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Barking Up the Wrong TreePosted 11/13/2000By Jeff Bach
As a Cadillac specialist, I don't get many Nissans in my shop. But when an old friend asked me to try to find the problem with her daughter's '86 Nissan hard body pickup, I couldn't turn her down - partly because the problem sounded interesting. Symptoms included no power, won't go over 30 mph, backfires and shoots a 20-foot flame out the tailpipe. Not being all that familiar with the fuel and ignition control systems of these critters, I began working on this jewel with a few time-burning misapprehensions. The problem was easy to duplicate: Just drive away from a stop and nail the throttle. Just as soon as the engine started to develop a little torque, it would shut down like a rev limiter. I scoped the injectors with my current probe during the shutdown and never lost the signal (see Figure 1). I tested the ignition system next, by getting a primary current waveform during the shutdown occurrence. I had the scope set to “wait for trigger” to capture the image in Figure 2. This lets the last pattern remain on the screen after you lose the trigger signal. The pattern in Figure 2 had me convinced I was on the right track; now I just needed to see why I was losing the primary current. Next, I scoped the trigger signal for the coil power transistor. I was also losing this signal as the miss was occurring (see Figure 3). I tested this signal at its origin (the ECU) and got the same results. This system uses an optical distributor with a high-resolution and a low-resolution signal to determine crankshaft position and cylinder number. Both of these signals looked good when the problem occurred. I realized that the signal was disappearing with a predictable consistency tied to the load, as if the ECU was dropping it deliberately. I thought that perhaps the coil may have been drawing too much current and the ECU may have been turning it off. With the current probe still hooked around the coil primary feed circuit, I set the scope to see 500 microseconds per division (half a millisecond), which gives me the best view of one primary event. The result, shown in Figure 4, looked normal. This is a very pretty “shark fin.” Notice the nice, even, 45-degree angle where the fin breaks the water. This is what I like to see in a coil primary pattern, but it didn't explain the cutting out. I could feel my head starting to itch a little. I then tested the other coil using the same scope settings. The results of this test revealed quite a different animal. The angle, created by the inductive charge curve, looked more like the top of a submarine than a shark fin (see Figure 5). This is usually an indication that this coil's primary windings have a short in them. This was good itch medicine, but as I would soon find out, the relief was only temporary. I wanted to get one more picture to show my friend before replacing the coil. I wanted it to show the bad coil primary current signal cutting out as the ECU dropped the trigger signal to limit the current. I started an IV in the trigger signal wire of the shorted coil, connected it to the lab scope and clamped the current probe around the coil feed wire at the transistor. What I got was the image in Figure 6, which caused my itch to return. At the time of the cutting out condition, I still had current and trigger signal to the shorted coil. Scratching now, I started to question whether I knew that I knew what I knew or not. Still triggering from the coil primary current, I moved the probe to the good coil feed circuit while leaving the coil transistor trigger signal from the shorted coil hooked to channel two of the scope and got the pattern in Figure 7. Seeing the trigger from the bad coil's transistor signal (which I was expecting to drop out) and watching the good coil primary current cut out (which I expected to remain), I started trying to make sense of what I was seeing. I checked the wiring diagram to be sure that the triggers for the coils were on the same circuit. I had already assumed this since they always fired simultaneously. Much to my dismay, they each had their own separate circuit to the ECU. I went against my nature and consulted my electronic information source for some diagnostic direction of the trouble tree sort. With little success finding a symptom chart closely resembling my problem, I turned to iATN and did an e-mail search of similar symptoms. I was surprised to see so many posts of a dot like my problem. There were a dozen or so with the exact same symptoms. The problem with polling the archives for the most popular fix is that (aside from feeling like you're lost and asking for directions) I couldn't find two posts with the same fix. How can so many different problems cause the same symptoms? My wife, Dianne, was at the shop and she and my mother were going to a fair or something so I told her I would drop her off at my mother's. I let her drive while I sat on the floor in the back of the cab reading over all the information I had printed pertaining to this truck. The seat had to be taken out earlier to access the computer. (What a great idea to put it there!) This was difficult, as Dianne hadn't quite got the hang of trying to drive this thing without getting it into the torque curve. Reading, riding and trying to concentrate is difficult enough, but try doing it while trailer-hitching up a winding hill. (This would be a good training routine for the cookie toss.) We made it to my mother's and I got in the truck to drive back to the shop. After hearing me puzzle over the problem with this truck, my wife offered a suggestion that I would frequently get from my mother when I was frustrated with a tough job. She said, “Did you try praying?” I sometimes forget where my gifts come from. I said a few cursory words asking for help from above to placate my wife and headed out the driveway. I hadn't even reached the street when something I had read earlier in the description of this two-coil system hit me, and the light came on ... something about the spark plug switching control system changing to a one-plug system under heavy load to reduce noise. Now I get it. The shorted coil is the one on the one-plug system when the other one cuts out ... under load. The bad coil stops producing spark under load, then the good coil is being cut by the ECU (to reduce noise) and viola ... no spark. I replaced the coil and all is well. Mom doesn't know much about fixing cars, but she always seems to know when it's time to call. Some of the most discouraging jobs I have dealt with, jobs that have had me the closest to giving up, were fixed moments after talking with her. To borrow a line from a Chris Smither song, “Why does it seem like such a deal to say help me now, I just can't do it alone.” I printed a copy of the two coils to show my friend. When my friend, Betty, arrived at the shop to pick up the truck, she was already running late for a wedding so I didn't even get a chance to tell her about my adventure. Judging from the coils' deteriorated condition and the mounting location next to that leaking exhaust pressure pulsation diaphragm, I figured that maybe one or two of you have had, or will have, a similar adventure with one of these birds and this story might be better told here.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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