Mode Description

The SCAN tool can be used in 3 different modes: the normal or open mode; the 10K, special or ALDL mode; the factory test, back-up or 3.9K mode. Most SCAN tools are programmed so that the system will enter the special (ALDL, 10K) mode directly, and the "normal" or "factory test" modes must be selected manually, if available. In the special mode, all information incorporated into an engine/ECM combination is available. However, in the special mode engine operating characteristics are modified as follows: The closed loop timers are bypassed, EST spark is at full advance, the IAC (if equipped) controls idle speed at 1000 RPM, the park/neutral restrict functions are disabled, and on some engines the canister purge solenoid is activated. Although not all engine/ECM "families" are able to transmit information on the serial data line with the SCAN tool in the normal (open) mode, on engines that can be monitored in the open mode the SCAN tool allows data on certain operating parameters to be obtained without changing engine operating characteristics. Selecting the factory test or back-up mode causes the ECM to operate on the fuel back up logic programming, designed to allow system operation in the event of an ECM failure. While selection of this mode verifies that the back up programming is functioning properly, parameters that can be checked with the tool in this mode are of little value in diagnosing system malfunctions.