WARNING
To avoid personal injury and/or vehicle damage, refer to the service precautions at the beginning of this section.
All late-model vehicles utilize computers to monitor and control the functions of on-board systems. These modules are known by many names such as Engine Control Unit (ECU), Engine Control Module (ECM), Powertrain Control Module (PCM), Transmission Control Module (TCM) and Vehicle Control Module (VCM). When problems occur in control circuits these modules record a diagnostic trouble code that can be used to help solve the problem. There is also freeze frame data and an OBD status report available to the technician. Over the years there have been many different variations of systems, each with their own unique way of retrieving these codes. On a good number of the older systems the stored codes were flashed on check engine lights (found in the dash cluster) once a small jumper wire was placed across the proper diagnostic terminals. However the use of a hand-held scan tool was still preferred for these systems by dealership service departments.
For some models produced during the 1995 model year and on almost every single 1996 and later model a new form of trouble code reporting was developed which required the use of a scan tool. On Board Diagnostic-II (OBD-II) compliant vehicles use a 5 digit, alpha-numeric code which would be difficult or impossible to read using a flashing light. Trouble code reading on an OBD-II compliant vehicle requires a scan tool. Flashing trouble codes are a thing of the past.
The scan tool allows any stored codes to be read from the ECM. The tool also allows the operator to view the data being used by the engine management computer while the engine is running, or with the engine OFF and the key ON. This ability has obvious diagnostic advantages; the use of the scan tool is frequently required for component testing. The scan tool makes collecting information easier; the data must be correctly interpreted by an operator familiar with the vehicles operating systems.
An example of the usefulness of the scan tool may be seen in the case of a temperature sensor which has changed its electrical characteristics. The computer module is reacting to an apparently warmer engine (causing a driveability problem) but the sensor’s voltage has not changed enough to set a fault code. The voltage signal being sent to the module may be viewed by connecting a scan tool. Comparison to normal values or a known good vehicle reveals the problem quickly.
| Typical aftermarket scan tool used to access diagnostic codes from the Electronic Control Module
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