The hottest diagnostic tool to come into the automotive technicians' toolbox in my opinion in the past 10 years has to be the lab scope. But recent developments in current probe technology have given us diagnostic tools that are coming in as a close second. The unbeatable combination of a lab scope and a current probe (scope on a rope) can unlock the secrets of circuits that in the past have left technicians guessing.

A prime example is this waveform (Figure 1) of a 1991 Cadillac Deville with a miss that many a seasoned veteran would swear was in the secondary ignition system. With a current probe, a quick check was made of the injector wave forms and the true culprit is quickly and correctly diagnosed as a bad PCM and two bad injectors.

These measurements were taken from the fuse box with an adapter in less than five minutes.

The normal routine for this type of miss if you are familiar with Cadillac's on-board diagnostic system would be to do an injector balance test through the climate control panel to pin down the cylinder. Otherwise you would probably hook up an ignition scope and do a power balance test. Once the dead cylinder was pinpointed (barring no obvious secondary pattern problems), you would likely pull the plug to get an idea of the condition inside the cylinder. And perhaps even a compression test might be in order. Showing no deviation from the norm, likely conclusions at this point would be either a rocker not opening, or an injector problem. The next logical step would be to dig up the injector specs (over 14 and under 20 ohms). Break out the ohmmeter and check the resistance of the injector for the dead cylinder. Once finding the shorted injector with the ohmmeter (being a good, thorough technician), it would only make sense to test the rest of the injectors since you have to pull the fuel rail to replace the one. After receiving the OK from the customer to replace low-reading injectors, you get the car back together, fire it up and find that it has the same miss it had when you started. You start to get that sick feeling and questions arise in your mind. Did I possibly reinstall one of the old injectors? Is one of the new injectors bad?

Out comes the ignition scope for another balance test.

Same cylinder still missing ... where did I put the ohmmeter? The injector tests good. Could there be an open in the power circuit to the injector? You check it for power and find system voltage. Could the PCM be bad also? You pull out the noid light, it blinks in time with the engine. Is it possible that this engine could have had a worn cam lobe or rocker pivot? It has a roller cam and it's not making any noise.

Do you pull the rocker cover from here or start questioning one of your previous diagnostic conclusions? You know the injector was shorted, and the new one reads well. Could the PCM be bad and still run the noid light? That is the most likely conclusion and trying a new one is a lot less trouble than pulling the rocker cover. But what if you are wrong? Bad enough to have to call the customer back to let them know the injectors they just authorized didn't cure the problem and you think they may need a PCM also.

Get ready to dance: "Well, did I really need the injectors? Are you sure this will fix it? Are you going to call me back and say something else is wrong?"

After a short waltz the customer gives the go-ahead and you order a new PCM from the dealer. Meanwhile you go about your regular, routine work while worrying about this car until the PCM arrives. You plug it in and give yourself a mental drum roll while you turn the key. The car starts and misses for a few seconds then smooths out. "Yes!" you say to yourself, feeling lucky - almost as though you have won something. A test drive confirms the fix.

You feel confident that you didn't waste your time or the customer's money on this job because it was a weird problem and you correctly diagnosed and repaired it. But you still dread having to waste time educating the customer on ohms' law, and shorts, and injector drivers, and how a shorted part can take out the computer. (Figure 2)

This is not at all an uncommon scenario, and I would venture to guess one that has happened to a few of those reading this article right now.

I have had dozens of these cars with various problems in the past few years that could have turned into some real time burners had it not been for my current probe. See Figure 1 for example. This car had been to a couple of other places for the usual "miss work." You know, plugs, wires cap and rotor.

With the gravy gone and the miss remaining, I get to be the proverbial "Lion's paw thorn puller." Thorn pullers, in my opinion, should be fairly compensated. By that I mean paid by the size of the thorn, not the difficulty in removing it. Cadillac injector misfire problems are not to be taken for granted and generally require some head scratching. The reason being that there are a number of things that can cause intermittent injector problems on these cars. (Figure 3)

This random miss made its way to us through the channels from driveway tuneup to chain store, then here. After checking the injectors with the current probe, I decided to get a trigger signal from No. 1 injector PCM side. What a strange pattern it turned out to have. It looks like a two-cupper (that's with cream and sugar). I printed out the waveform and went into the office to study.

I decided to check the cam signal next as it is what the PCM uses to trigger the injector sequence. The cam sensor - much to my surprise - looked normal even when the injector pattern was erratic. (Figure 4)

I felt confident that this one needed a PCM. The next one was one that threw me at first. (Figure 5)

It appeared to be a bad PCM and probably a shorted injector. I set the scope to trigger from the PCM side of the injector and expected to see a system voltage signal constant. Instead, I got the pattern in Figure 6. This was the first time I had seen a pattern like this; I had to stop and think about it for a while before continuing.

I wondered if it were possible to have a complete circuit from power source to PCM with the PCM driver doing its job and yet no current flowing in the circuit. I came to the conclusion that it would only happen this way if the resistance in the circuit was very high, which is what it turned out to be. This was the first Multech injector I had ever seen fail due to high resistance. This one was in the hundreds of ohms.

After thinking about it, it did look strange that there was no inductive kick in the voltage signal. High spikes alone don't always indicate a shorted injector. On some of these goofy PCMs you can have injectors stacking that make you think you have a shorted injector - when the real problem could be anything from a southbound cam sensor, to a loose engine ground.

A good tip on these is the pulse width. (See Figure 7) While the PCM can vary the pulse width from bank to bank - some of these engines use two O2 sensors with separate block learn and fuel integrator systems - it can't single out one injector. (Note the pulse width on the first injector past the cam signal in Figure 7). A true-shorted injector will show a high spike once every revolution, as seen in Figure 1. Bad PCMs and bad cam sensor signals can fool you as they have similar characteristics (random firing of injectors). Stairstepping is usually a sign of a bad PCM.

This condition also seems to occur right before or right after the cam sensor signal (Figure 8). It almost always turns out to be a bad PCM. This next one (Figure 9) was a run-of-the-mill job with shorted injectors; however, the customer needed to be forewarned of some things. Due to the high current from running the shorted injectors, and this car's weakness for PCM driver circuits, don't be surprised when it comes in for a miss on the cylinder with the tall spike. (The customer opted to take his chances.)

After showing all of these potential nightmares and alligators, I thought I should end with a positive image. Figure 10 shows what a good pattern looks like with new injectors.

One of the most satisfying things I get to do is hand the customer a clean bill of health for his or her car. I believe it has an immeasurable effect on the confidence level they have in us when they leave.( Figure 11)

Jeff Bach is the owner of CRT Auto Electronics, an ASA-member shop in Batavia, Ohio. For more information on this topic, contact Bach at (515) 732-3965. His e-mail address is northstarguy@zoomtown.com

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