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Patterns on Scope's Screen Can Help Pinpoint ProblemPosted 7/9/2004By Jeff Bach
There's an excitement and an energy that shines through the eyes of a person when they see something for the first time. It's like a light coming on. This "contagious twinkle" is what I witnessed recently in a technician who started working for me part time. He does drivability work for a living in another shop and has sent me a few cars that, after giving them his best shot, he felt were over his head. I knew he was pretty sharp from some of the phone conversations we had about other vehicles that had challenged him. He would call and consult with me to see if I could spark some fresh ideas. He loves his work and enjoys the challenge of today's drivability problems. I don't mind taking time to help a person who I feel is trying to go about diagnostics in an intelligent manner. The calls I find irritating are the ones from the guys who know you know what's wrong with the car they are working on and just try to pump the answer out of you so they can go back to making money. These guys don't care how it works or why it went bad ... they're just in it for the money, and the end justifies the means. I can usually come up with a technically correct statement for them that lets them know they aren't going to get the answers without doing some of the legwork themselves. If they call back after doing some testing, I may be willing to help, but usually they just send the problem to me after they've thrown all the parts on the car that show up on the ends of the trouble tree branches. I had a conversation with one of these leechlike characters last week that went something like this: " I heard you're the Cadillac expert in these parts." "I specialize in them," I replied. "Then what would make the O2 sensor keep setting a code after I put in all four new O2 sensors?" "Which code are you getting?" "The one for the O2 sensor in the back" (that narrows it to three O2s). "What's your long-term for that bank?" "I've had the thing two weeks and the customer's getting antsy, if you know what I mean." "Well, send it down and I'll take a look." I can't help this guy technically. He had a simple leaking fuel pressure regulator and he's chasing O2s. Now Mike, on the other hand (my "part timer"), is trying his best to understand the system and do logical repairs - kicking himself for not knowing what he doesn't know. I can work with him. He saw me figure one out with a current probe once and he bought one. He's trying to teach himself how to use it although that seems to be an exercise in futility. It's not something you can pick up easily on your own. You need to be taught to use one or you will likely end up disgusted with it (Mark Warren, Johnathan Riggle and Bob Pattengale do a great class if you can catch it). The first job we had on the first night he was here was a '90 Allante with a dead miss. Mike pointed out that this 14-year-old car had the original spark plug wires on it and suggested starting with a tuneup. "Good suggestion," I told him, "but I like to find the problem first so I know what the outcome is going to be before I start selling needed maintenance."
I could see from the scope screen that this drivability diagnostic was quickly turning into a mini scope class. "Now, how did you get that pattern to hold still for you?" Mike asked. By setting the scope's trigger to begin on the tallest injector, I got the pattern to stabilize - then I went in to the on-board diagnostic systems injector override function and killed No. 1 cylinder's injector. Figure 1 reveals some nice information with a single screen shot.
As I killed No. 7's injector the scope pattern disappeared. "Why did we lose the pattern?" Mike expectedly asked. "The pattern disappeared because I killed the injector that I had the scope set up to trigger on," I said. "The trace is set to start on anything more than 164 mV and that was the only injector drawing that much current." I lowered the trigger level until the trace came back, which brought up the image in Figure 2. This allows me to see the No. 1 injector also and know that its resistance is in the normal range.
It's amazing to me that it hadn't fried the driver inside the PCM for this injector. As you can see from Figure 3, the No. 7 injector (pulling well more than 10 amps) towers over the other injectors with more than five times their current. The customer went for the tuneup and the injectors since I made him a good deal on a couple of used ones I had. They had been sitting in an old fuel rail with
gas in it and were somewhat sticky when we first started the engine. I could tell they were sticking from the scope pattern when I blew up the pintle portion of the waveform. The pintle makes a dip in the curve of the waveform that to me resembles a seagull when it moves in the windings. When the injector doesn't open, the curve remains continuous. By setting the scope to trigger on the up slope of each waveform, you can get a pattern of good injectors with the occasional sticking one that animates the curve. It doesn't pay much but it's neat to see ... if you're in to that kind of thing.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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Using my Fluke 99 and the CRT current probe, I tapped in to the injector current at the fuse box under the ashtray and immediately had a nice picture of the injectors' current draw.
With No. 1 injector pattern missing, you can sequence that the No. 7 injector is shorted and No. 2 is on the way. We did some quick math to figure out the resistances, but I had to explain that the flat top in No. 7 is letting us know that the limit has been reached for the scale of the probe. So you can only say for sure that it's at least drawing a little more than 4 amps. Since the miss got worse when No. 1 was shorted, I went ahead and killed No. 7, which didn't change the miss - confirming that the cylinder with the shorted injector was the dead one.
To get a true picture of the actual current draw for the No. 7 injector, I changed the scale on the probe to its highest current reading and got the image in Figure 3.
