Body Control Module: Description and Operation
BODY CONTROL MODULEDESCRIPTION
A Body Control Module (BCM) (4) is concealed behind the driver side end of the instrument panel (2) in the passenger compartment, where it is secured to the fuse panel side of the Junction Block (JB) (3) with four screws. 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 (1) is enclosed in a molded plastic housing with two integral external connectors (3) that connect it to the vehicle electrical system through two take outs with connectors from the instrument panel wire harness. The BCM also has an integral interface connector (2) that joins it through a connector receptacle that is integral to the JB housing 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).
The BCM utilizes integrated circuitry and information carried on the Programmable Communications Interface (PCI) data bus along with many hard wired inputs to monitor numerous 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.
Vehicles equipped with the optional Electronic Stability Program (ESP) have a hybrid bus network. The hybrid network includes some electronic nodes that communicate using a Controller Area Network (CAN-C) protocol, in addition to nodes communicating on the PCI data bus. The BCM in vehicles with a hybrid bus is also equipped to serve as the Central Gateway and may be referred to as the BCMCGW. The central gateway allows communication between nodes on the two bus networks by translating messages between them.
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 in the heater-A/C control.
- Audio System Cabin Equalization - The BCM stores the cabin equalization curves for numerous optional speaker architectures for use by the radio. The equalization curve information allows the radio to optimize sound output for the unique combination of cabin and speaker architecture found within the vehicle. The BCM provides this information when requested by the radio over the PCI data bus.
- Cargo Lamp Disable - The BCM monitors an input from the cargo lamp switch to provide an interior lighting disable feature.
- Chimes - The chime tone transducer 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.
- Electronic Pinion Factor - On vehicles without the optional Antilock Brake System (ABS) the BCM provides a source voltage to the rear wheel speed sensor and monitors a returned vehicle speed signal input. The BCM is able to use the vehicle speed signal input to accurately calculate vehicle speed and distance information by applying an electronic pinion factor. This factor is based upon either a pre-programmed tire size or a Tire Revolutions per Mile (TIRE REV/MILE) value that compensates for multiple optional axle ratios and tire diameters. The correct electronic pinion factor must be programmed into the BCM using a diagnostic scan tool in order for the vehicle speed and distance information to be accurate. The BCM then transmits the correct vehicle speed information over the PCI data bus for use by the EMIC for control of the speedometer and odometer.
- Enhanced Accident Response Support - The BCM monitors an input from the Airbag Control Module (ACM) and, five seconds after a front or side airbag deployment will unlock all doors by activating the power unlock output if the power lock switch input remains inactive for two seconds. The BCM also turns on the interior lighting after an airbag deployment event, five 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 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 inputs from the tailgate cylinder lock switch, the tailgate handle switch, the Remote Keyless Entry (RKE) system and the rear wiper switch to provide control for the rear flip-up glass actuator.
- Fog Lamp Control - The 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 (ECO) 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 RKE system 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-after-wash 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 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 RKE system to provide control of the power lock motors through outputs to the lock, unlock, and driver unlock 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, RKE driver-door-only or unlock-all-doors, RKE optical chirp, and RKE audible chirp.
- Remote Keyless Entry - The BCM provides support for the 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.
- 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 BCM monitors and transmits the status of the optional remote radio switches.
- Self-Diagnostics - The BCM provides support for diagnostics through communication with a diagnostic 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 the troubleshooting of this unit.
- 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 Theft Security System - The BCM monitors inputs from the door ajar switches, the tailgate ajar switch, the flip-up glass ajar switch, the hood ajar switch (in required markets only), and the RKE system 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. Refer to the appropriate wiring information. The wiring information includes wiring diagrams, proper wire and connector repair procedures, further details on wire harness routing and retention, as well as pinout 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 a diagnostic 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 is damaged or faulty, the entire BCM unit must be replaced.
