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 7.0 volts nominal and inactive at ground potential. 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 class 2 serial data line on this vehicle is a star configuration. The powertrain control module (PCM) has an additional class 2 serial data circuit to BCM or to theft deterrent control module, if equipped. If one of the class 2 serial data circuits to the PCM opens, communication will not be interrupted. The following modules communicate on the class 2 serial data line:

• The BCM
• The communication interface module (OnStar®), w/UE1
• The digital radio receiver (DRR), w/U2K
• The driver door module (DDM)
• The driver seat module (DSM), w/AAB
• The DVD player
• The electronic brake control module (EBCM)
• The engine control module (ECM), w/5.3L
• The HVAC control module
• The HVAC control module - rear auxiliary
• The inflatable restraint sensing and diagnostic module (SDM)
• The instrument panel cluster (IPC)
• The liftgate control module (LGM)
• The passenger door module (PDM)
• The powertrain control module (PCM), w/4.2L
• The radio
• The transfer case shift control module (TCSCM), w/4WD
• The theft deterrent control module (VTD), w/BAE

The class 2 serial data line allows a scan tool to communicate with these modules for testing purposes, checking for DTCs, and to activate/enable/disable functions. These class 2 serial data circuits are bused together via 2 splice packs:

• SP205-Located behind the instrument panel (I/P) near the headlamp switch connector
• SP306-Located in the body harness near the rear of the right rear seat under the carpet