Body Control Module: Description
A Body Control Module (BCM) is concealed behind the driver side end of the instrument panel in the passenger compartment, where it is secured to the fuse panel side of the junction block (JB). The JB is the interface between the body, the instrument panel, and the headlamp and dash wire harnesses. The JB also contains the fuses and relays used for the interior electrical system of the vehicle. The BCM is enclosed in a molded plastic housing with two integral external connectors that connect it to the vehicle electrical system. The BCM also has an integral interface connector that joins it through a connector to the circuitry within the JB. This connector is referred to as the JB-BCM connector. The combined BCM and JB are sometimes referred to as the Junction Block Module (JBM).
There are two different versions of the BCM: base and premium. The base BCM is a subset of the components in the premium version. The base version BCM does not support the following features: electronic vehicle information center (EVIC), fog lamps (front and/or rear), remote keyless entry (RKE), remote radio switches, or vehicle theft security system (VTSS). Both versions of the BCM utilize integrated circuitry and information carried on the programmable communications interface (PCI) data bus network along with many hard wired inputs to monitor many sensor and switch inputs throughout the vehicle. In response to those inputs, the internal circuitry and programming of the BCM allow it to control and integrate many electronic functions and features of the vehicle through both hard wired outputs and the transmission of electronic message outputs to other electronic modules in the vehicle over the PCI data bus. Any time the BCM is replaced or reflashed for any reason, there may be subsystems that may need to be reprogrammed as well, such as Cabin Equalization. Follow the DRBIII(R) scan tool menu screen for details. The electronic functions and features that the BCM supports or controls include the following:
- A/C Select Switch Status - The BCM monitors an input from, and transmits the status of the A/C switch on the heater-A/C control.
- Audio System Cabin Equalization - Each time the BCM receives an electronic cabin equalization request message from the radio over the PCI data buss, it provides an electronic response to the radio containing the appropriate equalization curve information. Because there are numerous optional radios which are common to many platforms and available with various speaker architectures, each radio contains a Digital Signal Processing (DPS) microprocessor chip. This DPS chip uses the equalization curve information to optimize the radio's sound output for the unique cabin and speaker architecture found within the particular vehicle to which the radio has been installed.
- Cargo Lamp Disable - The BCM monitors an input from the cargo lamp switch to provide an interior lighting disable feature.
- Chimes - The chime tone generator is located on the ElectroMechanical Instrument Cluster (EMIC) circuit board, but the EMIC goes to sleep with the ignition switch in the Off position. The BCM provides a wake-up output to the EMIC based upon inputs from the key-in ignition switch or the exterior lighting switch, then sends electronic chime request messages to the EMIC for the headlamps-on warning and key-in ignition warning.
- Door Lock Inhibit - The BCM monitors the key-in ignition switch and the driver side front door ajar switch to provide a door lock inhibit feature.
- Exterior Lamp Load Shedding - The BCM provides a battery saver feature which will automatically turn off exterior lamps that remain on after a timed interval.
- Exterior Lamp Status - The BCM monitors the status of the park lamp, low beam, high beam or Daytime Running Lamp (DRL - Canada only), front fog lamp (optional), and rear fog lamp (in required markets only) relays.
- Exterior Lighting Control - The BCM provides exterior lamp control for standard head and park lamps, as well as Daytime Running Lamps (DRL - Canada only), front fog lamps (optional), and rear fog lamps (in required markets only). This includes support for features including optical horn (also known as flash-to-pass) and headlamp time delay.
- Flip-Up Glass Control - The BCM monitors the tailgate cylinder lock switch, the tailgate handle switch, the Remote Keyless Entry (RKE) module inputs and the rear wiper switch to provide control for the rear flip-up glass actuator.
- Fog Lamp Control - The premium BCM provides fog lamp control for front fog lamps (optional), and rear fog lamps (in required markets only).
- Front Wiper System Status - The BCM monitors the status of the front wiper motor park switch.
- Fuel Economy and Distance to Empty Calculations - The BCM calculates and transmits the fuel economy and Distance To Empty (DTE) data.
- Headlamp Time Delay - The BCM provides a headlamp time delay feature with the ignition switch in the Off position.
- Heated Rear Glass Control - The BCM provides control and timer functions for the heated rear glass feature and transmits the system status.
- Ignition On/Off Timer - The BCM monitors and transmits the elapsed ignition. On timer data and monitors the ignition Off time.
- Ignition Switch Position Status - The BCM monitors and transmits the status of the ignition switch.
- Instrument Panel Dimming - The BCM monitors and transmits the selected illumination intensity level of the panel lamps dimmer switch.
- Interior Lamp Load Shedding - The BCM provides a battery saver feature which will automatically turn off all interior lamps that remain on after a timed interval.
- Interior Lighting Control - The BCM monitors inputs from the interior lighting switch, the door ajar switches, the flip-up glass ajar switch, the tailgate ajar switch, the cargo lamp switch, the reading lamp switches, and the Remote Keyless Entry (RKE) module to provide courtesy lamp control. This includes support for timed illuminated entry with theater-style fade-to-off and courtesy illumination defeat features.
- Intermittent Wipe and Front Wiper System Control - The BCM monitors inputs from the front wiper and washer switch and the front wiper motor park switch to provide front wiper system control through the wiper on/off and high/low relays. This includes support for adjustable intermittent wipe, mist wipe (also known as pulse wipe), and wipe-afterwash features.
