Description and Use

Terminal "E" or "M" of the underdash ALDL connector allows access to a variety of system information known as "Serial Data" using SCAN type testers. Terminal "E" is used to transmit data on 1986 and 1987 thru 1990 models that retain the 160 baud transmission rate ECM. Terminal "M" is used on 1987 thru 1990 models with the 8192 baud transmission rate ECM. While using a SCAN tool to monitor serial data does not make use of diagnostic charts unnecessary, understanding what each position on the tester used measures and the function of the circuit involved can be useful in obtaining information that might otherwise prove time consuming or impossible to obtain using other test procedures.

Serial data access can be useful in obtaining information regarding intermittent malfunctions, as the SCAN tool can be installed and monitored while operating the vehicle under the conditions where the malfunction was reported to occur. If the malfunction seems to be related to specific functions that are monitored by the SCAN tool, the system can be checked in the applicable positions, under actual operating conditions. If the malfunction is not related to any specific area, the system can be monitored in all tester positions for a period of time or under a group of operating conditions. This allows the technician to observe the system and monitor changes in tester readings that may indicate a system malfunction.

Many diagnostic charts include provisions for the use of a SCAN type diagnostic tool to interpret serial data, and use of the SCAN tool is required to diagnose malfunctions on some 1987 thru 1990 models. The SCAN tool is capable of monitoring sensor inputs to the ECM and ECM commands to the various system operating elements, and provides the means to save time in diagnosis and prevent replacement of satisfactory components. Successful use of the SCAN tool for diagnosis requires an understanding of how the C-3 system functions, a thorough acquaintance with the tool manufacturer's recommendations and knowledge of the SCAN tool's limitations. Not all SCAN testers are compatible with every system. A SCAN tool that produces faulty readings should not be used, and problems should be reported to the tool manufacturer. Use of a faulty SCAN tool can result in improper diagnosis and unnecessary component replacement.