Circuit Description

Modules connected to the GMLAN High Speed 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 messages exchanged on the serial data circuits. The supervised and periodic messages are used by the receiving module as an availability indicator from the transmitter module. Each message contains the identification number of the transmitting module. When a message indicates availability of the transmitting module is not received, the receiving module sets a DTC Uxxxx. xxxx is equal to the 4-digit transmitter module identification number.

The control module ID number list below provides a method for determining which module is not communicating. A module with a GMLAN High Speed serial data circuits malfunction, or which loses power during the current ignition cycle, will set a Loss of Communication DTC by other modules requiring information from the failed module. The modules able to communicate will set a DTC Uxxxx indicating the module not communicating.

If any of the GMLAN High Speed circuits are shorted to battery voltage, ground or between them, no communication is possible on GMLAN High Speed serial data bus, and a DTC U0073 sets.