views

Description and Operation

Data Link Communications Description and Operation

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

Circuit/System Testing

  1. Verify the following DTCs are not set in the modules as follows:
  2. • SDGM - U1818
    • HPCM - U189A
    If any listed DTC is present, refer to Diagnostic Trouble Code (DTC) List - Vehicle to diagnose that DTC prior to this diagnostic.
  3. Ignition OFF, disconnect the harness connector of the yaw rate and lateral acceleration sensor.
  4. Verify that a test lamp illuminates between the ground circuit terminal 6 and B+.
  5. 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.
  6. Ignition ON, verify that a test lamp illuminates between the ignition voltage circuit terminal 1 and ground.
  7. If the test lamp does not illuminate, test the circuit for an open/high resistance. If the circuit tests normal, replace the EBCM.
  8. Ignition OFF, disconnect harness connector X1 of the SDGM.
  9. Test for less than 1.0 ohm between the SDGM and the yaw rate and lateral acceleration sensor on the following circuit terminals.
  10. • 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.
  11. If the circuits test normal, replace the SDGM.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  GMLAN Wiring Repairs
    •  Control Module References for module replacement, setup, and programming