U1818
DTC U1818
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle (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 U1818 00
- Lost Communication With ECM/PCM A on Bus B
Diagnostic Fault Information
Circuit/System Description
Modules connected to the 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.
DTC U1818 will set in the serial data gateway module (SDGM) when no communication is established with the ECM on the PTLAN serial data bus.
Conditions for Running the DTC
* The system voltage is between 9-16 volts.
* The vehicle power mode master requires serial data communication to occur from 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 are 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.
Diagnostic Aids
* If the ECM is inoperative due to an open in the voltage or ground circuits then DTC U0100 will be set current in the power steering control module (PSCM).
* Different DTCs may set in different modules along with this DTC depending on where the fault occurs and the vehicle operating conditions when the fault occurs.
Reference Information
Schematic Reference
* Data Communication Schematics (Electrical Diagrams)
* Control Module References (Programming and Relearning)
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 Verification
Verify the following DTCs are not set in the modules as follows:
* PSCM - U0100
* SDGM - U189A
If any listed DTC is present, refer to Diagnostic Trouble Code (DTC) List - Vehicle (Diagnostic Trouble Code Descriptions) to diagnose that DTC prior to this diagnostic.
Circuit/System Testing
1. Ignition OFF, disconnect harness connector X1 of the APM and the EBCM.
2. Test for less than 1.0 ohm between the APM and the EBCM on the following circuit terminals.
* PTLAN + APM terminal 5 and EBCM terminal 33
* PTLAN - APM terminal 4 and EBCM terminal 21
• If greater than the specified range, test the circuit for an open/high resistance.
3. Ignition OFF, disconnect harness connector X1 of the SDGM.
4. Test for less than 1.0 ohm between the EBCM and the SDGM on the following circuit terminals.
* PTLAN + EBCM terminal 34 and SDGM terminal 14
* PTLAN - EBCM terminal 22 and SDGM terminal 12
• If greater than the specified range, test the circuit for an open/high resistance.
5. If the circuits test normal, replace the SDGM.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* GMLAN Wiring Repairs (Component Tests and General Diagnostics)
* Control Module References (Programming and Relearning)for module replacement, setup, and programming