Scan Tool Does Not Communicate With Class 2 Device
Scan Tool Does Not Communicate with Class 2 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
The serial data circuit is the means by which the control modules connected to it communicate with each other. Once the scan tool is connected to the class 2 serial data circuit through the DLC, the scan tool can be used to monitor each control module for diagnostic purposes and to check for diagnostic trouble codes (DTCs). Class 2 serial data is transmitted on a single wire at an average of 10.4 Kbps. The bus is active at 7.0 volts nominal and inactive at ground potential. When the ignition switch is in RUN, each control module communicating on the class 2 serial data circuit sends a state of health (SOH) message to ensure that the control module is operating properly. When a control module stops communicating on the class 2 serial data circuit, for example if the control module loses power or ground, the SOH message it normally sends on the data circuit disappears. Other control modules on the class 2 serial data circuit, which expect to receive that SOH message, detect its absence; those control modules in turn set an internal DTC associated with the loss of SOH of the non-communicating control module. The DTC is unique to the control module which is not communicating, for example, when the body control module (BCM) SOH message disappears, several control modules set DTC U1064. Note that a loss of serial data DTC does not normally represent a failure of the control module that set it.
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're 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 low 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.
* An open between the DLC and the BCM will only effect the communication with the scan tool. The vehicle modules will still communicate, and the vehicle will start.
* The engine will not start when there is a total malfunction of the low speed GMLAN serial data circuit.
* 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:
- 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 ([1][2]Electrical Diagrams)
Connector End View Reference
Component Connector End Views (Connector Views)
Description and Operation
Data Link Communications Description and Operation (Description and Operation)
Electrical Information Reference
* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)
Circuit/System Testing
Note: Use the schematic to identify the following:
* Control modules the vehicle is equipped with
* Control module locations on the class 2 serial data circuit
* The class 2 serial data circuit terminals for each control module
1. Attempt to communicate with all control modules on the class 2 serial data circuit, refer to Data Link References (Data Link References). Communications should not be available with two or more control modules on the class 2 serial data circuit.
• If only one control module is not communicating, diagnose that control module only; refer to DTC U1001-U1254 (U Code Charts).
• If one or more control modules are communicating but not all, refer to Testing the Serial Data Circuit for an Open/High Resistance.
2. Disconnect the scan tool from the DLC. The following tests will be done at the DLC connector.
3. All access doors closed, ignition OFF for 60 seconds, test for less than 10 ohm between the ground circuit terminal 5 and ground.
• If greater than the specified range, test the ground circuit for an open/high resistance.
4. Ignition ON, test for less than 4.5 V between the serial data circuit terminal 2 and ground.
• If greater than the specified range, test the serial data circuit for a short to voltage; refer to Testing the Serial Data Circuit for a Short to Voltage.
5. Ignition OFF for 60 seconds, test for greater than 100 ohm between the serial data circuit terminal 2 and ground.
• If less than the specified range, test the serial data circuit for a short to ground; refer to Testing the Serial Data Circuit for a Short to ground.
6. Disconnect the appropriate harness connector at the BCM.
7. Test for less than 2 ohm between the DLC serial data circuit terminal 2 and the BCM connector.
• If greater than the specified range, test the serial data circuit for an open/high resistance. If the circuits test normal, replace the BCM.
Testing the Serial Data Circuits for a Short to Voltage
1. Ignition OFF, disconnect the appropriate harness connectors at the BCM.
2. Ignition ON, test for less than 4.5 V between the DLC serial data circuit terminal 2 and ground.
• If greater than the specified range, repair the short to voltage.
3. Test for less than 4.5 V between each serial data circuit at the BCM connectors and ground.
• If greater than the specified range, perform the appropriate test listed below:
* Serial data circuits with one control module, test the serial data circuit for a short to voltage. If the circuit tests normal, replace the control module.
* Serial data circuits with 2 or more control modules, test each section of the serial data circuit for a short to voltage. If the circuits test normal, replace the control module that causes the short to voltage when connected.
Testing the Serial Data Circuits for a Short to Ground
1. Ignition OFF, disconnect the appropriate harness connectors at the BCM.
2. Test for infinite resistance between the DLC serial data circuit terminal 2 and ground.
• If not the specified value, repair the short to ground.
3. Ignition OFF for 60 seconds, test for greater than 100 ohm between each serial data circuit at the BCM connectors and ground.
• If less than the specified range, perform the appropriate test listed below:
* Serial data circuits with one control module, test the serial data circuit for a short to ground. If the circuit tests normal, replace the control module.
* Serial data circuits with 2 or more control modules, test each section of the serial data circuit for a short to voltage. If the circuits test normal, replace the control module that causes the short to ground when connected.
Testing the Serial Data Circuit for an Open/High Resistance
1. Ignition OFF, disconnect the appropriate harness connectors at the BCM.
2. Install a 3A fused jumper wire between the disconnected BCM connector DLC terminal and a class 2 serial data circuit that is not communicating.
3. Ignition ON, verify the scan tool communicates with the control modules connected to the class 2 serial data circuit.
• If any control module does not communicate, test each section of the serial data circuit for an open/high resistance. If the circuits test normal, replace the control module that does not communicate when connected.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the repair.
* GMLAN Wiring Repairs (Component Tests and General Diagnostics)
* Control Module References (Programming and Relearning) for module replacement, setup, and programming