Class 2 Serial Data Line

Class 2 serial data is transmitted on a single wire at an average of 10.4 kbps. This value is an average, class 2 uses a variable pulse width modulation to carry data and depending on the message it may operate faster or slower. The bus will float at a nominal 7 volts during normal operation. Each module can pull this lower during the transmission. The bus is not at battery positive voltage or ground potential during normal operation. 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 inflatable restraint sensing and diagnostic module (SDM) SOH message disappears, several modules set DTC U1088. 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 body control module (BCM). 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 audio amplifier (AMP), w/UQ7 and Y91
• The body control module (BCM)
• The communication interface module (OnStar®), w/UE1
• The digital radio receiver (DRR), w/U2K
• The driver door module (DDM)
• The electronic brake control module (EBCM)
• The generator battery control module (GBCM)
• The HVAC control module
• The hybrid control module (HCM), w/HP2
• The inflatable restraint sensing and diagnostic module (SDM)
• The instrument panel cluster (IPC)
• The memory seat module (MSM), w/AN3
• The passenger door module (PDM)
• The powertrain/engine control module (PCM/ECM)
• The radio
• The rear seat audio (RSA) controller, w/UK6
• The rear seat entertainment (RSE) assembly w/U42
• The rear wheel steering control module (RWSCM), w/NYS
• The remote playback device - CD player (CDX), w/U1S
• The transfer case shift control module (TCSCM), w/NP1/NP8
• The transmission control module (TCM), w/M74

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 bussed together via 2 splice packs:

• SP205-Located behind the instrument panel (I/P) near the headlamp switch connector.
• SP207-Located in the I/P harness.

Refer to Computer/Integrating Systems Component Views .