LaCrosse FWD L4-2.4L (2011)
Information Bus: Symptom Related Diagnostic Procedures
Scan Tool Does Not Communicate With Low Speed GMLAN Device
Scan Tool Does Not Communicate with Low Speed GMLAN Device
Diagnostic Instructions
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Perform the Diagnostic System Check - Vehicle (See: Testing and Inspection/Initial Inspection and Diagnostic Overview/Diagnostic System
Check - Vehicle) prior to using this diagnostic procedure.
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Review Strategy Based Diagnosis (See: Testing and Inspection/Initial Inspection and Diagnostic Overview/Strategy Based Diagnosis) for an
overview of the diagnostic approach.
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Diagnostic Procedure Instructions (See: Testing and Inspection/Initial Inspection and Diagnostic Overview/Diagnostic Procedure Instructions)
provides an overview of each diagnostic category.
Diagnostic Fault Information
Circuit/System Description
The serial data is transmitted over a single wire to the appropriate control modules. The transmission speed for GMLAN low speed is up to 83.33 Kb/s.
Under normal vehicle operating conditions, the speed of the buss is 33.33 Kb/s. The control modules toggle the serial data circuit between 0-5 V during
normal communications. To wake the control modules connected to the GMLAN low speed serial data circuit, a voltage wake up pulse of 10 V is sent
out. If serial data is lost, control modules will set a no communication code against the non-communicating control module. A loss of serial data
communications DTC does not represent a failure of the module that set it.
Diagnostic Aids
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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:
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A control module on the data communication circuit was disconnected while the communication circuit is awake.
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Power to one or more modules was interrupted during diagnosis
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A low battery condition was present, so some control modules stop communicating when battery voltage drops below a certain threshold.
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Battery power was restored to the vehicle and control modules on the communication circuit did not all re-initialize at the same time.
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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.
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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.
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Communication may be available between the body control module (BCM) and the scan tool with the low speed GMLAN serial data system
inoperative. This condition is due to the BCM using both the high and low speed GMLAN systems.
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An open in the low speed GMLAN serial data circuit between the splice pack and a module will only affect that specific module. This type of
failure will set a loss of communication DTC for each module effected, and the other modules will still communicate.
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An open in the data link connector (DLC) ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle.
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The engine may not start when there is a total malfunction of the low speed GMLAN serial data circuit.
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Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed LAN
communications with the scan tool.
These conditions may be caused by the installation of an aftermarket navigation radio module (see bulletins). Some customers may comment of
one or more of the following concerns:
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Vehicle will not crank
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Vehicle cranks but will not start
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Vehicle stability enhancement system warning lights and messages
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PRNDL gear indicator position errors
