Onstar

OnStar is a new on-vehicle communications system which is a dealer installed option. The BCM acts as the interface between the OnStar and the vehicle architecture. It receives the following Class 2 messages from the Vehicle Interface Unit (VIU).

^ Door unlock or lock activation
^ Horn control
^ Interior light control
^ Exterior lighting; activates the parklamps and lowbeams. In the event the exterior lamps are active due to either DRL, twilight sentinel or wiper-on lighting, the message sent from the VIU will override these modes, turning the parklamps or lowbeams On or Off as required.
^ Requests the Alarm State of the BCM

The BCM sends Class 2 messages to the VIU on:
^ An ignition state change
^ When a door is opened or closed

The BCM is hard wired to the Inflatable Restraint Sensing Module. When you switch the ignition to the ON position, the restraint module sends voltage pulses as a test message and verifies the circuit to the BCM. In the event of a restraint system activation (in this case an airbag deployment), the BCM will continue to receive the voltage pulses but at a greater number and repetitive rate. The BCM then sends a Class 2 message to the VIU that a restraint system has activated.