Description and Use

Terminal "E" or "M" of the underdash ALDL connector allows access to a variety of system information known as "Serial Data." A SCAN type tester must be used to review this data. Terminal "E" is used to transmit data on models that retain the 160 baud transmission rate computer. Terminal "M" is used on models with the 8192 baud transmission rate computer. While using a SCAN tool to monitor serial data does not make the use of diagnostic charts unnecessary, understanding what each position on the tester is used for and what it measures, along with the function of the circuit involved can be useful in obtaining data 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. The SCAN tool can be installed and monitored while operating the vehicle under the conditions where the malfunction may occur. If the malfunction seems to be related to specific functions which are monitored by the SCAN tool, the system can be checked in the applicable positions, under the actual condition. 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 data 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 the use of the SCAN tool is required to diagnose malfunctions on some models. The SCAN tool is capable of monitoring sensor input to the computer and computer commands to the various system operating elements, and provides the means to save time in diagnosing to prevent replacement of good components. Successful use of the SCAN tool for diagnosis requires an understanding of how the Computer Command Control system functions, and a thorough acquaintance with the tool manufacturer's owner manual. 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.