Body Control Module: Description and Operation

BCM Module:

The Body Computer Module (BCM) is the heart of the computer network. The BCM has an internal microprocessor and uses a single data line to communicate with all the other components. The BCM also controls the on-board diagnostics of the vehicle. The BCM controls sub-systems through direct outputs or by data transmitted along the data line. The data line is a common circuit wire that is connected to all the sub-systems. This line allows communication between the BCM and the sub-systems.

The BCM receives information from various sensors on the vehicle and relays this information to the sub-systems that need to perform their function. The BCM produces this data by rapidly switching a 5.0 volt circuit on and off, this produces a digital signal. Each sub-system has its own code or address. When the BCM sends information on the data line it also assigns the address of the sub-system that it wants the information to be sent to. As a result only the sub-systems that are addressed will read and understand the information on the data line at that particular time. This process of reading and controlling information is continuous as long as the vehicle is running.

Electrical Flow Of Data:

A practical example of this process is as follows:

The BCM receives vehicle speed by monitoring the vehicle speed sensor, the speed sensors output is an analog signal (AC voltage). The BCM then converts this signal to a digital signal and assigns the addresses of the PCM and the IPC. The BCM now sends the signal out onto the data line. The PCM receives the vehicle speed and uses this input to control the transmission torque converter and other related engine functions. The IPC receives the input and displays vehicle speed on the instrument panel to inform the driver. Since vehicle speed is not needed for proper operation of the Heater A/C programmer and the ECC panel, the BCM did not assign their addresses to the information.

The data communication gives the BCM control over the PCM's self diagnostic capabilities in addition to its own. The BCM controls body sub-systems such as cooling fan control, fuel data display, outside temperature display, rear defogger, retained accessory power, self-diagnostics, and vacuum fluorescent display dimming. In order to access and control the BCM self-diagnostic features, the Instrument Panel Cluster (IPC) and Electronic Climate Control panel (ECC) are used.