Module, Body Control (BCM): Operation: Notes

WARNING: This page is about a different car, the 2017 Dodge Charger. However, it is still accessible from the selected car via links, so may be relevant.

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 (HSD), Low Side Drivers (LSD), 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 Cluster (IC) to illuminate the Low Fluid indicator based on the input signal received from the fluid level sensor.
Brake Lamps / CHMSL - The BCM will activate the output controls for the brake lamps / CHMSL upon receiving the brake signal status from the ESC module.
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 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
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- The headlamp beam select functionality is activated through the operation of the panel mounted headlamp switch for low beams, park lamps, fog lamps, and the left stalk switch on the SCCM for high beams. The panel mounted headlamp switch signal is an R-Mux'd signal to the BCM. Upon discerning the appropriate R-Mux signal request, the BCM will execute to turn on the headlamps, park lamps or the fog lamps by activating the corresponding HSDs or LSDs accordingly.
- The high beam function is a request received by the BCM from the SCCM. Once request is received, the BCM will activate/deactivate the corresponding HSDs accordingly. The BCM shall communicate the status of the switches through the CAN bus so other systems or modules can react accordingly.
- The left or right signaling direction shall be activated using the left steering column stalk switch input to the SCCM. The left and right stalk commands are read by the SCCM and sent to the BCM over CAN-C. Upon receiving the signal from the SCCM, and evaluating vehicle conditions, the BCM shall activate the turn light HSD in the BCM for the selected side, and provide CAN signal status. The BCM shall cycle the driver on/off to a frequency of 1.5 Hz +/- 5% and duty cycle of 50% +/- 5%. As soon as the SCCM stalk is returned to the neutral position, the HSD and CAN signal shall be disabled immediately.
- The BCM shall diagnose each of the front and rear turn lamp by directly monitoring each HSD output to each lamp, or by receiving an individual diagnostic feedback line from each the lamp. The BCM shall report the state of health of each turn lamp onto the CAN C bus so that the IC can receive and react accordingly and provide the customer with a visual and audible fault alert.
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 IC calculates the proper fuel gauge needle position and to control low fuel indicator operation based on these messages.
Hazard Lamp Circuit Control - The hazard switch is a hardwired switch input read by the BCM. When activated, the BCM sets the "Active" internal signal in order to actuate the hazard flasher lamps. The turn lamp HSDs are activated ON/OFF repeatedly by the BCM, and the BCM will also send the corresponding defined signals for hazard onto the CAN buses so that any respective modules can react accordingly to provide feedback to the customer or to activate other exterior lamps on the vehicle.
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 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 decklid ajar, 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, the Intelligent Battery Sensor (IBS), Humidity Sensor, Rear View Camera, the EVIC switches (Steering Column switches) and the Rain Sensor (RLS), then either acts on that information directly or places electronic messages on the CAN data bus for use by other modules.
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 - The telematics platform no longer has 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.
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 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 Instrument Cluster (IC) 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 IC 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.