Scope Basics

 

The oscilloscope is a must when diagnosing many of todays computerized engine control systems. Learning to use a scope seems a bit daunting at first, but it is really a simple tool to use once you understand what it really is. Many of the informative articles on d-tips.com are written with the assumption that the reader has a basic knowledge of the oscilloscope and how to read waveforms. In this article I will show you how to use a scope so that you can begin the learning process of diagnosing todays complicated engine management systems and get more out of the other articles on d-tips.com.

 

Many of the scopes seen in automotive shops are smaller hand held devices. Most technicians seem to prefer the mobility of these small scopes, not to mention some very nice features that are not available on most of the older cabinet style ignition scopes. These smaller digital scopes are actually called DSO, which stands for Digital Storage Oscilloscope. The older CRT (cathode ray tube) scopes were true live oscilloscopes but are quickly fading into a thing of the past for automotive applications. The DSO is not a true live image, but is instead data that has been saved in digital format and then converted to an image on the screen. The amount of time it takes to save the data to a buffer and then draw it on the screen, however, is so small (a very small fraction of a second) that it is impossible to tell that the image is not live. A great bonus of the DSO is that since the data is all in digital format, it is very easy to record a movie, freeze frame, scroll through patterns, zoom in and out on saved images etc. These are all things that very few CRT scopes had the capability to do and the primary reasons the DSO has become the favorite in many shops today.

 

So, what exactly does an oscilloscope do? The scope is really nothing more than a fancy voltmeter with a few extra controls and pretty pictures. Instead of displaying voltage in numeric format like a voltmeter, it graphs the voltage so that you can see what the voltage is over a specified period of time and not JUST what the voltage reading is right now. When using a voltmeter on a car battery, you would see a value around 12.6 volts and the value would be steady unless a load was applied to the system such as cranking the engine over. If the vehicle were running, the value on the voltmeter would likely be around 14 volts, indicating the alternator is charging normally. The oscilloscope would read the same as the voltmeter, but instead of giving a numeric value of 14, it would plot the voltage on a graph over a specified period of time.

 

 

For now, ignore all the added messages and just look at the solid green line that goes from one side of the picture to the other. Looking at the voltage scale on the left side, you can see that this line falls just under 14. This scope has an added feature in that it also gives a numeric value at the top of the display for each channel. Since only channel two is being used in this capture, you can see this vehicle has a charging voltage of about 13.72 volts. By looking at the green line you can also tell that the voltage remained unchanged for the duration of the sweep which was 10 seconds in this capture.

 

The green numbers going up the left side of the image are voltage and the numbers across the bottom is time. Currently, the voltage scale is set to 20 volts top to bottom or 2 volts per division. This means that every horizontal line on the grid in the background is 2 volts. Note that the 0 volt line does not start at the bottom and instead starts on the first grid line. Using the current configuration of the scope, the bottom of the image would actually be 2 volts and the top would be 18 volts, which gives a total voltage of 20 volts. The zero volt line can be moved around to adjust for an image for optimal viewing. The numbers 1 through 10 across the bottom indicate seconds with the current configuration. The total sweep time is 10 seconds, or 1 second per division. With most automotive applications you will be selecting a time scale much faster than 10 seconds, usually around the 2 to 10 millisecond range per division.

 

Below is an example of a commonly tested component on any fuel injected vehicle, the throttle position sensor. The throttle position sensor (TPS) steadily increases voltage as the throttle is opened. If you were to view the TPS signal on a standard voltmeter, you would see the voltage start out around .7 volts and steadily increase to around 4.5 volts as the throttle is slowly depressed. Using a scope is much easier than a voltmeter since it will graph the voltage for you making it unnecessary to watch the numbers constantly change on the voltmeter.

 

 

 

 

 

In the image above we can see that the TPS signal started at just under a volt and steadily moved up to just over 4.5 volts at wide open throttle. The zero volt line has been moved to the bottom of the screen and the voltage and time scales are different than the first image. This image shows a voltage setting of 5 volts, or 500mv per division (500mv = .5 volts). The sweep time has been changed to 5 seconds or .5 seconds per division as seen across the bottom. Looking at the image you can see that the throttle was in the idle position until about 1.4 seconds are reached. At that point the throttle begins to open and voltage goes up until wide-open throttle is reached at about the 1.8-second mark. The throttle is held wide open for almost half a second and then begins to close, reaching the fully closed position around the 3-second point. The total time taken to open and close the throttle is about 1.6 seconds. Take a look at the image of a bad TPS below. The voltage does not rise steadily and drops out several times when the throttle is depressed.

