Powertrain Systems Description

The recent trend in the past few years is to equip new vehicles with on-board computer systems. This is done because manufacturers of new vehicles must meet certain fuel economy and emission standards set fourth by the EPA, along with improved performance and reliability demanded by the consumer. The manufacturer of such vehicles found that the only way to meet such demands was to incorporate high tech computer systems to control fuel, ignition and other systems that improve performance and reliability.

The Buick Riviera uses a computer system network that uses electronic devices to control various engine and body functions. The "brain" of the system is the Body Computer Module (BCM). The BCM is used to relay information to the sub-systems of the network and to control on-board diagnostics of the vehicle. The sub-systems are the Instrument Panel Cluster (IPC), Cathode Ray Tube Controller (CRTC), Electronic Engine Control Module (ECM), and the Heater A/C Programmer. Each of the sub-systems control a specific function, the Instrument Panel Cluster is used to display information to the driver such as vehicle speed, engine rpm, fuel level, etc. The Cathode Ray Tube (CRT) is used by the driver to control the air conditioning and heating system. The heater A/C programmer receives information from the ECC and carries out the commands necessary to obtain the correct in-car tempature. The Electronic engine Control Module controls the fuel and ignition system to obtain good performance and low emissions.

Computer Network:

Because the BCM and its sub-systems are electronic devices a central power supply (CPS) is used. The CPS takes power from the vehicles battery and provides a voltage output that is isolated from electrical interference that may be produced during normal vehicle operation. It provides voltage output to the BCM and the IPC and provides an isolated ground for the ECC, IPC, BCM and the heater A/C programmer. Fig 1.

Electrical Flow Of Data:

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 it to perform their function. The BCM produces this data by rapidly switching a 5 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. A practical example of this process is as follows: Fig. 2 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 ECM and the IPC. The BCM now sends the signal out onto the data line. The ECM 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.

How to Enter "Service Mode":

The data communication gives the BCM control over the ECM's self diagnostic capabilities in addition to its own. When a sub-system circuit exceeds pre-programmed limits a system malfunction is sensed by the BCM. The BCM then relays this information to the driver by way of a warning lamp displayed on the instrument panel cluster. In order to access and control the BCM self-diagnostic features the CRT is used as the communication link between the technician and the BCM. Fig. 3

The CRT display has a 20 character display area called the information center. During normal engine operation this display is used to control various functions such as radio and climate control operation. During diagnostic mode this area of the CRT is used to display ECM, BCM, and CRT diagnostic codes, along with ECM, BCM, and IPC data parameters, and descrete input and output values.

The CRT display becomes the controller to enter diagnostics and access the BCMs information. This communication process allows the BCM to transfer any of its available diagnostic information to the CRT display during service. By pressing the appropriate buttons on the CRT, data messages can be sent to the BCM over the data line requesting the specific diagnostic features desired. For more specific diagnostic information refer to "SELF DIAGNOSTIC PROCEDURES".