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Engine Scopes (Digital Storage Oscilloscopes)

Copyright AA1Car Adapted from an article written by Larry Carley for Underhood Service magazine

Fluke oscilloscope ..

A "digital storage oscilloscope" (DSO), also known as a "dual trace scope," is unquestionably one of the most useful pieces of diagnostic equipment that is available today. What makes them so valuable as a diagnostic aide is their ability to capture and display electronic signals as waveforms on a screen.

If a picture is worth a thousand words, then a waveform ought to be worth at least several pages of diagnostic flow charts and God knows how many hours of time testing individual components and circuits in an attempt to identify and isolate a problem. One waveform can often give you a clear picture of exactly what is happening inside a sensor or the onboard computer system. That is the power of a scope.

WHAT SCOPES CAN & CANNOT DO

First things first. A scope is not a substitute for a scan tool, an engine analyzer, an exhaust analyzer, a digital multimeter, a breakout box or any other piece of diagnostic equipment you may already own. These are all essential tools for underhood diagnosis today. A scope complements all of these other tools by giving you yet another means of peering into the inner workings of the onboard electronics.

A scan tool can display fault codes or messages as well as voltages and other values by translating the serial data that comes out of the vehicle onboard computer. You can look at all kinds of numbers and data, but the numbers alone don't always give you a complete picture of what is really going on inside the system, especially when you are dealing with an intermittent fault or a momentary fault. What's more, many problems won't even set a fault code. So you may not have a clue as to where to start if you are trying to fix a driveability or emissions problem.

The drawback to using scan tool serial data to diagnose sensor problems and other faults within the onboard electronics is that serial data is not "real" data. It is the computer's interpretation or report of what it thinks it sees, which may not necessarily be what is really going on electronically in its input and output circuits.

For example, say a car has a hesitation problem and you suspect the throttle position sensor (TPS). You look at the output voltage of the TPS with your scan tool and watch the numbers increase then decrease as you open and close the throttle. The TPS seems to be okay, but is it? If there is a momentary dead spot in the TPS (which typically occurs between idle and part throttle where wear is greatest), the serial data that you are seeing may not reveal the dead spot in the TPS. Even if you are using an analog voltmeter to read the TPS directly, the needle may not respond fast enough to detect a momentary dead spot. Sometimes a TPS will read fine when opening and closing the throttle slowly, but skips when the throttle is snapped opened quickly. But you may never see the glitch unless you have a means of viewing the TPS output signal itself.

What a scope does is translate an electronic signal into a pattern or waveform on a screen. As the waveform is traced across the screen, it creates a signature of the signal's characteristics (more on this subject in a minute). The waveform reveals a tremendous amount of useful information about what is actually going on electronically within the sensor or circuit. So once you know what to look for, you can quickly distinguish good signals from bad ones.

A LEARNING EXPERIENCE

Reading waveforms isn't something you are going to pick up overnight, however. If you thought learning how to use a scan tool was fun, wait until you are confronted with a scope for the first time.

The initial setup procedure can be intimidating, but the menu driven setup screens in most scopes helps simplify the process. Instead of entering vehicle year, make and model or VIN number as you would with a scan tool, you tell the scope how to display the signal data. This includes setting the voltage scale and time base.

A scope displays voltage on the vertical scale and time along the horizontal scale. You pick a voltage scale and time base that allows you to see the entire waveform and also makes it large enough so you can see all the important details.

Next, you have to tell the scope when to start displaying the signal unless this is done automatically (which it is on some scopes). This point is called the "trigger level" and is set to a specific voltage value. You also have to tell the scope which way to draw the pattern (up or down) when the signal voltage passes the trigger level.

All this may sound rather confusing to the uninitiated, but once you have figured out how the scope works it is no more difficult to setup and use than a scan tool. It is getting over the initial hump that scares off many would-be scope users.

Another thing that is different with a scope is how you hook it up to the vehicle. Unlike a scan tool that simply plugs into a diagnostic connector somewhere on the vehicle, the scope leads require you to either backprobe connectors or pierce the wiring of individual sensors or circuits. Most vehicle manufacturers d not like technicians poking holes in wires. Even so, if you use "Hirschmann" style probes, they make only tiny holes in the wiring which can be easily resealed afterwards with a dab of nail polish (just what every technician carries in his toolbox, right?).

