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 hard wired 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 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  - 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 switch is turned to the OFF position, at which time the interior lighting returns to normal operation and control. These Enhanced Accident Response System (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 headlamp switch position to activate or deactivate the exterior lighting. The headlamp 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 headlamp beams were last selected with the multi-function switch, and energizes those beams by default the next time the headlamps are turned ON. If the vehicle is equipped with optional automatic headlamps 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 hard wired input from the rain sensor to turn the exterior lighting ON and OFF automatically while the ignition switch is in the ON position. The BCM also controls the fog lamps, cargo lamp, CHMSL, tail/stop lamps, reverse lamps and turn signals.
• 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 modules 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 park 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.
• Headlamp Washer Control  - On vehicles equipped with headlamp washers, the BCM controls a relay in the Power distribution Center (PDC) based on electronic messages received over the Can data bus.
• Ignition On and Ignition Accessory/On Relay Control  - The BCM monitors electronic ignition switch status  messages received over the CAN bus from the RF hub and a hard wired input from the ignition switch on the instrument panel, and provides high side driver 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 liftgate ajar, and liftgate flip-up glass ajar switches (where applicable), 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.
• Local Interface Network Master Module  - The BCM is the master module for the LIN data bus. In this role it gathers information from the compass sensor, and the Intelligent Battery Sensor (IBS), then either acts on that information directly or places electronic messages on the CAN data bus for use by other modules.
• Power Inverter Support  - The BCM monitors a hard wired input from the power inverter to determine the inverter status, then transmits electronic inverter status  messages to other electronic modules in the vehicle over the CAN data bus.
• 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 CAN 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  - The new FCA Telematics Platform (FCATP) vehicles no longer have an 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.
• Steering Wheel Switch Support  - The Steering Column Control Module (SCCM) microcontroller receives electronic message inputs from the steering wheel switches on the steering wheel over the CAN data bus to control and configure many of the EVIC displays and functions.
• Vehicle Theft Security System Control  - The BCM monitors inputs from the door ajar switches, and 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 stores and compares vehicle configuration data with other Electronic Control Units (ECU) in the vehicle. This process is referred to as Programming Of Configuration of Systems Integrated (PROCSI) (also known as PROXI). If a configuration mismatch is detected, the BCM sets a Diagnostic Trouble Code (DTC). A configuration mismatch will require the performance of a Restore BCM PROXI Configuration routine, or a PROXI Configuration Alignment routine using a diagnostic scan tool.

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.

The hard wired inputs and outputs of the BCM may be diagnosed using conventional diagnostic tools and procedures. Refer to the appropriate wiring information. However, conventional diagnostic methods will not prove conclusive in the diagnosis of the BCM electronic controls or the communication between modules and other devices that provide some features of the BCM-controlled systems. The most reliable, efficient and accurate means to diagnose the BCM or the electronic controls and communication related to BCM-controlled systems operation requires the use of a diagnostic scan tool. Refer to the appropriate diagnostic information.