U2175
DTC U2105-U2199 (Except U2142 and U2143)
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptors
DTC U2105 00
-
Lost Communication with Engine Control Module
DTC U2106 00
-
Lost Communication with Transmission Control Module
DTC U2107 00
-
Lost Communication with Body Control Module
Diagnostic Fault Information
Circuit/System Description
Control 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 control modules. The control 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.
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
A supervised periodic message that includes the transmitter module availability has not been received.
Action Taken When the DTC Sets
The control module(s) is never signaled, therefore the specific subsystem(s) 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
* Sometimes, while diagnosing a specific customer concern or after a repair, you may notice a history U-code present. However, there is no associated "current" or "active" status. Loss-of- communication U-codes such as these can set for a variety of reasons. Many times, they are transparent to the vehicle operator and technician, and/or have no associated symptoms. Eventually, they will erase themselves automatically after a number of fault-free ignition cycles. This condition would most likely be attributed to one of these scenarios:
- A control module on the data communication circuit was disconnected while the communication circuit is awake.
- Power to one or more modules was interrupted during diagnosis.
- A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.
- Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialize at the same time.
- If a loss-of-communication U-code appears in history for no apparent reason, it is most likely associated with one of the scenarios above. These are all temporary conditions and should never be interpreted as an intermittent fault, causing you to replace a part.
* A control module may have a U-code stored in history that does not require any repairs. Issues with late or corrupted messages between control modules can be temporary with no apparent symptom or complaint; this does not mean the control module is faulty. Do not replace a control module based only on a history U-code.
* Do not replace a control module reporting a U-code. The U-code identifies which control module needs to be diagnosed for a communication issue.
* Communication may be available between the BCM and the scan tool with either the low or high speed GMLAN serial data system inoperative. This condition is due to the BCM using both the low and high speed GMLAN systems.
* Use to determine if the module uses high or low speed GMLAN serial data communications.
* Some control modules may not have internal protection for specific control circuits and may open a B+ or ignition fuse. If a fuse is open and the B+ or ignition circuit is not shorted to ground, ensure none of the control circuits are shorted to ground before replacing the control module.
* This diagnostic can be used for any control module that is not communicating, regardless of the type of serial data circuit it is connected to, providing the vehicle is equipped with the control module.
Reference Information
Schematic Reference
* Data Communication Schematics Electrical Diagrams
* Control Module References Programming and Relearning
Connector End View Reference
Component Connector End Views 1-2 Shift Solenoid (SS) Valve (MN5)
Description and Operation
Data Link Communications Description and Operation Description and Operation
Electrical Information Reference
* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs
Scan Tool Reference
Control Module References Programming and Relearning for Scan Tool Information
Circuit/System Testing
Note:
Use the schematic to identify the following:
* Control modules the vehicle is equipped with
* Control module locations on the low and high speed GMLAN serial data circuits
* The control modules B+, ignition, ground, accessory wakeup serial data, serial data communication enable, and serial data circuit terminals
1. Ignition OFF, disconnect the harness connector at the control module that is not communicating.
2. Test for less than 10 ohms between each ground circuit terminal and ground.
• If greater than the specified range, test the ground circuit for an open/high resistance.
3. If equipped, ignition ON, verify that a test lamp illuminates between each B+ circuit terminal and ground.
• If the test lamp does not illuminate, test the B+ circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the control module for a short to ground. If the circuits test normal, replace the control module.
4. If equipped, ignition ON, verify that a test lamp illuminates between each ignition circuit terminal and ground.
• If the test lamp does not illuminate, test the ignition circuit for a short to ground or an open/high resistance. If the circuit fuse is open, test the control circuits of the control module for a short to ground. If the circuits test normal, replace the control module.
Note:
Only the high speed GMLAN modules have a serial data communication enable circuit OR an accessory wakeup serial data circuit, and the body control module is the output for these circuits. Refer to the control module schematics to identify which control modules have these circuits.
5. If equipped, ignition ON, verify that a test lamp illuminates between the serial data communication enable circuit terminal OR the accessory wakeup circuit terminal and ground.
• If the test lamp does not illuminate, test the entire circuit, including the B+ circuit to the K9 body control module (BCM), for an open/high resistance or short to ground. If the circuits test normal, replace the K9 BCM.
6. Ignition OFF for 60 seconds, test for less than 5 ohms between the serial data circuit terminals and the appropriate X84 data link connector (DLC) terminal listed below:
* Low speed GMLAN serial data terminal 1
* High speed GMLAN serial data terminal 6 or 14
• If greater than the specified range, test the serial data circuit for an open/high resistance between the non communicating control module and the control module setting the DTC or a serial data splice pack.
7. If all circuits test normal, replace the control module that is not communicating.
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
* GMLAN Wiring Repairs GMLAN Wiring Repairs