In addition to learning how to use the scope itself, you also have to learn about electronic signals and waveforms. For starters, you have to know what the five basic types of electronic signals are: direct current (DC), alternating current (AC), fixed pulse width (variable frequency), pulse width modulated and serial data. Then you have to learn what each of these signals looks like on a scope as well as the "critical dimensions" that are important for each signal's waveform (what you are supposed to look at in other words). This includes signal amplitude, frequency, shape, pulse width and overall pattern.

Still with me? I hope so. Then you have to learn what the basic waveforms for each type of sensor and other device are supposed to look like. This is the hard part because waveforms vary a great deal depending on the vehicle application.

Different types of fuel injector drivers, for example, produce different waveform signatures. Some produce a single spike when the computer opens the ground circuit (saturated switch type injector drivers like those used with Bosch multiport systems), some produce a double spike (peak and hold type drivers such as those used with GM throttle body injectors), and some produce an inverted spike (like the "backwards" style Jeep 4.0L multiport injectors that are normally grounded and open when energized with voltage through the computer driver circuit). Why do you have to know all this stuff? Because the height of the voltage spike as well as where it occurs on the waveform can reveal electrical problems within the injector solenoid or computer driver circuit. A shorter than normal spike, for example, would be characteristic of a partially shorted injector solenoid. A simple resistance check with an ohmmeter might not reveal such a problem. This is just one example of the many things you can't see with a scan tool or multimeter but you can see with a scope.

You will also find the learning curve requires a certain amount of hands-on experience with a variety of vehicles. A relatively inexperienced user can easily identify a "flat liner" or an obviously bad sensor signal. But it takes an experienced eye to distinguish a marginal waveform that may be causing trouble. What may be normal "noise" in the waveform on one application may be unacceptable "hash" in the waveform of another. This is especially true when looking at oxygen sensor waveforms which give you more diagnostic insight into the overall health of the onboard computer system than any other individual signal. So until you develop an eye for reading waveforms, it helps to have a library of good waveforms for comparison and reference.

The fastest way to get up to speed on scopes is to attend a clinic put on by one of the scope manufacturers, or to take a self-study course. One of the best self-study courses I have seen is NAPA's "Using the digital storage oscilloscope to master driveability and emissions diagnosis" training program (part number 6212). The workbook and video tape will really open your eyes to what a scope can do, and even includes repair case histories that show how a scope can be used to diagnose kinds of problems.

SNAP SHOTS

One of the attributes of a digital storage scope that makes it such a valuable diagnostic tool is its ability to capture and store signal waveforms. Analog lab scopes with cathode ray tube (CRT) displays are good at showing signal patterns in real time, which is why they have long been used with engine analyzers to display ignition patterns. But using an analog scope to display electronic signals has some drawbacks.

One is that analog scopes can't be configured to slow or freeze a waveform display. If a momentary glitch occurs, there is no way to capture and analyze it. It is there, then zip it is gone. And it may have happened so quickly that you didn't even notice it.

A digital scope, by comparison, does not display a signal in real time. There is a slight delay, the length of which varies depending on how "fast" the scope is. Though this may seem like a disadvantage, it is actually an advantage when it comes to capturing events that happen very quickly because it can slow the events down so we can see them more easily.

Unlike an analog scope that uses the input signal to continuously shape the waveform display, a digital scope takes little samples or snippets of information from the signal, then processes the bits to paint a waveform picture dot by dot across the screen. The end result is a "cleaner" waveform with less noise to garbage it up. This makes it much easier to analyze the waveform and compare it to other previously captured and stored waveforms.

The sampling rate with a digital scope is normally around 25 million samples per second, which is fast enough to catch even the most momentary glitch. Depending on the scope, this can usually be increased to an even higher rate. Some scopes offer a "spike detect" mode which jumps the sampling rate up to once every billionth of a second! At this rate, the waveform contains much more detail and noise, but also reveals problems that might be overlooked in the normal sampling mode.

Most digital scopes are also hand-held units with LCD displays, which makes them easily portable. If you think a scan tool or flight recorder can help you catch a momentary glitch during a test drive, you have not seen anything until you have taken along a scope. You will see things you have never seen before, and catch problems you would have never caught before.

