GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2009 🢒 STS 🢒 Driver Information and Entertainment 🢒 Cellular, Entertainment, and Navigation 🢒 DTC U1820

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 Communications With Antilock Brake System (ABS) Control Module on Bus B

Circuit/System Description

Modules connected to the high 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. The supervision time-out period is 250 ms. Each message contains the identification number of the transmitter module.

Conditions for Running the DTC
    • Voltage supplied to the modules is in the normal operating voltage range.
    • The vehicle power mode requires serial data communication to occur.

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 module uses a default value for the missing parameter.

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, without a repeat of the malfunction.

Reference Information

Schematic Reference

Data Communication Schematics

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

Scan Tool Reference

Control Module References for scan tool information.

Circuit/System Verification

With a scan tool, attempt to communicate with all modules.

Circuit/System Testing
  1. With a scan tool, verify DTC U0100-U0299 or U2105-U2199 is not set as current.
    2. ⇒If DTC is set, refer to DTC U0100-U0299or DTC U2105-U2199
  2. Ignition OFF, test for less then 5 Ohms of resistance between the applicable modules ground circuit and ground.
    3. ⇒If greater then the specified range, test the circuit for an open/high resistance.
  3. Ignition OFF, disconnect the applicable modules harness connector.
  4. Ignition ON, verify a test lamp illuminates between the following circuits of the applicable module and ground.
    5. • The battery positive voltage input circuits.
    • The battery positive voltage output circuits.
    • The ignition voltage input circuits.
    • The ignition voltage output circuits.
    • The switched battery positive voltage supply circuits.
    ⇒If the test lamp does not illuminate, test the circuit for an open/high resistance. If the circuit fuse is open, test the circuit for a short to ground. If the circuit tests normal, replace the applicable control module.
  5. Ignition OFF, test for less then 5 Ohms of resistance between the non-communicating modules GMLAN serial data circuits and the module reporting DTC U1820.
    6. ⇒If greater then the specified range, test the circuit for and open/high resistance.
  6. If all circuits test normal, replace the applicable control module.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

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