Scheme 1
Scheme 2
Scheme 3
Circuit/System Description
The high speed GMLAN serial data circuits are high speed serial data buses used to communicate information between the control modules. Typical data transmission speeds must be high enough to ensure that a required real-time response is maintained. The high speed GMLAN serial data circuits also connect directly to the data link connector (DLC).
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 modules. The 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 serial data communication enable circuit by applying voltage when the ignition key is in ACC, ON or START. The serial data communication enable circuit wakes up the electronic brake control module (EBCM), all wheel drive module, and the electronic power steering control module if equipped for buss communication.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
- 1: A module which loses power during an ignition cycle will cause other modules to set Lost Communication DTCs.
- 2: A module which loses ground during an ignition cycle will cause other modules to set Lost Communication DTCs.
- 3: The malfunction is due to an open in the class 2 serial data circuit or an open in the module.
- 6: The module which was not communicating may have set Lost Communication DTCs for those modules that it was monitoring.
- 8: The modules which can communicate indicate the module which cannot communicate. You must clear the DTCs from these modules to avoid future misdiagnosis.
| Step | Action | Yes | No |
|---|---|---|---|
| Schematic Reference: Data Communication Schematics Connector End View Reference: Master Electrical Component List | |||
| 1 | Test the battery positive voltage and ignition B+ circuits of the module that is not communicating for an open or a short to ground. Refer to the following: Circuit Testing Wiring Repairs Did you find and correct the condition? | Go to Step 8 | Go to Step 2 |
| 2 | Turn OFF the ignition. Test the ground circuits of the module that is not communicating for an open. Refer to the following: Circuit Testing Wiring Repairs Did you find and correct the condition? | Go to Step 8 | Go to Step 3 |
| 3 | Test the class 2 serial data circuit of the module that is not communicating for an open or short to ground. Refer to Circuit Testing and Wiring Repairs. Did you find and correct the condition? | Go to Step 6 | Go to Step 4 |
| 4 | Inspect for poor connections at the battery positive voltage circuits, the ignition B+ circuits, the ground circuits, and the class 2 serial data circuit of the module that is not communicating. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs. Did you find and correct the condition? | Go to Step 6 | Go to Step 5 |
| 5 | Replace the module which is not communicating. Refer to Control Module References for replacement, setup and programming for the appropriate repair instructions. Did you complete the replacement? | Go to Step 8 | |
| 6 | Install a scan tool Turn ON the ignition leaving the engine OFF. Select the Display DTCs function for the module which was not communicating. Does the scan tool display any DTCs which do not begin with a "U"? | Go to Diagnostic System Check - Vehicle | Go to Step 7 |
| 7 | Use the scan tool in order to clear the DTCs. Did you complete the action? | Go to Step 8 | |
| 8 | Select the Display DTCs function for the modules which had the Lost Communication with xxx DTC set. Does the scan tool display any DTCs which do not begin with a "U"? | Go to Diagnostic System Check - Vehicle | Go to Step 9 |
| 9 | Use the scan tool in order to clear the DTCs. Continue diagnosing or clearing the DTCs until all the modules have been diagnosed and all the DTCs have been cleared. Did you complete the action? | System OK | |
The number below refers to the step number on the diagnostic table.
- 4: If the battery positive voltage and ground circuits of the DLC are functioning properly. The malfunction must be due to the scan tool.
| Step | Action | Yes | No |
|---|---|---|---|
| Schematic Reference: Data Communication Schematics Connector End View Reference: Component Connector End Views | |||
| 1 | Test the battery positive voltage circuit of the data link connector (DLC) for an open or a short to ground. Refer to the following: Circuit Testing Wiring Repairs Did you find and correct the condition? | Go to Diagnostic System Check - Vehicle | Go to Step 2 |
| 2 | Test the ground circuits of the DLC for an open or high resistance. Refer to the following: Circuit Testing Wiring Repairs Did you find and correct the condition? | Go to Diagnostic System Check - Vehicle | Go to Step 3 |
| 3 | Inspect for poor connections and terminal tension at the DLC. Refer to the following: Testing for Intermittent Conditions and Poor Connections Connector Repairs Did you find and correct the condition? | Go to Diagnostic System Check - Vehicle | Go to Step 4 |
| 4 | The scan tool may be malfunctioning. Refer to the scan tool user guide. Did you obtain a properly operating scan tool? | Go to Diagnostic System Check - Vehicle | |
The numbers below refer to the step numbers on the diagnostic table.
- 3: A partial malfunction in the class 2 serial data circuit uses a different procedure from a total malfunction of the class 2 serial data circuit. The following modules communicate on the class 2 serial data circuit. Communication interface module (OnStar®) Electronic brake control module (EBCM) Inflatable restraint sensing and diagnostic module (SDM) Instrument panel cluster (IPC) Radio
- 5: DLC terminals 2 and 5 provide the connection to the class 2 serial data circuit and the signal ground circuit respectively.
- 7: A poor connection at splice pack S232 will cause this condition but will not set a DTC.
- 8: An open in the class 2 serial data circuit between the DLC and splice pack S232 will prevent the scan tool from communicating with any module on the class 2 network. This condition will not set a DTC.
| Step | Action | Yes | No |
|---|---|---|---|
| Schematic Reference: Data Communication Schematics Connector End View Reference: Component Connector End Views | |||
| 1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle |
| 2 | Turn ON the ignition, with the engine OFF. Perform the Vehicle DTC Information function on the scan tool under Computer/Integrating Systems menu. Are any DTC's retrieved from any module? | Go to Diagnostic Trouble Code (DTC) List - Vehicle | Go to Step 3 |
| 3 | Did the scan tool communicate with any module on the class 2 serial data circuit? | Go to Step 4 | Go to Step 5 |
| 4 | Diagnose the non-communicating module by referring to the appropriate section. Did you complete the action? | Go to Diagnostic System Check - Vehicle | |
| 5 | Turn OFF the ignition. Disconnect the scan tool from the data link connector (DLC). Inspect for poor connections at the DLC terminals 2 and 5. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs. Did you find and correct the condition? | Go to Diagnostic System Check - Vehicle | Go to Step 6 |
| 6 | Test the DLC ground circuit at terminal 5 for an open. Refer to Circuit Testing and Wiring Repairs. Did you find and correct the condition? | Go to Diagnostic System Check - Vehicle | Go to Step 7 |
| 7 | Disconnect the splice pack S232. Inspect for poor connections at the splice pack SP205. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs. Did you find and correct the condition? | Go to Diagnostic System Check - Vehicle | Go to Step 8 |
| 8 | Repair the class 2 serial data circuit between the DLC and the splice pack S232 for an open. Refer to Circuit Testing and Wiring Repairs. Did you complete the action? | System OK | |
Data Link Communications Description and Operation
The scan tool accesses the following data lines
- The brake pressure control valve (BPMV)-On vehicles with antilock brake systems.
- The powertrain control module (PCM)
- The inflatable restraint sensing and diagnostic control module (SDM)
- The radio.
The BLU/RED wire at cavity 7 of the data link connector (DLC) provides access to the BPMV, SDM, and to the PCM and the radio. The RED/BLK wire at cavity 16 provides voltage to the scan tool. The WHT/BLK wire at cavity 4 and the BRN/RED wire at cavity 5 provides ground to the scan tool.
The data lines monitor the module for diagnostic purposes and for diagnostic trouble codes (DTCs).
See also:
• Master Electrical Component List