Description & Operation

The Body Control Module (BCM) performs the following functions: battery guard, door lock and unlock (including keyless entry), exterior lighting control, delayed interior lighting control, various indicator light controls in instrument cluster and headlight switch, retained accessory power, theft deterrent, exterior and interior lighting control, trunk release, fuel door release, and class 2 diagnostic protocol.

Signals that activate the BCM are known as "wake-up" signals. BCM wake-up signals include any ignition state transition, door switch activated or trunk opened with ignition switch in LOCK position, or any class 2 communication requiring BCM interaction. BCM also has a low power current sense feedback, which will wake-up BCM if current feedback sense is over 2.4 amps.

After BCM is awakened, it will enter a high power mode. If no further activity occurs within 15 minutes, it will enter a low power "asleep" mode. After 3 days and 15 minutes, BCM will convert to a "no controlled power" mode. Both low and high power will be deactivated until a wake-up signal is received.

Battery guard feature provides protection from excessive battery drain caused by high current loads that are accidentally left on. BCM controls battery power regulation by using a low power driver and a high power control of battery guard relay to the following components: radio receiver, Radio Interface Module (RIM), fuel door relay coil, trunk release relay coil, and phone (dealer installed).

Retained Accessory Power (RAP) feature allows use of the entertainment and phone systems after ignition is turned off. If ignition switch is still in LOCK position after 10 minutes, RAP feature is disabled. If any door is opened or closed during the 10-minute period, RAP feature will be disabled. RAP feature will also disable if ignition switch is turned to START position.

Catera incorporates 4 types of protocol: Class 2, controlled area network (CAN), Keyword 82 and Keyword 2000. See Fig 1 .

1. Class 2 Device
   
   Class 2 serial data is transferred at a rate of 10,400 bits per second and voltage is toggled between 0 and 7 volts to represent communication activity on the device (0 representing no activity and 7 representing activity). Messages on the class 2 device are pulse width modulated and prioritized. This allows better utilization of data line and ensures messages of higher priority override messages of lesser importance. Control modules on the class 2 device are:
   • Body Control Module (BCM).
   • Cellular Telephone.
   • Radio Receiver.
   • Remote Control Door Lock Receiver (1st Design).
   • Sound Processor Interface.

1. Controlled Area Network (CAN)
   
   Serial data on the CAN is transferred at a rate of 125 kilobytes-1 megabyte per second to ensure "real-time" response is maintained. Each message sent on this data line has a unique identifier to classify the content and a priority assignment to denote importance. With this embedded information, only those modules on the CAN that have access to the message can receive the message. When more than one module on the network trys to transmit simultaneously, the message with highest priority gets access first. Control modules on the CAN are:

• Electronic Brake/Traction Control Module (EBTCM).
• Engine Control Module (ECM).
• Transmission Control Module (TCM).

1. Keyword 82 Device
   
   This data line is a single wire, bi-directional communication link. The message structure on this line is a request and response format. On this line, a voltage value less than 20 percent of battery voltage is considered a logic "0" and a voltage value greater than 80 percent of battery voltage is considered a logic "1". Control modules on the Keyword 82 device are:
   • Automatic Level Control (ALC) Sensor.
   • Memory Seat Module.
   • Instrument Cluster.
   • Heater & A/C Control.
   • Supplemental Inflatable Restraint (SIR) Module.
   • Power Steering Control Module.
   • Sun Roof Actuator.
   • Theft Deterrent Module (Immobilizer System).

1. Keyword 2000 Device
   
   This data line is a single wire, bi-directional communication link. The message structure on this line is a request and response format. On this line, a voltage value less than 20 percent of battery voltage is considered a logic "0" and a voltage value greater than 80 percent of battery voltage is considered a logic "1". Control modules on the Keyword 2000 device are:
   • Engine Control Module (ECM).
   • Transmission Control Module (TCM).
   • Electronic Brake/Traction Control Module (EBTCM).

BCM acts as an interface between the optional OnStar(R) vehicle communication system and the vehicle architecture. The Radio Interface Module (RIM) will send the following class 2 messages to the BCM: door unlock or lock activation, horn control, interior light control and exterior light control (will override daytime running light, twilight sentinel or wiper-on lighting modes). BCM is hard-wired to air bag restraint sensing module. If air bag is deployed, BCM will send a class 2 message to RIM to indicate a restraint system was activated.