O2 WAVEFORMS

One of the most powerful uses for a scope, however, is one we have not even mentioned yet: looking at the oxygen sensor signal. A scope can tell you if the O2 sensor is capable of producing a good signal even if the sensor is reading rich or lean. The scope can also allow you to use the O2 sensor waveform to verify that the computer feedback fuel control loop is functioning properly. Of course, you can do the same thing with a scan tool, but not with the same degree of accuracy as a scope. And what if a vehicle has no data stream output for a scan tool or the O2 sensor? Then what?

When you look at an O2 sensor's output with a scan tool, you see only a voltage value or a rich or lean indication. You can also look at cross counts to see if the sensor is flip-flopping back and forth from rich to lean at an acceptable rate. You can also check the sensor's rich and lean response by making the fuel mixture rich (by feeding propane into the intake manifold) and then lean (by pulling off a vacuum hose) to see if the sensor responds as it should. Yet a sensor that passes all these tests may still be causing problems if its waveform is bad or full of noise. That is where a scope comes in. It shows you everything you need to know about the sensors output in one simple picture.

You can see at a glance if the sensor is reading rich or lean, what the sensor's peak and minimum voltages are, if the sensor is flip-flopping from rich to lean at a normal rate, and how it responds to changes in the fuel mixture. You can also see if the signal is clean or full of noise. If the scope has dual trace capability, you can also display the injector driver waveforms at the same time to see if the feedback loop is changing injector duration in response to changes in the the O2 sensor signal.

snap-on Solus scope

AN EKG FOR THE ENTIRE SYSTEM

Like an EKG reveals an irregular heartbeat, the O2 sensor waveform will also reveal any underlying problems such as vacuum leaks, ignition misfire, injector imbalance and even compression losses. Each of these conditions will produce a characteristic type of hash in the sensor waveform. Anytime a cylinder misfires or leaks compression, unburned oxygen enters the exhaust. This shows up as a momentary dip in the O2 sensor's output voltage. So if the O2 sensor's waveform contains lots of little inverted spikes, it tells you the engine is misfiring or leaking compression. You can then use your other diagnostic equipment to nail down what is causing the problem.

OTHER USES

A scope can also be used as a repair verification tool. If you "baseline" a vehicle before repairs are made (capture the O2 or other sensor waveform that reveals a problem), you can then compare "before" and "after" waveforms to make sure the problem has been corrected.

An O2 sensor and feedback loop diagnostic scope check should be part of every tune-up as well as every driveability and emission repair you do. Those who use a scope this way say it eliminates most comebacks.

You can also use a scope to check the "V-ref" voltage in sensor circuits. Unlike a digital voltmeter that only gives you a number, the V-ref voltage on a scope appears as a flat horizontal line. Though not very interesting to look at, it can reveal hidden problems if the line is full of noise, has spikes or breaks up. The same technique can also be used to check battery voltage and wiring continuity. If the line breaks up or dips when you wiggle a connector, it tells you there is a problem.

A scope can also help you spot bad alternator diodes. The normal AC output pattern of the alternator should look like top of a picket fence. If any tops missing, it indicates one or more bad diodes.

You can even use a scope to read serial data, though not in the same way as a scan tool. A scan tool has software that converts serial data into letters and numbers we can read. A scope cannot do that. All it will show you is an irregular square waveform as the data stream passes by. Even so, this can tell you the computer is producing a data stream (should your scan tool not be displaying anything). It can also tell you if the computer is shutting down when the ignition is turned off. If you continue to see data stream up to a minute or more after the key has been switched off, the computer is not powering down as it should. The power relay is probably stuck on and may be draining the battery.

Buying a Scope

Digital storage scopes are not cheap. Most cost $2200 to $2800 or more, which is more than some professional grade scan tools. So it is hard to justify making such an investment unless you are a professional technician and are doing a lot of diagnostic work. The least expensive way to buy a scope is to buy a combination professional grade scan tool that also has a built-in scope.




Related Articles:

Scan Tool Help
Check Engine Light OnBoard Diagnostics
More on Check Engine Lights & Diagnostics
Diagnostic Tips for Trouble Codes
Scan Tool Diagnostics
Decoding Onboard Diagnostics
TROUBLE-CODES (list of generic OBD2 codes and definitions)
Understanding OBD II Driveability & Emissions Problems
Zeroing in on OBD II Diagnostics
Mode 06 Diagnostics

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