Module, Body Control (BCM): Operation: Notes

A Body Control Module (BCM) is an electronic control unit with a microcontroller that controls and integrates many of the main body electronic functions and features of the vehicle. Many of the functions and features provided by the BCM are possible because of numerous hardwired inputs and outputs, but most of these features are only possible or are enhanced because the BCM communicates electronically with other electronic modules in the vehicle as well as with a diagnostic scan tool using the Controller Area Network (CAN) data bus.

The BCM is a gateway between the high and low speed CAN data bus networks as well as a Local Interface Network (LIN) master node. This method of communication allows the sharing of sensor information, which reduces wire harness complexity, internal controller hardware and component sensor current loads. At the same time, this system provides increased reliability, enhanced diagnostics and allows the addition of many new feature capabilities. This method of communication is used by the BCM to acquire vehicle configuration data, including customer programmable features.

Software programming allows the BCM microcontroller to monitor all of these inputs and provide the appropriate outputs through high side drivers, low side drivers, on-board relays, Pulse-Width Modulation (PWM) and electronic messages to other modules in the vehicle. Some of the functions and features that the BCM supports or controls include:

• Brake Fluid Level  - The BCM continuously monitors the brake fluid level sensor through a hard-wired input to monitor the brake fluid level. The BCM transmits an electronic message over the CAN data bus to the Instrument Panel Cluster (IPC) to illuminate the Low Fluid indicator based on the input signal received from the fluid level sensor.
• Enhanced Accident Response Support (EARS)  - The BCM monitors an input from the Occupant Restraint Controller (ORC) and, following an airbag deployment, will immediately disable the power lock output, unlock all doors by activating the power unlock output, then enables the power lock output if the power lock switch input remains inactive for two seconds. The BCM also monitors an input from the Powertrain Control Module (PCM) to automatically turn ON the interior lighting after an airbag deployment event 10 seconds after the vehicle speed is zero. The interior lighting remains illuminated until the ignition is turned to the OFF position, at which time the interior lighting returns to normal operation and control. These EARS features are each dependent upon a functional vehicle electrical system following the vehicle impact event.
• Exterior Lighting Switch Support  - The BCM continuously monitors the headlight switch position to activate or deactivate the exterior lighting. The headlight switch provides the appropriate resistor multiplexed output hardwired to the BCM. The BCM reads and responds to this input by energizing or de-energizing the right and left park lamp feed circuits and the right and left high or low beam driver circuits through internal High Side Drivers (HSD) and by sending an electronic confirmation message back to the IPC, which controls the high beam indicator as appropriate. The BCM also remembers which headlight beams were last selected with the multi-function switch, and energizes those beams by default the next time the headlights are turned ON. If the vehicle is equipped with optional automatic headlights and the A  (Automatic) position is selected, the BCM also monitors an electronic ambient light level  message received over the CAN data bus from the Heat, Ventilation and Air Conditioning (HVAC) control module based upon a hardwired input from the rain sensor to turn the exterior lighting ON and OFF automatically while the ignition is in the ON position. The BCM also controls the fog lamps, Center High Mounted Stop Lamp (CHMSL), front combination lamps, and the rear combination lamps.
• Fuel Level Data Support  - The BCM provides a current source for and receives a hard-wired analog input from the fuel level sending unit located on the fuel pump module in the fuel tank. Based upon this input, the BCM uses electronic messaging to transmit this data over the CAN data bus for use by other Electronic Control Units (ECUs) in the vehicle. The IPC calculates the proper fuel gauge needle position and to control low fuel indicator operation based on these messages.
• Hazard Lamp Circuit Control  - The BCM monitors an input from the hazard switch and receives a hard-wired analog input from the switch. The BCM reads and responds to this input by energizing or de-energizing the right and left turn lamp feed circuits through internal HSD units and by sending an electronic confirmation message to the IPC over the CAN data bus, which controls the hazard light indicators as appropriate.
• Ignition ON and Ignition ACCESSORY/ON Relay Control  - The BCM monitors electronic ignition switch status  messages received over the CAN bus from the Radio Frequency Hub (RF-Hub) and provides HSD outputs to control both the ignition ON and ignition ACCESSORY/ON relays in the Power Distribution Center (PDC) as appropriate.
• Interior Lamp Load Shedding  - The BCM provides a battery saver feature which will automatically turn OFF all interior lamps if they remain ON after a timed interval of about eight minutes.
• Interior Lighting Control  - The BCM monitors electronic messages and hard-wired inputs from the interior lighting switch, the door ajar switches, the decklid ajar switch, the reading lamp switches and the RF-Hub to provide courtesy lamp control. This includes support for timed illuminated entry with theater-style fade-to-OFF and courtesy illumination DEFEAT features.
• LIN Master ECU  - The BCM is the master module for the LIN data bus. In this role it gathers information from the compass sensor, the Intelligent Battery Sensor (IBS), Humidity Sensor, Rear View Camera (RVC), the Electronic Vehicle Information Center (EVIC) switches and the Rain Sensor (RLS) and then either acts on that information directly or places electronic messages on the CAN data bus for use by other ECUs.
• Power Lock System Control  - The BCM monitors inputs from the power lock switches and the RF-Hub to provide control of the power lock motors through high side and low side driver outputs. This includes support for rolling door locks (also known as automatic door locks), automatic door unlock, and a door lock inhibit mode.
• Remote Radio Switch Support  - The BCM receives electronic message inputs from the remote radio switches on the steering wheel over the LIN data bus, then provides electronic radio request messages over the CAN data bus to support the remote radio switch function.
• Remote Start System Support  - The BCM receives electronic message inputs from the RF-Hub and then displays the appropriate remote start system textual reminder messages to the vehicle operator within the EVIC display.
• Shipping Mode  - This vehicle no longer has an Ignition Off Draw (IOD) fuse to use when transporting or storing for a long period of time. The BCM has a mode that takes the place of pulling the IOD fuse called "Shipping Mode"  that is easily enabled or disabled.
• Vehicle Theft Security System (VTSS) Control  - The BCM monitors inputs from the door ajar switches and from the RF-Hub, on vehicles so equipped. The Intrusion Module provides electronic horn  and lighting request  messages to the BCM for the appropriate VTSS alarm output features.
• Washer Fluid Level  - The BCM continuously monitors the washer fluid level sensor through a hard-wired input to monitor washer fluid level. the BCM transmits an electronic message over the CAN data bus to IPC to illuminate the Low Washer Fluid indicator based on the input signal received from the fluid level sensor.

The BCM uses On-Board Diagnostics (OBD) to monitor all of the systems and circuits it controls, then sets active and stored Diagnostic Trouble Codes (DTC) for any monitored system faults it detects. The BCM will also send electronic message requests to the IPC for the display of certain textual warning messages related to some detected system conditions or faults.