- Key-In-Ignition Switch Status - The BCM monitors and transmits the status of the key-in-ignition switch.
- Panic Mode - The BCM provides support for the Remote Keyless Entry (RKE) system panic mode feature.
- Parade Mode - The BCM provides a parade mode (also known as funeral mode) that allows the interior Vacuum Fluorescent Displays (VFD) to be illuminated at full intensity while driving in daylight with the exterior lamps On.
- Power Locks - The BCM monitors inputs from the power lock switches and the Remote Keyless Entry (RKE) module (optional) to provide control of the power lock motors through outputs to the lock, unlock, and driver unlock (RKE only) relays. This includes support for rolling door locks (also known as automatic door locks) and a door lock inhibit mode.
- Programmable Features - The BCM provides support for several standard and optional programmable features, including: rolling door locks, headlamp time delay interval, Remote Keyless Entry (RKE) driver-door-only or unlock-all-doors, RKE optical chirp, and RKE audible chirp.
- Remote Keyless Entry - The premium BCM provides the optional Remote Keyless Entry (RKE) system features, including support for the RKE Lock, Unlock (with optional driver-door-only unlock, and unlock-all-doors), rear flip-up glass control, Panic, audible chirp, optical chirp, and illuminated entry modes, as well as the ability to be programmed to recognize up to four RKE transmitters.
- Rolling Door Locks - The BCM provides support for the power lock system rolling door locks feature (also known as automatic door locks).
- Tailgate and Flip-Up Glass Ajar Status - The BCM monitors and transmits the status of the tailgate and rear flip-up glass ajar switches.
- Remote Radio Switch Interface - The premium BCM monitors and transmits the status of the optional remote radio switches.
- Self-Diagnostics - The BCM provides support for diagnostics through communication with the DRBIII(R) scan tool over the PCI data bus network. Each analog and digital input can be verified, and each output can be actuated through the use of this diagnostic protocol. The BCM also stores Diagnostic Trouble Codes (DTCs) to assist in troubleshooting this unit.
- Tire Size - The BCM calculates vehicle speed based upon a programmed Tire Revolutions per mile (TIRE REV/MILE) value. The correct tire size must be programmed to the BCM using the DRBIII(R). Using the DRBIII(R), select Body Computer, then Program Tire Size. The BCM must be programmed with one of the available tire sizes in the DRBIII(R) menu or by programming the correct REV/MILE value.
- Vacuum Fluorescent Display Synchronization - The BCM transmits panel lamp intensity data which allows modules with Vacuum Fluorescent Displays (VFD) to coordinate their illumination intensity.
- Vehicle Speed System - ABS Equipped - The CAB provides 12VDC hard wire speed sensor supply to the Left and Right Wheel speed sensors and the Rear Wheel Speed Sensor the speed sensor output hard wire square wave signals back to the CAB. The CAB outputs a hard wire square wave back to the BCM. The BCM calculates vehicle speed based upon a programmed Tire revolutions per mile (TIRE REV/MILE) value. This calculation is based upon tire circumference revolutions per mile. This calculation must be programmed into the BCM. New service BCM's are shipped in default mode that prevents speedometer indication until a valid tire size is programmed. The BCM outputs a hard wired square wave Vehicle Speed Output signal to the PCM. The PCM transmits vehicle speed to the instrument cluster via the PCI bus.
- Vehicle Speed System - NON-ABS Equipped - The BCM provides a 12VDC hard wire speed sensor supply to the Rear Wheel speed sensor. The speed sensor outputs a hard wire square wave signal back to the BCM. The BCM calculates vehicle speed based upon a programmed Tire Revolutions per mile (TIRE REV/MILE) value. This calculation is based tire circumference revolutions per mile. This calculation must be programmed into the BCM. New service BCM's are shipped in default mode that prevents speedometer indication until a valid tire size is programmed. The BCM outputs a hard wired square wave Vehicle Speed Output signal to the PCM. The PCM transmits vehicle speed to the instrument cluster via the PCI bus.
- Vehicle Theft Security System - The premium BCM monitors inputs from the door cylinder lock switches, the tailgate cylinder lock switch, the door ajar switches, the tailgate ajar switch, the flip-up glass ajar switch, the hood ajar switch (in required markets only), and the Remote Keyless Entry (RKE) module to control the features of the optional Vehicle Theft Security System (VTSS).
Hard wired circuitry connects the BCM to the electrical system of the vehicle. These hard wired circuits are integral to several wire harnesses, which are routed throughout the vehicle and retained by many different methods. These circuits may be connected to each other, to the vehicle electrical system and to the BCM through the use of a combination of soldered splices, splice block connectors, and many different types of wire harness terminal connectors and insulators. See BODY CONTROL MODULES in SYSTEM WIRING DIAGRAMS article. The wiring information includes wiring diagrams, proper wire and connector repair procedures, further details on wire harness routing and retention, as well as pin-out and location views for the various wire harness connectors, splices and grounds.
Many of the electronic features in the vehicle controlled or supported by the BCM are programmable using a customer programming procedure or the DRBIII(R) scan tool. In addition, the BCM software is Flash compatible, which means it can be reprogrammed using Flash reprogramming procedures. However, if any of the BCM hardware components is damaged or faulty, the entire BCM unit must be replaced.