Global Diagnostics

General Motors has been in the electrical business for years. They were the first ones to come out with a starter motor! Technicians, by association, have also been in the electrical business for years. To help with diagnosis and keep techs ahead of the curve, trouble trees were used to bring you to a resolution for a customer concern. The concept was simple; start at the top of the tree and (depending on test results) you would work your way down branch to branch to arrive at what was hopefully the repair or faulty component that would put a smile back on your customer's face and some folding currency in your pocket. The problem-going to the "root" without following the branch to branch philosophy. In this brief article I will try and assuage some of the trepidation of using Lean Diagnostics by looking at what it provides the tech and how to apply it to a diagnostic procedure.  For more on the new diagnostic format, please see the November 2005 issue of Techlink magazine.

Enter the trouble chart! Still a step by step walk through of problem resolution, however it was more difficult to go tumbling into the "let's change this part and see what happens" shot gun approach to vehicle repair. These charts, whether they be for a Diagnostic Trouble Code (DTC) or Symptom Chart, were a carefully orchestrated menu designed to eliminate the items that can be tested in the service stall and lead you to the things that cannot. For example; you are working on a vehicle with a DTC for a faulty Throttle Position Sensor (TPS), looking at the graphic below, what are the items that can be checked for part of your diagnosis and what item(s) are out of reach?

Figure 1, the extremely average TPS Circuit courtesy General Motors

In order to diagnose, we need to look at what we have in our arsenal as far as tools and equipment. 

    Tech 2 - Great way to peek into the heart of an Electronic Controller like a Powertrain Control Module (PCM). Is the module getting good info and is it sending out the right commands.

    Digital Multi Meter (DMM) - Your right hand man in the service bay! A great way to check circuitry and ensure the module and Tech 2 are not giving you bad information. 

    Schematics - The "blueprint" for electrical architecture as well as connector end views.

    Terminal Test Probes - MANDATORY for checking any component's or harness electrical terminal. It is enough of a challenge  to have to fix the vehicle without designing in your own unintentional faults. So, right tool for the right terminal. (Enroll in the Terminals and Connectors Class - 18043.07 (if you need training in this area).

  Realize that the approach we are going to take here is no different than when you use a Troubleshooting Tree or Chart, process of elimination. The first order of business would probably be to look at the Data List for the TPS parameter and see what the PCM is seeing. Voltage too high, too low or any fault in between. Creating a list of what is possible to test we can see that circuit testing is well within our realm. Checking Reference Voltage and Sensor Ground as well as the Voltage coming in on the Sensor Signal line can all be verified by the use of a DMM. Of the components involved that we can check; TPS - a sweep test with an ohmmeter may show a glitch. PCM - not something we can really test but if all other components check out and circuits check out, it is the most likely candidate.   

  Using the above example we looked at the former way of diagnosing this same DTC (on a 98 Cavalier) and then jumping to today's format (2006 Impala). The Cavalier chart offered some 17 steps total for a complete chart, the Impala DTC page only 11. Let's do a little "side by side" comparison to see what's happening. Using the table below check out what was on the Diagnostic Flow Charts and what is available to the tech now.  

     Some of the Global Format Pages have tables with specifications included as well as Symptom Number information. More than what your getting before! For more detailed information on using the Global Diagnostic format check out SI Document ID# 1814794. 

What about the bullet (Þ)
 A source of confusion for many techs has been when and where to go when navigating through the chart(s). The repair arrow bullet will show the way when a test does not meet spec. Let's look at an example below;

If I make the measurement called out in step 2 and get a reading of  125 ohms what would my next step be. Click below on which way you would go in this scenario. 

A. Repair the short to ground

B. Replace the BCM

C. Replace the Under-hood Fuse Block

Bottom Line;               

  New ideas and technologies take time to get used to and this will be no exception. Just remember to;

• Test what you can

• Use Service Information Available for the concern (don't forget bulletins and PIs)

• Most of all STAY UP ON YOUR TRAINING BECAUSE KNOWING HOW SOMETHING WORKS WILL MAKE IT MUCH EASIER TO DIAGNOSE IT!

Mik Stubing
Lead Instructor-Northeast Region

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