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For 1990-2009 cars only
Makes 🢒 Cadillac 🢒 2008 🢒 XLR 🢒 Driver Information and Entertainment 🢒 Cellular, Entertainment, and Navigation 🢒 DTC U1001-U1254

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 U1001-DTC U1254: Class 2 Data Link

Circuit/System Description

Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module receives a message for a critical operating parameter, the module records the identification number of the module which sent the message for State of Health monitoring. A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter.

The following modules communicate on the class 2 serial data circuit:

    • The body control module (BCM)
    • The communication interface module (OnStar®)
    • The digital radio receiver (DRR)
    • The driver door module (DDM)
    • The driver door switch (DDS)
    • The driver position module (DPM)
    • The fold top controller (FTC)
    • The head up display (HUD)
    • The HVAC control module
    • The inflatable restraint sensing and diagnostic module (SDM)
    • The instrument panel cluster (IPC)
    • The passenger door module (PDM)
    • The radio
    • The remote control door lock receiver (RCDLR)
    • The steering column lock control module (SCLCM)

The BCM emulates all devices that are on high speed GMLAN serial data circuits to the modules on class 2 serial data circuit. The BCM transmits multiple Node Alive messages using source IDs that correspond to each of the GMLAN nodes. This is in addition to the BCM behaving itself as a real node.

    • The electronic brake control module (EBCM)
    • The electronic suspension control module (ESC)
    • The engine control module (ECM)
    • The transmission control module (TCM)

Conditions for Running the DTC
    • Voltage supplied to the module is in the normal operating voltage range of 9-16 volts.
    • DTCs U1300, U1301 or U1305 do not have a current status.
    • The vehicle power mode requires serial data communication to occur.

Conditions for Setting the DTC

A message containing a critical operating parameter has not been received within the last 5-10 seconds after establishing class 2 serial data communication.

Action Taken When the DTC Sets

The module uses a default value for the missing parameter.

Diagnostic Aids

When a malfunction such as an open fuse to a module occurs while modules are communicating, a DTC  U1001-1254 Loss of XXX Communications is set current. When the modules stop communicating the current DTC U1001-1254 Loss of XXX Communications is cleared but the history DTC remains. When the modules begin to communicate again, the module with the open fuse will not be learned by the other modules so U1000 or U1255 is set current by the other modules. If the malfunction occurs when the modules are not communicating, only U1000 or U1255 is set.

Reference Information

Schematic Reference

Data Communication Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

Body Control System 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 verify communication with each class 2 module is available.

Circuit/System Testing
  1. With a scan tool, attempt to communicate with all class 2 modules.
  2. Ignition OFF, disconnect the harness connector of the non communicating control module containing the ground circuit.
  3. Ignition OFF, scan tool disconnected, test for less than 5 ohms between the non-communicating control module ground circuit(s) and ground.
    4. ⇒If greater then the specified range, test the circuit for an open/high resistance.
  4. Ignition OFF, disconnect the harness connector of the non communicating control module containing the following circuits:
    5. • The battery positive voltage supply circuits
    • The switched battery positive supply circuits
    • The battery positive voltage output circuits
    • The ignition voltage input circuits
    • The ignition voltage output circuits
  5. Ignition ON, verify a test lamp illuminates between the following circuits of the non-communicating control module and ground:
    6. • The battery positive voltage supply circuits
    • The switched battery positive supply circuits
    • The battery positive voltage output circuits
    • The ignition voltage input circuits
    • The ignition voltage output circuits
    ⇒If the test lamp does not illuminate, test the circuit for a short to ground, or an open/high resistance. If the circuit fuse is open, test the circuit for a short to ground. If all circuits test normal, replace the non-communicating control module.
  6. Ignition OFF, test for less than 5 ohms between the class 2 serial data communication terminal of the non-communicating control module, and the splice pack.
    7. ⇒If greater than the specified range, test the circuit for an open/high resistance.
  7. If all circuits test normal, replace the non-communicating control module.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for control module replacement, programming, and setup