Modules connected to the 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 programmed information about what messages are needed to be exchanged on the serial data circuits, for each virtual network. The messages are also supervised and some periodic messages are used by the receiver module as an availability indication of the transmitter module.

A supervised periodic message that includes the transmitter module availability has not been received

The module uses a default value for the missing parameter.

Schematic Reference

Connector End View Reference

Data Communication Connector End Views

Description and Operation

Data Link Communications Description and Operation

Electrical Information Reference

Scan Tool Reference

1. With a scan tool, verify communication with 1 or more modules is present.

⇒	If no communication is available refer to Scan Tool Does Not Communicate with Low Speed GMLAN Device.

2. Ignition OFF, disconnect the X4 harness connectors at the BCM.
3. Ignition OFF, test for less than 1 ohms of resistance between the ground circuit terminal E 12 and ground.

⇒	If greater than the specified value, test the circuit for an open/high resistance.

4. Ignition OFF, disconnect the X1 harness connectors at the BCM.
5. Ignition OFF, test for less than 1 ohms of resistance between the ground circuit terminal 61 and ground.

⇒	If greater than the specified value, test the circuit for an open/high resistance.

6. Reconnect the X1 and X4 harness connector and disconnect the X3 harness connector at the BCM.
7. Ignition ON, verify a test lamp illuminates between the B+ between the B+ circuit terminal D1 and ground.

⇒	If test lamp does not Illuminate, test the circuit for a an open/high resistance. If the circuit fuse is open, test the circuit for a short to ground, if all circuits test normal, replace the BCM.

8. Ignition OFF, disconnect the harness connector of the non communicating module.
9. Ignition OFF, test for less than 1 ohms of resistance between the applicable modules ground circuit terminal and ground

⇒	If greater than the specified value, test the circuit for an open/high resistance.

10. Ignition ON, verify a test lamp illuminates between the applicable modules B+ circuit and ground.

⇒	If the test lamp does not illuminate, test the circuit for a short to ground or an open/high resistance.

11. Individually disconnect the harness connectors at each control module on the GMLAN serial data line, beginning with the module furthest from the DLC as noted on the system schematic. After each disconnection, verify the scan tool is unable to communicate with the BCM.

⇒	If the scan tool is able to communicate with the BCM, test the GMLAN serial data line at the most recently disconnected module.

12. If all circuits test normal, replace the applicable module.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.