Scan Tool Does Not Communicate with Low Speed GMLAN Device
Scan Tool Does Not Communicate with Low Speed GMLAN Device
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.
Diagnostic Fault Information
Circuit/System Description
Modules connected to the low speed GMLAN serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules when the ignition switch is in any position other than OFF. The low speed GMLAN serial data circuit must be operational for the vehicle to start so the vehicle theft deterrent (VTD) module and body control module (BCM) can communicate. The low speed GMLAN serial data circuit uses SP 205 as the low speed splice pack or star connector.
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.
* 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 the high speed GMLAN serial data system inoperative. This condition is due to the BCM using both the high and low speed GMLAN systems.
* An open in the DLC ground circuit terminal 5 will allow the scan tool to operate but not communicate with the vehicle. When the scan tool attempts to communicate with the vehicle a message "no CANdi module detected" will be displayed.
* The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus.
* Technicians may find various Local Area Network (LAN) communication Diagnostic Trouble Codes (DTC) and no low speed GMLAN communications with the scan tool.
* The engine will not start when there is a total malfunction of the high speed GMLAN serial data bus.
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:
- Vehicle will not crank.
- Vehicle cranks but will not start.
- Vehicle stability enhancement system warning lights and messages.
- PRNDL gear indicator position errors.
- Tire Pressure Monitor (TPM) system warning lights.
Reference Information
Schematic Reference
Data Communication Schematics (Electrical Diagrams)
Connector End View Reference
Component Connector End Views (Connector End Views By Name)
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
1. Ignition OFF, disconnect the harness connector at the low speed splice pack.
2. Ignition ON, test for less than 1.0 volt between the low speed GMLAN serial data circuit of the DLC and ground.
• If greater than 1.0 volt, repair the serial data circuit for a short to voltage.
3. Test for infinite resistance between the serial data circuit of the DLC and ground.
• If less than infinite resistance, repair the serial data circuit for a short to ground.
4. Test for less than 1.0 ohm of resistance in the serial data circuit between the DLC and the splice pack.
• If greater than 1.0 ohm, repair the serial data circuit for an open/high resistance.
5. Install a 3-amp fused jumper wire between pin A and pin M of the splice pack. Install another 3-amp fused jumper wire to pin M.
6. Using the extra jumper wire from pin M attempt to establish communications with any other module connected to the splice pack. At least one module should be able to communicate.
• If communications can't be established with any module, test the serial data circuit of the BCM for a short to voltage, short to ground or open/high resistance. If the circuit tests normal, replace the BCM.
7. Using the extra jumper wire at pin M, continue testing each serial data circuit at the splice pack for communications. All modules should be able to communicate.
• If there is only one module connected, test the serial data circuit for a short to voltage or a short to ground. If the circuit tests normal, replace the faulty module.
• If there are multiple modules, test each serial data circuit for a short to voltage or a short to ground, with the modules disconnected. If the circuit tests normal, install each module one at a time starting with the closest module to the splice pack until communications stop. Replace the faulty module.
8. If all circuits test normal, test or replace the splice pack.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the repair.
* Control Module References (Programming and Relearning)for module replacement, setup, and programming
* GMLAN Wiring Repairs (GMLAN Wiring Repairs)