Body Control Module: Notes

A Body Control Module (BCM) is concealed below the driver side end of the instrument panel. The BCM utilizes 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. The BCM is serviced only as a complete unit. Many of the electronic features in the vehicle controlled or supported by the BCM are programmable using either the Electronic Vehicle Information Center (EVIC) user interface, or the DRBIII(R) scan tool. In addition, the BCM software is Flash compatible, which means it can be reprogrammed using Flash reprogramming procedures.

The body system consists of a combination of modules that communicate over the PCI bus (Programmable Communication Interface multiplex system). Through the PCI bus, information about the operation of vehicle components and circuits is relayed quickly to the appropriate module(s). All modules receive all the information transmitted on the bus even though a module may not require all information to perform it's function. It will only respond to messages "addressed" to it through a binary coding process. This method of data transmission significantly reduces the complexity of the wiring in the vehicle and the size of wiring harnesses. All of the information about the functioning of all the systems is organized, controlled, and communicated by the PCI bus. See COMMUNICATION  .

The vehicle systems that are part of the BODY system are: Airbag, Audio, Chime, Communication, Electrically Heated System, Exterior Lighting, Heating and A/C, Instrument Cluster, Interior Lighting, Memory Seat, Overhead Console, Power Door Lock/RKE, Power Mirrors, Power Sunroof, Power Windows, Tire Pressure Monitoring (TPM), Vehicle Theft Security System (VTSS) and Windshield Wiper and Washer. See Fig 1 .

Fig 1: Locating Body Control Module Components

Courtesy of DAIMLERCHRYSLER CORPORATION

The Body Control Module (BCM) supplies vehicle occupants with visual and audible information and controls various vehicle functions. To provide and receive information, the module is interfaced to the vehicle's serial bus communication network (Programmable Communication Interface or PCI bus). This network consists of the Powertrain Control Module (PCM) or Engine Control Module (ECM) (diesel), the Memory/Heated Seat Module (MHSM, MSM or HSM), the Sentry Key Immobilizer Module (SKIM), the Transmission Control Module (TCM), the Electro/Mechanical Instrument Cluster (MIC), the Driver Door Module (DDM), the Passenger Door Module (PDM), the Airbag Control Module (ACM), the Controller Antilock Brake (CAB), the optional Electronic Vehicle Information Center (EVIC), the optional PCI radio, the optional CD changer, the optional Automatic Zone Control (AZC) module, the shifter assembly (diesel only), the optional power amplifier, the optional adjustable pedal module, the optional rain sensor, and the optional intrusion transceiver module (export only). The BCM is operational when battery power is supplied to the module. In addition, ignition switch power is needed for ignition switched functions. The BCM provides the following features:

1. Full control of all exterior and interior lamps, including these features:
   • A Battery Saver automatically turns off all exterior and interior lamps within 8 minutes after the ignition is turned off, if they are not turned off by the driver. This includes the front and rear reading/courtesy lamps and the cargo area dome lamp, even if they are turned on manually.
   • Theater-style interior lighting dims gradually when the doors are closed.
   • Interior courtesy lamp operation may be "defeated" using the dimmer control ring on the left multi-function switch.
   • Instrument panel light intensity may be maximized for daytime visibility with the head lamps or parking lights by using the dimmer control ring on the left multi-function switch.
2. An Accessory/Sunroof Delay allows continued operation of the sunroof and the power windows after ignition is OFF.
3. These accessories can be operated up to 45 seconds after ignition is turned OFF or until door is opened.
4. Beep tones for warnings and programming actions associated with the Sentry Key Immobilizer system and the EVIC.

The BCM also provides the following features:

• Intermittent wipe and low speed wiper control.
• Wiper system status.
• Ignition on/off timer.
• VF displays synchronization.
• Rear door and liftgate ajar status.
• Acquisition and transmission of external temperature.
• A/C select switch status.
• Chime.
• Courtesy lamps with fade-to-off feature.
• Reading lights/glove box lamp load shedding control.
• Exterior lighting control including automatic headlamps, fog lamp control, rear fog lamp control and parking lamps.
• Exterior lamp status.
• Headlamp time delay upon ignition off and headlamps switch off.
• Parade mode.
• Illuminated entry.
• Instrument panel dimming.
• Vehicle theft security system with panic, horn and hazard lamp activation.
• Key-in ignition switch interface.
• Brake fluid level switch/park brake switch interface.
• Seatbelt reminder.
• Speed sensitive intermittent wipe control.
• Remote radio switch interface to radio.
• Vehicle ignition status.
• BCM self diagnostics.
• Liftgate courtesy lamp disable.
• Rear window defogger control.
• Accessory delay control (windows and sunroof).
• Fuel economy and Distance To Empty (DTE) calculations.
• Heated seat switch status.
• Exterior lamp load shed.
• Support of EVIC programmable features.

The BCM receives information over the PCI bus from the PCM in order to support certain features. The required information is engine RPM, engine and battery temperature, injector on time and distance pulses, vehicle speed, engine model, fuel tank level and vehicle identification number. The BCM provides the PCM with information on the A/C switch status. There are several programmable features which can be enabled/disabled by using the EVIC or the DRBIII(R).

The BCM is designed to control and integrate many of the electronic features and functions of the vehicle. The microprocessor-based BCM hardware and software 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 PCI data bus network. The internal programming of the BCM microprocessor allows the BCM to determine the tasks it needs to perform and their priorities. The BCM programming then performs those tasks and provides features through both PCI data bus communication with other electronic modules and hard-wired outputs to a number of relays. These relays provide the BCM with the ability to control numerous high current accessory systems in the vehicle. The BCM circuitry operates on battery current received through fuses in the junction block on a non-switched fused B (+) circuit, a fused ignition switch output (start-run) circuit, and a fused ignition switch output (run-accessory) circuit. This arrangement allows the BCM to provide some features regardless of the ignition switch position. The BCM circuitry is grounded through the chassis beneath the center console. 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 DRBIII(R) scan tool. See SELF-DIAGNOSTIC SYSTEM  .