Training Tomorrow's TechsPosted 11/16/2004
By Brian Manley
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
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:
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?"
"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.
"Trust No One"
This is another of my favorite sayings, borrowed from agent Fox Mulder of The X-Files, that I was about to put to good use. I checked the resistance spec on our information system. Sure enough, the student had the correct vehicle entered, and the spec was 1.2 to 2.2 ohms.
We then grabbed the GM service manual off of the shelf and searched for chart A-3: "Engine Cranks But Won't Run." This chart has the injector driver circuit diagnostic procedure, complete with a resistance spec for the injector: 15 to 16 ohms.
So now my question was, "Which specification is correct?" As we pondered that question, I asked another, "Do No. 1 and No. 4 injectors have anything in common?" As we scrutinized the wiring schematic, we found that injectors No. 1 and No. 2 fired in pairs as did No. 3 and No. 4.
There was no direct electrical relationship there, but one of the student's observations led to an important lesson in Ohm's Law. He noticed that the engine ran better with the No. 4 injector unplugged.
"Why," I asked. Then came the response I had hoped for: "Because the resistance is different?"
Low impedance injectors range from 2-4 ohms. High impedance injectors range from 12-16 ohms. The impedance of the injector determines what type of driver in the PCM is used. Low impedance injector drivers (peak and hold) allow more current to flow to open the injector faster, and ramp it down to hold it open. High impedance injectors use a saturated switch-type driver. They will open a little slower, 5-l ms, and the current will stay more constant once the coil is saturated. The point being is that not any injector can be used with any PCM.
The No. 4 injector was using more current than it was supposed to, robbing No. 3 of enough current to stay open long enough. When the shorted injector was disconnected, the No. 3 injector resumed normal operation, causing a smoother idle.
More Testing Methods
We pulled out our Fluke 98 and demonstrated both methods of waveform analysis, each one identifying the No. 4 injector as inoperative.
From Ohm's Law to Waveform Analysis
Our case study provided many lessons for my students, and I hope it helped to remind many veterans to stop and consider the basics before branching off to an erroneous conclusion. As a teacher and a technician, I am constantly searching for opportunities to move further down the path. If I don't keep moving down the path, I'll get run over.
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