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 U2105: Lost Communications With Engine Control SystemDTC U2107: Lost Communications With Body Control System
DTC U2108: Lost Communications With ABS/TCS Control System
Diagnostic Fault Information
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
|---|---|---|---|---|
Class 2 Serial Data | 1 | U2105-U2199 | 1 | -- |
Battery Positive Voltage Circuits | -- | U2105-U2199 | -- | -- |
Ignition Voltage Circuits | -- | U2105-U2199 | -- | -- |
Module Ground Circuits | -- | U2105-U2199 | U2105-U2199 | -- |
1. Scan Tool Does Not Communicate with Class 2 Device | ||||
Circuit/System Description
Modules connected to the GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. The modules have programmed information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are also supervised and some periodic messages are used by the receiver module as an availability indication of the transmitter module. The supervision time-out period is 250 milliseconds. Each message contains the identification number of the transmitter module. When a message that indicates the availability of the transmitter module is not received, the receiver module sets a DTC 21xx where xx is equal to the 2-digit identification number of the transmitter module.
The DTC descriptors listed below provide a method for determining which module is not communicating. A module with a GMLAN serial data circuit malfunction or which loses power during the current ignition cycle will have a Loss of Communication DTC set by other modules that depend on information from that failed module. The modules that can communicate will set a DTC indicating the module that cannot communicate.
Conditions for Running the DTC
| • | Voltage supplied to the modules is in the normal operating voltage range. |
| • | The vehicle power mode master requires serial data communication to occur. |
| • | DTC U2100, U0073 or U1000 does not have a current status. |
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 module uses a default value for the missing parameter.
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, 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
Connector End View Reference
Description and Operation
Data Link Communications Description and Operation
Electrical Information Reference
Scan Tool Reference
Control Module References for scan tool information
Circuit/System Testing
- Using the DTC Descriptor and Diagnostic Aids above, determine the module that is not communicating that should be tested.
- Ignition OFF, disconnect the harness connector of the module that is not communicating.
- Ignition ON, verify that a test lamp illuminates between the module voltage circuit and ground.
- Test for less than 5 ohm between the module ground circuits and ground.
- Test for less than 1 ohm in the Class 2 serial data circuit between the non-communicating module and the module that displays the DTC.
- If all circuits test normal, replace the module that is not communicating.
| ⇒ | 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, also test the positive voltage outputs of the module for a short to ground. If the circuits test normal, replace the faulty module. |
| ⇒ | If greater than 5 ohm, test the ground circuit for an open/high resistance. |
| ⇒ | If greater than 1 ohm, test the Class 2 serial data circuit for open/high resistance. |
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for module replacement, setup, and programming |
