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How I developed critical thinking skills to solve diagnostic dilemmas

Monday, January 1, 2018 - 09:00
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My wife often comments that I have book sense but not a lick of common sense. I guess she’s right, to an extent. I learned a long time ago that feigning ignorance was a surefire way of getting out of those “honey-do” items I really didn’t want to do.

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But maybe she has a point. Maybe that’s why I was only an OK and not a great diagnostic tech. Don’t get me wrong, I won more than I lost, but often it took me a while to nail down the cause of an especially irritating customer concern. To be a great diagnostic technician, you need to constantly add to your knowledge (through quality training from any source) and be able to apply what you know to any given situation. You have to be able to understand how things interact, and how one element of a system can impact another. These are all critical thinking skills and some are born with a greater abundance of these skills than others.

But that doesn’t mean you can’t improve on what you were born with. Let me share a few tales that helped me improve my critical thinking skills. If I can do it, I know you can!

A Pontiac Grand Am

If you’ve been reading my work for any length of time, you know how I harp on voltage drop testing. The reason I am so adamant that every technician master this technique is because of the hundreds of concerns I’ve personally been faced with where this technique came into play. And this Pontiac is one early case where I wished I had known then what I know now!

The customer’s complaint was unusual. While driving, the car would simply shut off and fail to restart. It was an intermittent failure that the shop where I worked at the time had tried to address several times before without success. Every time the car came in on a hook, it would start and run just fine. We never were able to duplicate the complaint.

Figure 1

I drew the ticket this time around and got a break. This time the car would not start. More accurately, it was a “crank no-start.” The spinning engine seemed healthy enough and a mechanical issue didn’t sound right anyway for the concern. I checked for fuel pressure at the rail and was rewarded with a good squirt from the Schrader valve. Last item was spark, and sure enough no spark was present. Was I dealing with a fundamental ignition problem or was there something else? A quick check with a screwdriver to the ear revealed no injector operation either. On this 3.8 V-6, it had to be something related to the CKP/CMP (Crankshaft Position Sensor/Camshaft Position Sensor) circuit or signal. That was the only thing I could think of right off the bat that would cause no spark or injector pulse.

To do any more testing, I needed my scope. At the time, I had a UEI 2-channel scope (Figure 1) that had no record capability so whatever it was causing the problem, I had to be able to see it when it happened or catch it with the freeze screen button on the tool. I backprobed the signal wires at the ignition module, located under the DIS coil pack, and had another tech crank it over so I could see the signal. The first go round didn’t look right at all — very “hashy” as if I didn’t have good contact with the terminal pins. On the second attempt, the car unfortunately started and the patterns looked normal.

I tugged and pulled on the wiring leading to the sensors in an attempt to duplicate the problem without success. According to the history, the sensors had been replaced, but even if they hadn’t, an intermittent drop out or one that looked “hashy” is not a symptom that usually resulted from a failed component. What was more likely was an issue in the wiring or the ground circuit. In other words, a voltage drop of some kind.

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