Auto Class members work on a diagnosis for a '91 Geo Storm.

One of my favorite sayings is, "We're all on the same path, but we're just at different points on the path." The saying comes in handy as I teach auto technology. I've taught basic electrical fundamentals to teenagers and adults alike, and I've come to understand that the "path" is not age-related. We move forward on the path as we add to our arsenal of knowledge and enhance our ability to diagnose and repair vehicle systems.

Each year, I teach Level II (Advanced Auto) and Level III students (Auto Specialists). My Level II students begin with our General Motors Specialized Electronic Training (SET) boards, learning the basics of series and parallel circuits, voltage drop testing, current flow, resistance measurement and meter usage. We move through most of GM's SET curriculum in a few weeks, building dozens of different circuits on the boards. When the students are ready, they apply their skills in the shop.

My Level III students tackle most of our customer vehicles that come into our shop. These young men and women are further down the path. They can perform some advanced electrical diagnosis and use advanced features on our scan tools. These students can grab a lab scope and diagnose a throttle position sensor (TPS) or an O2 sensor just about as fast as I can. If an auto tech does not have a firm grasp of these basic concepts and procedures, the results are incorrect diagnoses and parts swapping.

Case Study: 1991 Geo Storm 1.6 L SOHC

Information provider gives a resistance specification of 1.2. to 2.2 ohms.

This little black coupe came to us recently with a complaint of a misfire, along with poor engine performance. Longing to give my Level II students a crack at a customer car, we pulled the coupe in cold and began diagnosing.

The engine had a rough idle - a hard misfire - and some overt signs that it lacked maintenance. There were some oil leaks, worn drive belts, corroded battery cables and a dented fender. But it only had 78,000 miles. I took each student to the tailpipe and showed them what a dead miss feels and sounds like. I also explained to my Level II students that even though this was an engine misfire, what they have learned about electrical fundamentals would play a role in diagnosing the root cause of this car's problem.

I wanted to turn some of the diagnostic control over to some sharper techs-in-training, so with two of my Level III students leading the diagnosis, they began to check for answers to the following questions:

• What is the engine's mechanical ability?
• How is the ignition system?
• Is there fuel delivery to the injectors?
• Could any of the emission control devices be at fault?
• Is the power control module (PCM) in fuel control?

Meter reading show 1.7 for the number 4 injector.

All of these systems needed to be confirmed as operating properly, so I suggested that they figure out a way to power balance the engine. One student used the pull-the-wires-off-and-watch method, while another rolled our ignition analyzer over and hooked it up for a safer power-balance test. It turns out that both methods revealed the same suspect: the No. 4 cylinder was not contributing, although all cylinders had adequate firing lines, spark duration and coil capacity.

Next, one of the specialists pulled out the spark plugs and performed a compression test. All cylinders were 135-145 pounds per square inch (PSI). For demonstration purposes, I had the students perform a running compression test as well, to show how much more cylinder pressure is created during cranking, as opposed to idling. Again, all cylinders idled at around 60 PSI and snap-throttled to about 100. These results were enough to convince them that the engine had adequate compression to do its job.

"So," I asked, "What's next?"

Meter reading shows 17.0 ohms for the number 1 injector (a "good" injector).

Before moving on, one student plugged a noid light into the injector harness connector for the No. 4 injector to test for trigger; we had fire. Next, they decided to blame the injector, so I asked them how they were going to either confirm proper operation or condemn it as being faulty.

"Check resistance," said one of my Level II students.

I knew we'd work electrical fundamentals into this diagnosis somehow! They tested the No. 4 injector, which read 1.7 ohms, while another student went to our information system for specs. Information can be difficult to find, so when he came back saying "1.2 to 2.2 ohms," I was puzzled. "Why is the spec so low," I thought. I had them test another injector - in this case, the No. 1 injector. The reading came back "17 ohms."

So I asked the class which one was defective. A couple of students piped up, saying, "the No. 1 injector must be bad." "How can that affect the No. 4 cylinder?" was my response. I found the spot on the path of inquisition where, in this situation, all of my students were stumped.