How I developed critical thinking skills to solve diagnostic dilemmas

Jan. 1, 2018
To be a great diagnostic technician, 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.

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.

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.

I was able to remove the section of harness leading to the sensors and in doing so, I noticed where they had been misrouted. It was very tight in spots, so much so the mating connector to the main harness was out of position by at least two inches.  Luckily, this harness was easy to remove, and after stripping away the insulation, here is what I found (Figure 2). This is the ground splice from the ICM to the sensors, and there was literally only one strand of wire left to complete this circuit.

Figure 2

Testing this circuit path with an ohmmeter would have resulted in a passing reading. The single strand would measure the same static resistance as the complete multiple strand wire would have and could have caused my diagnostic process to go off in a completely different direction. At the time, I was not familiar with the use of a headlight as a substitute load but today, I would have used that method to verify my suspicions and then I would have dismantled the harness. I fixed the car, but the process was as much luck as it was skill.

A Nissan Sentra

While working for the same shop, I had the opportunity to improve my luck/skills ratio on this 2002 Sentra. The young lady who owned the car was complaining of an intermittent stalling on initial start up and the MIL light was on when I got in the car to drive it into my bay.

As soon as I started the car to drive in, I understood the customer’s complaint. It started rough, almost like it’s flooded, and at least one cylinder out of the four was not home. The MIL was on, but not flashing. It’s only about 50 feet to my bay, so I decide to limp it in. In the time it takes to pull in, the miss and rough running are gone and the car is as smooth as silk.

Checking for codes revealed nothing surprising. There was a P0300 (Random Misfire) stored and the related freeze frame data painted a clear picture of a problem occurring when the engine was cold. This lined up with the customer’s concern and my observations when I drove the car in. Opening the hood, I took a quick look around the engine compartment and noted that the coolant level in the reservoir was low but not empty. Am I dealing with a head gasket issue?

Pulling out the UEI scope, I did a relative compression test that showed no appreciable loss of compression. My next step was to check for cooling system leaks, looking for the reason the reservoir was low. I pressurized the system and let it sit but surprisingly no loss of pressure was noted.

Of course, now the engine was slightly warm and getting warmer every time I started it up. I tried a combustion leak detector test (that passed) and looked for signs of the pressure rising with the engine running (also passed). My gut insisted it was a head gasket problem, but so far I was having a tough time proving it!

Here’s when I started a diagnostic step I’ve used ever since. When I think I’ve reached a roadblock, I take a moment to step back and review what I’ve learned to that point. Keep in mind, knowing what isn’t causing a problem can be as significant as knowing what may be the cause of a problem. The engine, warm and above, was doing just fine. What about when it was cold?

Figure 3

I left my radiator pressure tester installed on the now fully warmed up engine, and let the car sit for the rest of the day (Figure 3). Periodically, I would walk over to check the pressure reading. Slowly, the pressure dropped, and I knew I was on to something! Once the pressure dropped to nothing, I removed all four spark plugs and peered down into the combustion chambers.

Looking back at me in cylinders 2 and 3 were droplets of green coolant! My hypothesis is justified!

My idea was this – when the engine was warm, the head gasket would begin to leak ever so slightly. Running, this was no problem. The small amount of coolant entering the chamber would be burned off with the fuel charge and wasn’t enough to cause a drivability issue. But once the engine shut off, the leak would allow coolant to pool on the piston tops, waiting for that restart. Replacing the head gasket cured the complaint. No luck here! Just good old fashioned common sense!

About the Author

Pete Meier | Creative Director, Technical | Vehicle Repair Group

Pete Meier is the former creative director, technical, for the Vehicle Repair Group with Endeavor Business Media. He is an ASE certified Master Technician with over 35 years of practical experience as a technician and educator, covering a wide variety of makes and models. He began writing for Motor Age as a contributor in 2006 and joined the magazine full-time as technical editor in 2010. Pete grew the Motor Age YouTube channel to more than 100,000 subscribers by delivering essential training videos for technicians at all levels. 

Connect with Pete on LinkedIn.

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