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| • | Perform the Diagnostic System Check - Vehicle prior to using the diagnostic procedure. |
| • | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
| • | Diagnostic Procedure Instructions provides an overview of each diagnostic category. |
Refer to Control Module U Code List .
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
|---|---|---|---|---|
Module B+ | U2105-U2199 | U2105-U2199 | -- | -- |
Module Ignition | U2105-U2199 | U2105-U2199 | -- | -- |
Accessory Wakeup Serial Data | U1814 | U2105-U2199 | -- | -- |
Serial Data Communication Enable | U2099 | U2105-U2199 | -- | -- |
High Speed GMLAN Serial Data (+) | 2 | U2105-U2199 | 2 | -- |
High Speed GMLAN Serial Data (-) | 2 | U2105-U2199 | 2 | -- |
Low Speed GMLAN Serial Data | 1 | U2105-U2199 | 1 | -- |
Module Ground | - | U2105-U2199 | -- | -- |
1. Scan tool does not communicate with low speed GMLAN device 2. Scan tool does not communicate with high speed GMLAN device | ||||
Control modules connected to the high and low 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.
The body control module (BCM) activates the high speed GMLAN serial data wake up and the accessory wake up serial data circuits by applying voltage when the ignition key is in ACC, ON or START. The high speed GMLAN serial data wake up circuit wakes up the electronic brake control module (EBCM) for bus communication and the accessory wake up serial data circuit wakes up the engine control module (ECM) and transmission control module (TCM).
| • | The system voltage is between 9–16 V. |
| • | The vehicle power mode requires serial data communication to occur. |
A supervised periodic message that includes the transmitter module availability has not been received.
The control module uses a default value for the missing parameters.
| • | A current DTC clears when the malfunction is no longer present. |
| • | A history DTC clears when the control module ignition cycle counter reaches the reset threshold of 50, without a repeat of the malfunction. |
| • | If there are multiple non-communicating modules choose the one closest to the data link connector (DLC). |
| • | 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 control 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-initialise 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. |
| • | Use Data Link References to determine if the control module uses high or low speed GMLAN serial data communications |
| • | Some control modules may not have internal protection for specific voltage outputs and may open a battery positive voltage or ignition voltage source fuse. If a voltage input fuse is open and no short is found in that circuit, ensure that no control module output voltage circuit is shorted to ground before replacing the control module. |
| • | Some intermittent communication concerns may be caused by fretting corrosion on the serial data circuit terminals. Inspect all connectors at the control module that set the communication DTC, the control module that the communication DTC was set against, and any inline harness connectors between the two control modules. Do not replace a control module based only on fretting corrosion. Refer to bulletin 09-06-03-004 for assistance with the diagnosis and repair of this condition. |
| • | This diagnostic can be used for any control module that should communicate with high or low speed GMLAN serial data providing the vehicle is equipped with the option that uses that control module. |
Data Link Communications Description and Operation
Control Module References for scan tool information
| ⇒ | If any of the DTCs are set, refer to Diagnostic Trouble Code (DTC) List - Vehicle . |
Note: Terminal and connector identifiers will be different depending on the individual control module being tested. Use Schematic Reference and Connector End View Reference to identify control module's connector, B+, ignition, earth, communication enable and serial data circuit terminals.
| ⇒ | If greater than specified range, test the ground circuit for an open/high resistance. |
| ⇒ | 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, also test the control circuits of the control module for a short to earth. If the circuits test normal, replace the appropriate control module. |
| ⇒ | 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, also test the control circuits of the control module for a short to earth. If the circuits test normal, replace the appropriate 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. If the control module that is not communicating does not have a serial data communication enable circuit OR an accessory wakeup serial data circuit, proceed to step 6.
| ⇒ | If the test lamp does not illuminate, test the entire circuit, including the B+ circuit to the K9 body control module, for an open/high resistance or short to ground. If the circuits test normal, replace the K9 body control module. |
| ⇒ | If greater than specified range, test the GMLAN serial data circuit for open/high resistance. |
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
| • | Control Module References for control module replacement, programming and setup |
| © Copyright Chevrolet. All rights reserved |
| © Copyright Chevrolet. All rights reserved |
