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Keeping Up with Current Probe TechnologyPosted 5/10/1999By Jeff Bach
I consider myself fortunate to have a job that is both challenging and rewarding. It keeps me in a constant state of learning while at the same time giving me the opportunity to reap the rewards of my experience. This industry, with its constantly changing technological advancements, has made it very difficult for today's technicians to stay current with the technology and tools necessary to accurately diagnose and repair modern automobiles. Like the woodcutter that moved through the grove of poplars and came upon a stand of oaks, he soon learned you need to sharpen your axe more frequently to cut the tighter grained trees. Learning to sharpen the axe becomes as important as the technique of using it. The best axe-sharpening tool for me has been the current probe. I constantly find myself saying, "I can't imagine how much time I would have wasted on this job trying to figure out the problem when here it is in black and white." Here is a good illustration of my point. Figure 1 (Figures will open up in a new browser window) shows the current waveform of the Torque Converter Clutch solenoid as the ECM is grounding the circuit and turning on the solenoid. The ramp, or inductive curve as I like to refer to it, is showing the time it takes for the relay coil to reach full current. Note that about two-thirds of the way up the ramp a rather distinctive dip in the waveform occurs. This is a fluctuation in the magnetic field caused by the movement of the solenoid's plunger as the field becomes strong enough to overcome the spring pressure and lift it to the closed position allowing the fluid pressure in the torque converter to build up to the locked position. (I call this the "Gull Effect"). After the transmission heats up (usually after a good 20-minute road trip) occasionally the solenoid plunger would stick in the locked position. This would lead to holding fluid pressure in the torque converter so that the engine would lug down, stalling or almost stalling when coming to a stop. Upon restart the car would have the feel of no power as the stall speed of the torque converter was severely lowered. This condition, while easy to detect once it gets to the point of stalling or severely lugging the engine at stops, is often misdiagnosed as a timing issue, clogged exhaust, or lean condition in its early stages of sticking. The TCC solenoid waveform in Figure 2 is from a car not yet lugging the engine, but merely losing power going up hills and on take-off when warm. I, myself, had not yet ruled out fuel or engine aspiration as a possibility when I collected this waveform. My test drive was not long enough for the condition to become acute, but it was long enough to exhibit the characteristic symptoms that caused me to hook the current probe to the TCC circuit as an experiment. What I saw made me realize that this was not only a valid test for TCC solenoid operation, but could be used as an early detection test for these driveability symptoms, at least as valid as testing the fuel pump for the same symptoms. I find that exciting! (I must admit I do lead a boring life.) Needless to say, this test now ranks on the list of valid tests that we routinely use to determine the cause of driveability symptoms such as low power, surging hesitating etc., which cause complaints from customers and have value. (Figure 3) The fuel pump current test (Figure 4), along with injector waveforms (Figure 5) and primary ignition patterns (Figure 6) are as common here as temperature, pulse rate and blood pressure tests at the doctor's office. I can imagine the day not too far away when these will become standard and a customer will have a breakdown out of town while on vacation. The attending technician will call the customer's regular automotive repair facility to have the charts faxed or e-mailed so that he may have a detailed history of the charted progress of the measured components. This kind of technology is something I can sink my teeth into. It takes very few computer skills to be able to capture a scope screen to a waveform program, edit the screen to personalize it for my customer, then print it out to give them a detailed, unquestionable color picture of something that has tangible value. For example, I would say to the customer, "Here was your car broken," as I show them Figure 7 with bad injectors, 1992 Cadillac 4.9. "And here it is fixed," I say, as I present Figure 8. (All I need now is one of those backlit X-ray reader lights to display the pictures, so I can consult with my colleague on the tough ones.) I still have one of those 1200 LB, rides on a monorail, cost $36,000 new, Fun 2001 Diagnostic Computers sitting in a barn collecting mouse nests. Knowing what I know today, I would have traded even for this tool back then. After four-plus years of current probe testing with a lab scope, I am starting to get people in here that want to have their cars tested on that new high-tech test machine from which their friend showed them the printout. The funny thing is, they are already sold on us and we didn't even have to go through the usual interview that skeptical new customers like to give. You know, "How long you been doing this?" "I guess you guys have to go to a lot of schools to learn how to fix these new cars huh?" and then there's "Do you guys have any kind of guarantee?" One of my favorites is, "What if this doesn't fix it?" I can see a time in the foreseeable future where the customer will undergo an interview before he can get an appointment for his car. I also believe that if technology continues to change at its current pace, it will be a thing of the past that any shop with a vendor's number can legally throw enough parts at most cars to fix them and make the customer pay the bill. You can bet when the first case comes to court in my town for "Practicing Automobology without a license," I will be among the spectators (perhaps even on the witness list).
Regardless of which techniques your shop uses to approach your particular diagnostic dilemmas, the current probe and lab scope are as important to today's automotive technician, as a stethoscope and sphygmomanometer are to a physician.
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