Scan tool diagnostics began in the mid 1980's on GM vehicles with the introduction of computer-controlled carburetors. Using the now-famous 12-pin ALDL (Assembly Line Diagnostic Link) connector under the driver's side instrument panel, technicians could get a view into what the magic box was seeing and doing to control fuel and spark. As on-board systems progressed, more Electronic Control Units (ECUs) were added to the vehicle, and many were wired to the ALDL so they could be accessed via the scan tool. By the end of the 1980's, most GM vehicles had a PCM, ABS and SRS ECU which could be accessed via the scan tool.
In order to make diagnostic information consistent, systems were classified under the following main groups in the Tech 1A scan tool menu: Powertrain, Chassis and Body. Under those groupings, many new ECUs were added, but the structure has remained in place to this day. Service procedures and training curriculum have been written around this model, and any technician familiar with scan tool usage is intimately familiar with its structure and can navigate the scan tool quickly to access the system being diagnosed. This design is so prevalent that it was adopted by the SAE as part of the DTC definitions in the OBDII standard (P codes, C codes, B codes).
The old adage, "if it ain't broke, don't fix it" may apply here, but there has been a change in the scan tool menu on select models to a design called Functional Diagnostics. (Fig. 6) Users of the Tech 2 scan tool may have noticed that beginning with the 2006 model year some vehicles do not present the traditional P-C-B selections.
Fig. 6
In an effort to bring a unified system diagnostic approach to the technician, the menuing for the Tech 2 is being updated to reflect the way the GM electronic Service Information (SI) is laid out. (Fig. 7)
Fig. 7
These diagnostic procedures assist the technician through a logical sequence of steps to check the entire system, as opposed to focusing on individual components. The real motive behind this strategy comes from the proven understanding that today's vehicle networks are intimately connected components, and they share data, processing and problems. Although pinpoint diagnostics is used at the end of the flow chart, the high-level system diagnosis is important to make sure that related components and controls are not overlooked. Wasted time in the flow chart may lead to inaccurate diagnosis and lost revenue.
One additional change worth noting is the addition of the Vehicle Control Systems selection on the Tech 2 menu and the SI menu. Although part of the Service Information for some time, it was introduced in the Tech 2 on vehicles with a High-Speed CAN network and replaces the Diagnostic Circuit Check function that has been around since 1998 on the GM Class 2 network. This test allows the technician to "poll" the network and find out which ECUs are present and which ones are not.
Once the module has responded, it can display if there is communication and the number of DTCs stored. In addition, once all modules have responded, the list is reorganized based on the priority of the problems reported. (Fig. 8) No communication will take priority and the ECUs with the most DTCs come next.
Fig. 8
The DTC information displayed includes symptom numbers that identify the type of failure, such as lost communication.
Symptom numbers reduce the number of DTCs for each vehicle. For example, symptom 03 may indicate a short to ground and symptom 07 may indicate an open or short to voltage.
- Thanks to Bob Augustine
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