The data link connector (DLC) allows a scan tool to communicate with the class 2 serial data line. The 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 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.

On some vehicles, if the powertrain control module (PCM) is unable to communicate with the vehicle theft deterrent (VTD) system after the vehicle has started, the PCM will consider the VTD system to be malfunctioning. The PCM will enter a fail enable state and will command the security indicator to illuminate. When the PCM is in a fail enable state, the vehicle will NOT stall or stop running. If the PCM is in a fail enable state when the ignition is switched OFF, the PCM will remain fail enable until communications with the VTD system has been restored. When the PCM is in a fail enable state, the VTD system is NOT active and the vehicle will start. This feature is NOT available on all GM vehicle lines.

The data link connector (DLC) is a standardized 16 cavity connector. Connector design and location is dictated by an industry wide standard, and is required to provide the following:

The class 2 serial data line is a ring/star configuration with discrete lines entering and leaving each module on the ring. This allows communications to the modules on the ring if one of the two discrete circuits is open.

The control modules on the ring portion of the class 2 serial data circuit can include the following modules:

The following modules are connected to the star portion of the class 2 serial data circuit by splices and have only 1 communication line connection to the class 2 serial data circuit. Therefore if the class 2 serial data circuit is open to any of these modules communication will not occur with that module:

The power door serial data circuit does not communicate with the scan tool via the DLC. The power door serial data information is interpreted by the driver door module (DDM) and transmitted on the Class 2 data link by the DDM.

The door system serial data circuit is used for serial data communication between the four door modules as well as the driver door switch assembly (DDSA).

Refer to Diagnostic System Check - Door Systems in Doors for diagnosis of the power door serial data circuit.