Mode Description

The SCAN tool can be used in 3 different modes: the normal or open mode; the 10K, special or ALCL mode; the factory test, back up or 3.9K mode. Most SCAN tools are programmed so that the system will enter the special (ALCL, 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 by-passed, 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.
In the generally used special or normal test modes the SCAN tool allows a quick check of sensors and switches which provide inputs to the ECM, but the data only updates once every 1.25 seconds making the tool less effective than a voltmeter for locating intermittent conditions which may last for less than 1.25 seconds. However, the SCAN tool does allow the technician to manipulate wiring harnesses or underhood components while observing the SCAN readout. This aids in locating intermittent malfunctions while the engine is not running.
It is also important to note that while the SCAN tool may indicate that a component has been activated by the ECM, this indication only represents the ECM command to the specific circuit. The only way to verify that the command has been transmitted and carried out is to observe operation of the specific component activated by the ECM, or by noting changes in the system that the device controls. However, the SCAN tool allows the technician to compare operating parameters of a poorly operating engine with the parameters of an engine which is operating properly. Therefore, system malfunctions that may not ordinarily set a trouble code in the ECM memory, such as a shift in sensor values or excessive resistance in an operating element, can be uncovered by comparing operation of like systems.