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 U1814 00: Powertrain High Speed Communication Enable Circuit

Diagnostic Fault Information

Circuit

Short to Ground

Open/High Resistance

Short to Voltage

Signal Performance

Accessory Wake Up Serial Data

U1814

U0100-U0299

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Circuit/System Description

The accessory wake up serial data circuit is a positive voltage signal to wake up multiple control modules. The signal is activated in 2 ways; any time the ignition switch is in the ACC or RUN position, or anytime the body control module (BCM) determines that communication is needed from the powertrain. When the ignition switch is in the ACC or RUN position, the accessory voltage circuit input to the BCM is internally connected to the accessory wake up serial data circuit output, and voltage is supplied to the modules. The BCM also supplies voltage to the accessory wake up serial data circuit when it determines that communication is necessary with the powertrain and the ignition is not in the ACC or RUN position such as during remote start.

Modules activated by the accessory wakeup serial data circuit are as follows:

    • Engine control module (ECM)
    • Transmission control module (TCM)
    • Transfer case control module (TCCM) (RPO NP8)
    • Serial data gateway module (SDGM) (RPO HP2)

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 BCM senses a short to ground on the accessory wake up serial data circuit.

Action Taken When the DTC Sets

    • The modules use a default value for the missing parameters until the next ignition cycle.
    • The module(s) is never signaled, therefore the specific subsystem(s) will not function.
    • The vehicle may not start.

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

Short to ground may also set multiple no communication codes for GMLAN high speed modules.

Reference Information

Schematic Reference

    • DLIS Fuse and Ignition Switch in Power Distribution Schematics
    •  Body Control System Schematics
    •  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

Circuit/System Testing

  1. With the ignition OFF, disconnect the module harness connectors listed below one at a time. After each disconnect, retest for current DTCs. DTC U1814 should remain current.
  2. • Engine control module (ECM) - X1
    • Transmission control module (TCM) - X1
    • Transfer case control module (TCCM) (RPO NP8) - X2
    • Serial data gateway module (SDGM) (RPO HP2) - X2
    If the DTC becomes history, replace the module disconnected before the DTC became history.
  3. Ignition OFF, disconnect the X4 harness connector at the BCM.
  4. Test for infinite resistance between the accessory wake up serial data circuit terminal 19 and ground.
  5. If less than the specified range, test the serial data circuit for a short to ground.
  6. If the circuit tests normal, replace the BCM.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for module replacement, setup, and programming