Class 2 Serial Data Circuit

WARNING: This page is about a different car, the 2007 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.

The data link connector (DLC) allows a scan tool to communicate with the class 2 serial data line. This serial data line is the means by which the microprocessor-controlled modules in the vehicle communicate with each other. Once the scan tool is connected to the class 2 serial data line through the DLC, the scan tool can be used to monitor each module for diagnostic purposes and to check for diagnostic trouble codes (DTCs). 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 7.0 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 circuit on this vehicle consists of ring configuration. Each module on the ring has 2 class 2 serial data circuits connected to it. If there is one open on the ring serial data communication will still take place. If there is more than one open on the ring, the modules not connected to the class 2 serial data circuit will not communicate. The only exception from the ring configuration is the communication interface module (OnStar), with UE1, that is connected to splice S203. The following modules are connected to the class 2 serial data circuit in a ring fashion

Body control module (BCM)
Instrument panel cluster (IPC)
Power steering control module (PSCM)
The inflatable restraint sensing and diagnostic module (SDM)