OPERATION
The microprocessor-based Body Control Module (BCM) monitors many hard wired switch and sensor inputs as well as those resources it shares with other electronic modules in the vehicle through its communication over the Programmable Communications Interface (PCI) data bus or the hybrid bus network. The internal programming and all of these inputs allow the BCM microprocessor to determine the tasks it needs to perform and their priorities, as well as both the standard and optional features that it should provide. The BCM programming then performs those tasks and provides those features through both PCI data bus or hybrid bus communication with other electronic modules and through hard wired outputs through a number of driver circuits, relays, and actuators. These outputs allow the BCM the ability to control numerous accessory systems in the vehicle.
The BCM operates on battery and ignition voltage inputs received through several fuses in the Junction Block (JB) on a non-switched fused B(+) circuit, through another fuse in the JB on a fused ignition switch output (run-start) circuit, and through a third fuse in the JB on a fused ignition switch output (run-acc) circuit. This arrangement allows the BCM to provide some features regardless of the ignition switch position, while other features will operate only with the ignition switch in the On, Start, and/or Accessory positions. All of the battery voltage circuits are connected to the BCM through the JB/BCM connector.
The BCM receives ground through five separate circuits. Three of these circuits are connected to the BCM through a connector of the instrument panel wire harness on three separate ground circuits, while the other two circuits are connected to the BCM through the JB/BCM connector. Each of these circuits receives a path to ground through the instrument panel wire harness with an eyelet terminal connector that is secured by a nut to a ground stud on the driver side instrument panel end bracket near the JB.
The BCM monitors its own internal circuitry as well as many of its input and output circuits, and will store a Diagnostic Trouble Code (DTC) in electronic memory for any failure it detects. These DTCs can be retrieved and diagnosed using a diagnostic scan tool.
INPUT AND OUTPUT CIRCUITS
HARD WIRED INPUTS
The hard wired inputs to the BCM include the following:
- A/C on/off control
- BCM flash enable
- Door lock switch mux
- Driver door ajar switch sense
- Flip-up glass ajar switch sense
- Flip-up glass release switch sense
- Front fog lamp switch sense
- Front wiper park switch sense
- Front wiper switch mux
- Fused B(+)
- Fused ignition switch output (run-acc)
- Fused ignition switch output (run-start)
- Headlamp switch mux
- High beam switch sense
- Hood ajar switch sense - with VTSS - in markets where required only
- Key-in ignition switch sense
- Left rear door ajar switch sense
- Lightbar switch sense - Renegade with light bar only
- Panel lamps dimmer switch mux
- Passenger front door ajar switch sense
- Radio control mux - with remote radio switches only
- Rear courtesy lamp control
- Rear window defogger control
- Rear wiper intermittent driver
- Rear wiper on driver
- RHD input - connected to ground on right-hand drive models only
- Right rear door ajar switch sense
- Tailgate ajar switch sense
- Washer pump driver
- Vehicle speed signal
Refer to the appropriate wiring information for additional details.
HARD WIRED OUTPUTS
The hard wired outputs of the BCM include the following:
- Accessory delay relay control - power sunroof only
- Courtesy lamp driver
- Courtesy lamp load shed
- Door lock relay control
- Door unlock relay control
- Driver door unlock relay control
- Flip-up glass release motor driver
- Front fog lamp relay control
- Front wiper high/low relay control
- Front wiper on/off relay control
- Fused B(+) - lock, unlock and driver unlock relay feed
- Hazard lamp control
- High beam relay control
- Horn relay control
- Instrument cluster wake up signal
- Low beam relay control
- Park lamp relay control
- Rear fog lamp relay control - with rear fog lamps in markets where required only
- Rear window defogger relay control
- Tailgate lock driver
- Tailgate unlock driver
- Vehicle speed output
- Vehicle speed sensor supply - except with Antilock Brake System (ABS)
- VTSS indicator driver - with VTSS only
Refer to the appropriate wiring information for additional details.
GROUNDS
The BCM receives ground through five separate circuits, and also supplies a clean ground path to several switches through the following hard wired circuits:
- Door lock switch ground
- Multi-function switch ground
- Radio control mux return
- Tailgate switch ground
Refer to the appropriate wiring information for additional details.
COMMUNICATION
The BCM has the following communication circuits:
- PCI bus
- CAN-C High bus - with ESP only
- CAN-C Low bus - with ESP only
Refer to the appropriate wiring information for additional details.