Class 2 Circuit Description

The data link connector (DLC) allows a scan tool to communicate with the class 2 serial data circuit. Class 2 serial data is transmitted on a single wire at an average of 10.4 Kbps. The bus is active at 7 volts nominal and inactive at ground potential. When the ignition switch is in RUN, each module communicating on the class 2 serial data line sends a state of health (SOH) message every 2 seconds to ensure that the module is operating properly. When a module stops communicating on the class 2 serial data line, for example if the module loses power or ground, the SOH message it normally sends on the data line every 2 seconds disappears. Other modules on the class 2 serial data line, which expect to receive that SOH message, detect its absence; those modules in turn set an internal DTC associated with the loss of SOH of the non-communicating module. The DTC is unique to the module which is not communicating, for example, when the body control module (BCM) SOH message disappears, several modules set DTC U1064. Note that a loss of serial data DTC does not normally represent a failure of the module that set it.

The BCM emulates the GMLAN modules on the class 2, embedding the original transmitter ID in GMLAN messages to class 2. Therefore the class 2 modules can set specific non communications DTCs against GMLAN modules.

The class 2 serial data communications circuit on this vehicle is in a star configuration. The star has 2 splice packs, located as follows:

• The splice pack SP205 is located under the left side of the instrument panel, near the data link connector.
• The splice pack SP208 is located under the right side of the instrument panel.

The following modules communicate on the class 2 serial data circuit:

• The splice pack SP205 attaches to:
  • The driver door module (DDM)
  • The driver door switch (DDS)
  • The driver position module (DPM)
  • The head up display (HUD)
  • The instrument panel cluster (IPC)
  • The inflatable restraint sensing and diagnostic module (SDM)
  • The steering column lock control module (SCLCM)
• The splice pack SP208 attaches to:
  • The body control module (BCM)
  • The communication interface module (OnStar®)
  • The digital radio receiver (DRR)
  • The folding top controller (FTC)
  • The HVAC control module
  • The passenger door module (PDM)
  • The radio
  • The remote control lock door receiver (RCDLR)