U1814

DTC U1814

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (with HP2) (Initial Inspection and Diagnostic Overview)Diagnostic System Check - Vehicle (without HP2) (Initial Inspection and Diagnostic Overview) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Initial Inspection and Diagnostic Overview) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Initial Inspection and Diagnostic Overview) provides an overview of each diagnostic category.

DTC Descriptor
DTC U1814 00
- Powertrain High Speed Communication Enable Circuit

Diagnostic Fault Information





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 (Power Distribution Diagrams)
* Body Control System Schematics (Electrical Diagrams)
* Data Communication Schematics (Electrical Diagrams)

Connector End View Reference

Component Connector End Views (Connector Views)
Description and Operation

Data Link Communications Description and Operation (Description and Operation)
Electrical Information Reference

* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)

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.

* 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.

2. Ignition OFF, disconnect the X4 harness connector at the BCM.
3. Test for infinite resistance between the accessory wake up serial data circuit terminal 19 and ground.

• If less than the specified range, test the serial data circuit for a short to ground.

4. If the circuit tests normal, replace the BCM.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

Control Module References (Programming and Relearning) for module replacement, setup, and programming