Self-Diagnostic System

WARNING: This page is about a different car, the 1990 Cadillac Fleetwood and 1990 Cadillac DeVille. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: The terms Electronic Control Module (ECM) and Power Train Control Module (PCM) refer to the same system and are often used interchangeably. In this article the term ECM is used throughout.

The Body Control Module (BCM) and the Electronic Control Module (ECM) are the 2 major components of the self-diagnostic system. The BCM is the heart of the system, controlling a multitude of vehicle functions through monitored sensors and switch inputs. In addition, the BCM provides self-diagnostic capabilities. Likewise, the ECM provides control and self-diagnostic capabilities in relation to the various engine and emission related components it monitors.

Between the BCM and ECM a communication process has been incorporated which allows these units to communicate with each other and share information. The BCM, upon receiving information from the ECM or one of its own related subsystems, compares the received information with programmed instructions within system memory. In this way, the BCM provides monitoring of the individual subsystems and their related sensors and switches.

Should a subsystem exceed pre-programmed limits, the BCM will recognize a malfunction and, in response, may act to control the malfunctioning subsystem. To control a particular subsystem, the BCM rapidly switches an internal circuit between zero and 5 volts, converting programmed control information into a series of pulses which represent coded data messages. These messages are transmitted to the malfunctioning component which interprets the information and responds accordingly.

As a result of interactions between the BCM and a malfunctioning component, an alpha-numeric code, known as a trouble code, is often set in the BCM's memory. This trouble code identifies the malfunctioning component and can be accessed by a service technician as an aid to diagnostic procedures. All trouble codes are displayed on the Fuel Data Center (FDC). See Figure .

In addition to monitoring the self-diagnostic system and displaying trouble codes, the BCM can be programmed by the service technician to perform specific diagnostic tests on individual components and systems. Depressing the appropriate buttons on the Electronic Climate Control (ECC) panel will request the BCM to provide specific diagnostic information for display on the FDC. See Figure .