Body Control Module
DESCRIPTION
The Body Control Module (BCM) (1) is an electronic control unit that manages the main body functions of the vehicle thus performing gateway functions between high and low speed Controller Area Networks (CAN) and between Local Interface Network (LIN) and CAN networks. Moreover, the BCM is an interconnecting unit that provides a protected power supply to many electrical loads by means of fuses. The BCM is the configuration master for the vehicle. As the configuration master, the BCM broadcasts configuration data over the bus whenever the ignition is in the RUN state and performs the following operating characteristics:
- Receives hardwired input from sensors and switches
- Provides logic and output control of certain electrical systems
- Provides load shedding logic and control
- Supports flashing (reprogramming) of itself
- Is located behind the left kick panel
Some advantages of the BCM include:
- CAN message optimizations
- Controls all exterior lighting outputs
- Intelligent Battery Sensor (IBS) connection on LIN to BCM
- Interior Lighting and Dimmable "Mood" Lighting
OPERATION
The BCM uses semiconductors or solid-state electronics, including transistors, to perform some of its required functions.
A transistor:
- Is similar to a relay
- Is used to turn an electrical circuit ON or OFF
- Is controlled by another circuit
- Has no moving parts
- Is an integral part of the Electronic Control Unit (ECU) circuit board
- Is often referred to as a "driver"
In the case of the BCM, the following drivers are used to distribute power to various electrical circuits:
- High Side Driver (HSD)
- Low Side Driver (LSD)
If a HSD or LSD is used, it will often be indicated on the wiring diagram by the word "ELECTRONICS" or "PCB" next to a dashed-box inside the ECU.
HSD
:
- Provides battery voltage (+) to power the load device requiring operation
- Can be used by an ECU to switch power (+) to an electrical circuit, which is typically grounded through its harness and associated circuitry
Some common examples of how the BCM uses a HSD include the:
- Horn
- Starter relay
- Ignition Run-Start relay
- Washer Pump
LSD
:
- Provides a ground (-) for the load device requiring operation
- Can be used by an ECU to switch ground (-) to an electrical circuit, which is typically powered through its harness and associated circuitry
BCM FUNCTION
The BCM will gate messages among all modules on the following bus networks:
- CAN-Chassis (CAN-C)
- CAN-Interior High Speed (CAN-IHS)
- LIN
The BCM provides the following diagnostic features:
- Diagnoses and reports hundreds of Diagnostic Trouble Codes (DTCs)
- Monitors the CAN bus for failures and logs network DTC "U" communication codes
LIN MASTER - The BCM is a LIN master module to the following components:
- Intelligent Battery Sensor (IBS)
- Steering Wheel Switches (SWS) via the Steering Column Control Module (SCCM)
- Electronic Overhead Module (EOM)
- Humidity Light Rain Sensor Module (HLRSM)
- Integrated Center Stack (ICS) module
- Tailgate Module
- Power Inverter Module (PIM)
- Heated Seat Switches
- Wireless Charging Pad Module
- Inside Rearview Mirror
OPERATION CONTROL
The BCM operates the following functions:
- Exterior lighting
- Washer and Wiper control
- Ignition control
- Horn
- Power Locks
- Interior Lighting
- Power windows
Several inputs are used to alert the BCM to become active. The hardware wake-up inputs include:
- Headlamp Switch
- Hazard Switch
- Vehicle Access Point (VAP) ajar switches
- Interior lamp load detection
- The BCM will also wake-up from module messages from the CAN-C, CAN-IHS, or LIN communication bus networks
STARTING SYSTEM MASTER
The BCM is the starting system master:
- Uses ignition position bus messages from the Radio Frequency Hub (RF-Hub) module to generate vehicle ignition position (commanded ignition position).
- Works with the Powertrain Control Module (PCM) and various starting system components to start the vehicle.
- Signals the RF-Hub module that the starting attempt has begun or has completed (engine may or may not be running).
MODES OF OPERATION
Customer Mode:
- All features will work as designed for the customer
- This mode is set by the dealer prior to vehicle delivery
Ship Mode:
- The BCM mode will be set to "Ship Mode" by the Certification test at the end of the vehicle assembly process
- This mode will be set after all electrical testing is completed and the vehicle is ready to be shipped
- In Ship mode many non-critical vehicle features are disabled, minimizing any unnecessary battery drainage during shipping and storage
- The BCM will broadcast the "Ship Mode" message over the vehicle CAN bus so all other vehicle modules will receive the "Ship Mode" message and respond accordingly
The different BCM operating modes can be changed and verified using the scan tool. In addition to using the scan tool to verify and change the BCM mode, you can set a vehicle to/from Ship Mode by following these steps:
- Place the ignition in the RUN position.
- Turn on the four-way emergency flashers
- Press and hold the left steering wheel "up" button for approximately 5 seconds.
- The vehicle will switch modes.
The modes can be switched back and forth by using this method.
POWER ACCESSORY DELAY:
The BCM manages the Power Accessory Delay (PAD) system for the vehicle.
The PAD feature provides power to some accessory features such as power windows and sunroofs so that they can remain active after the ignition is cycled off. This allows the customer to finish closing windows, sun roofs etc. without having to keep the ignition system active. On vehicles equipped with a customer selectable options, the PAD time is programmable. The feature will time out after a specified period of time and is terminated when either front door is opened. PAD does not function when the vehicles ship mode feature is active.