Grabbing a waveform is as easy as turning on a scope, adjusting the settings and connecting the probes. Understanding what that waveform is trying to tell you, however, is much more difficult. And without that understanding, it's impossible to use a scope's power to help you fix cars.
Being able to analyze waveforms correctly is not a new problem. Since the 1940s, TV repair guys and other scope users have struggled to figure out what the waveforms they were looking at meant. The variations in scopes, TV design and probes meant wide variations in the patterns displayed. Years back, Sun Electric, among others, actually had plastic templates with waveforms printed on them for use as screen overlays for their automotive scope testers. But while comparison is a good way to determine if a waveform is "wrong" or "different," it doesn't necessarily lead to a successful diagnosis.
Basic understanding of waveforms begins with an understanding of what you're looking at on the scope's screen. Properly set up, a scope provides a visual picture of how voltage in a circuit changes in relation to time. The key is to understand why the voltage behaves as it does, and what it ultimately means regarding the performance of the vehicle.
For almost all automotive applications, what we'll be looking at are pulse waveforms. A pulse waveform can be a single event such as a glitch, or a repetitive sequence, or pulse train, such as you'd see on a computer data bus or at the ignition coil primary.
Unlike sine waves, pulse waveforms contain many frequencies within the pulse. A good example is the common square wave. Even though it looks like the DC voltage is simply switched off and on, a square wave is actually a complex waveform made up of many different sine wave frequencies that, when combined together, give it its square shape.
The waveforms you see on a scope screen are a combination of the energy pulse that's applied and the effect the circuit, and its impedance, has on it. Impedance consists of inductive and capacitive elements, as well as DC resistance. Since both inductance and capacitance are frequency-sensitive, there's an interaction with the frequency content of the waveform.
An excellent example of this is a tank circuit. Imagine a capacitor in parallel with an inductor. If we start with a charged capacitor, then connect the inductance, the capacitor will discharge its current through the inductor, which builds a magnetic field. When the current from the capacitor stops, the magnetic field collapses, sending its energy back to the capacitor. If you were to put a scope probe on this circuit, you'd see a sine wave.
Waveform interpretation becomes useful when we can connect changes in vehicle performance with changes in a waveform that result from changes in a particular circuit. As an example, let's take a look at the ramp of the ignition coil primary shown on the opposite page. This waveform was grabbed using a current probe to convert changes in current into a voltage that can be displayed. At the beginning of this waveform, we see a turn-on of the current and the initial damped sinusoidal oscillations, which are normal and associated with the charging of the secondary inductance. Note that the change in current here is enough in some coils to induce a feed-forward voltage in the secondary coil of a couple of thousand volts. Some coils contain a diode to block this voltage from accidentally firing the spark plugs out of time.
If these sinusoidal oscillations are not present, it indicates that something is wrong in the coil secondary. For instance, if the line goes straight up (no oscillations) before turning into a ramp, it's an indication of a full or partial short in the secondary coil. In this case, it's the change in the shape of the waveform that has the diagnostic information in it.
In an ignition coil, the resistance and the inductance of the primary determine the time the ramp takes to get from its starting point to its highest point. A coil with a shorted primary will have lower inductance and resistance, thus its current will rise faster. In this case, it's the scope's ability to measure time that's useful in diagnosis.
At the top of the ramp, the coil current may or may not show a flat spot. Older systems traditionally have this flat spot, which represents coil current-limiting. Most coils have primary currents in the range of 6 amps. With primary resistances in the vicinity of .5 ohm, battery voltage could drive nearly 30 amps through the primary if the current was not somehow limited. In waveforms with a flat top, a power transistor in the module is being turned partially off to limit the flow of current. For the period of time that it's limiting current to the coil, the transistor driver may have to dissipate as much heat as a light bulb (66 watts).
More popular these days, and for good reason, is the ramp-and-fire system. In this system, battery voltage and ambient temperature information are used to calculate how soon before the spark is needed it is necessary to turn on the coil. If the accuracy of the prediction is good enough, the dwell current will just be passing through 6 amps when the coil needs to fire. This type of system avoids the heat in the power transistor caused by it being used as a current-limiter. The diagnostic information is shown by the shape of the waveform and your ability to measure the amount of primary current at the firing point.
The falling edge of the coil primary current ramp represents the turn-off of the ignition coil by its driver. Although it looks like a completely vertical line, there really is some slope to it. Usually the fall time is in the 3- to 4-microsecond range. If the time is very much longer than that, the output of the coil will be affected. A slow falling edge typically indicates that something is wrong with the transistorized coil driver.
The falling edge of the coil current is also quite interesting in another way. If you've ever wondered how an ignition coil really works, this is the part of the waveform that tells the story. By Faraday's law, the voltage induced into a coil depends on the number of turns of wire multiplied by the change in flux divided by the change in time. The faster the flux moves, the greater the induced voltage.
It's the speed at which the coil turns off that's ultimately responsible for the voltage output of the coil primary. As an example, 14.8 volts (battery voltage) applied to the primary is converted to 350+ volts by the sudden collapse of the magnetic field. It's this 350-volt spike that's actually sent across the transformer into the secondary. With the typical 100:1 step-up turns ratio in the coil, the approximate secondary output voltage can be as high as 35kV.
The falling edge of the primary current waveform can also give you information about the condition of the secondary circuit. Once the magnetic field collapses, its energy has to go somewhere. If the secondary coil can't accept it, you'll see current oscillations as the energy fluctuates between the inductance of the primary and the distributed capacitance of the coil windings. The waveform on page 20 shows this exact circumstance. The oscillations in the primary current can be seen in the area right after the initial switching.
As you can see, there's a lot of diagnostic information in a coil primary current waveform. That doesn't mean that everything you might want to know can be seen there. Still, in these days of coil-on-plug ignitions where there's no secondary wire around which to put an inductive pickup, a great deal can be learned from the primary circuit.
Waveform analysis is something that has to be studied and learned. The hard part has always been knowing what a good waveform is supposed to look like. It would be terrific if manufacturers would print appropriate waveforms as a part of their service literature, or at least post them on their websites. In the meantime, there are variety of other resources available to you. For example, the International Automotive Technicians Network (iATN) has some 4400 stored waveforms that members can download. Some scope makers include a library of sample waveforms with their scopes. And scope seminars often include a book of waveforms as a part of the course material. Finally, if you have a spare $500, there are books of waveforms for sale on the Internet.
Probably the best way to learn about waveforms is to collect your own set of known-good and known-bad patterns. By building your own library, you can limit the number of waveforms to the kind of work you do, on the kind of vehicles you most often work on. Not a bad deal, is it?