Self-Diagnostic System

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

The self-diagnostic system consists of 6 components. These components include the Body Control Module (BCM), the Heating, Ventilating and Air Conditioning (HVAC) programmer, the Diagnostic Energy Reserve Module (DERM), the Supplemental Inflatable Restraint (SIR) controller, the Powertrain Control Module (PCM), the Climate Control Driver Information Center (CCDIC) and the Instrument Panel Cluster (IPC).

In addition to monitoring a particular set of sensors and switches, the BCM functions as the heart of the self-diagnostic system, maintaining continuous communication with each of the other 5 components of the system. These other 5 components, known as subsystems, also monitor sensors and switches, reporting system conditions to the BCM.

The BCM, upon receiving information from a subsystem, compares the received information with programmed instructions within its memory, monitor 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 alphanumeric 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 CCDIC.

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. The results of these tests are displayed on the CCDIC. This article covers accessing trouble codes and programming the self-diagnostic system to perform specific diagnostic tests on system components.