User:Mhewitt2000/sandbox/1988 Cadillac Computer System Diagnosis

Computer System Diagnosis - 1988 Cadillac new article content ... Coming from the 1988 Cadillac Service Information Manual for Deville and Fleetwood models: Computer System Diagnosis General Description: Aboard this vehicle are several electronic components which can be controlled, by the service technician, to provide valuable self-diagnostic information. These components are part of an electronic network designed to control various engine and body subsystems. A description of each of these subsystems will be found in their appropriate service manual section, however, this section will provide a description of the overall electronic network and the on-car diagnostic capabilities which have been designed to aid the service technician in system repair. At the heart of the self-diagnostic system is the Body Computer Module (BCM.) The BCM is located behind the glove compartment opening and has an internal microprocessor which is used to control various vehicle functions based upon monitored sensor and switch inputs. All vehicles are equipped with a 4.5L Digital Fuel Injections (DFI) and have in addition to the BCM an Engine Control Module (ECM) which provides microprocessor control for various engine and emission related functions. The ECM, located to the right side of the instrument panel, plays the major role in providing self-diagnostic capabilities for those subsystems which it controls. A communication process has been incorporated which allows the two modules to share information and thereby provide for additional control capability. In a method similar to that used by a telegraph key operator, each module's internal circuitry rapidly switches a circuit between 0 and 5 volts. This process is used to convert information into a series of pulses which represent coded data messages understood by the other component. This data transfer is illustrated in Figure 8D-1 and shows a partial list of information each component is provided during system operation. One of the data messages transferred from the BCM is a request for specific ECM diagnostic action. This action may affect an ECM controlled output or require the ECM to transfer some information back to the BCM. This communication gives the BCM control over the ECM's self-diagnostic capabilities in addition to its own. In order to access and control the self-diagnostic features available to the BCM, two additional electronic components are utilized by the service technician. Located to the right of the steering column is the Climate Control Panel (CCP.) Located to the left of the steering column is the Fuel Data Center (FDC.) These devices provide displays and keyboard switches used with several BCM controlled subsystems. This display and keyboard information is transferred over single wire data circuits which carry coded data messages back and forth between the BCM and display panels. This communication process allows the BCM to transfer any of its available diagnostic information to the instrument panel for display during service. By depressing the appropriate buttons on the CCP, data messages can be sent to the BCM requesting the specific diagnostic features required. First, enter Diagnostic Mode 1. Turn ignition on. 2. Depress the "OFF" and "WARMER" buttons on the CCP simultaneously and hold until all display panel segments illuminate which indicates the beginning of the diagnostic readout.

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