 

 

Spotting a problem with a bad TPS is very easy with a scope as seen above. Many scopes will allow you to turn the background grid on and off. Turning the grid off like in the picture above does nothing to the waveform, it is only a user preference setting.

 

Learning to set the time and voltage divisions are crucial steps in becoming proficient with a scope. When testing a TPS the voltage settings will be low in most cases, since most TPSs do not achieve over 5 volts. The time division setting will be high when compared to most components you will be testing. A fuel injector, for example, can achieve over 100 volts with its inductive kick and the pulse width of an injector is measured in milliseconds, not seconds. Looking at the capture of a fuel injector below you can see the voltage scale is set to 100 volts, or 10 volts per division and the time scale is now at 10 milliseconds or 1 millisecond per division.

 

 

Adjust the voltage and time settings to view the entire pattern. If the pattern fills the entire screen and then some, slow down the sweep time. If the voltage rises off the top of the screen, raise the voltage scale. If the image appears very small and difficult to read, increase the sweep speed and decrease the voltage scale. Look at the capture below of a TPS with a total sweep time of 1 second or .1 seconds per division. With this setting, only the first part of the pattern can be seen. Therefore the sweep time needs to be decreased so that the scope will draw the pattern a little slower and fit the entire image on the screen.

 

 

Setting the voltage and sweep time are not enough; you must also learn how to set a trigger point. A trigger is the point at which the scope will begin to draw the graph on the screen. For example, a fuel injector is only on for a few milliseconds at a time leaving a lot of idle time that would read a steady flat line battery voltage. If you were to try and view a fuel injector pattern without setting a trigger point to begin the drawing, you would see a flat line 90% of the time and the actual injector pattern would flash by so fast it would be unreadable. To illustrate the trigger setting I will once again use a TPS signal shown in the image below.

 

 

The small yellow cross hair represents the trigger point. The trigger value in this capture is 1.69 volts and 301 milliseconds and the slope is set to a rising slope. With this setting , the scope monitors the voltage coming in on channel 1. As soon as the scope sees a voltage rise (rising slope) above 1.69 volts from any point below 1.69, it will begin to draw a graph at the 301-millisecond mark. (it also fills in what happened BEFORE the trigger point was reached in the 0 to 301 millisecond range.)

 

 

The scope will continue to graph the voltage until the screen is full and then it will STOP drawing until the trigger point is reached again. Remember, with a rising slope the voltage must rise above the trigger point from anywhere below the trigger point. Look at the next two images that have identical settings except for the trigger slope. The first one is set to a rising slope, the second image is set to a falling slope.

 

 

 

With a falling slope set on the trigger, the scope will begin to draw whenever the voltage falls below the trigger point from anywhere above the trigger point. Although the example scope I am using allows for the trigger to be moved vertically (voltage) as well as horizontally (time), many scopes will only allow voltage and slope settings for the trigger point. When using one of these, it is important to understand slope and how to properly adjust it to fully view a pattern on the display.

 

Thus far only one channel at a time has been shown in the example images. Oscilloscopes come in varying prices, sizes and with varying options. Some have only one channel, some have two and others have four or more. Each channel can display a different pattern so that more than one signal can be viewed at the same time. There are many times it will be necessary to view more than one signal when troubleshooting a drivability or electrical problem. Below is an example with all four channels turned on and in use.

 

 

 

Aliasing

 

Be careful when using a scope for high frequency patterns. If the sweep time is set too slow for the pattern being viewed, the scope can alias the image on the screen and lead to an incorrect diagnosis. The DSO is taking millions of samples per second. Obviously, the display cannot draw a point for ever sample taken. If the signal being sampled changes voltage at a very high frequency, the display will not be able to properly display the pattern if a low sweep time is selected. This can cause the pattern to appear erratic.

 

In the example below, the yellow trace is from a Chrysler crank sensor. When viewed at idle at 100ms or faster sweep time, the pattern appears to be normal.

 

 

But what happens if we slow the scope down to a 200ms sweep time?

 

 

 

If we slow the sweep time down even more, the repeating pattern of 4 becomes unrecognizable.

 

 

The image below is a good example of what happens when you select too slow of a sweep time. These samples are all of the same signals, just different sweep times set up on the scope. At 1 ms, the yellow square waves are rather large, and the green square wave fills the entire screen. Increase the sweep time to 2ms, and the square wave sizes cut in half and you can see twice as many on the screen...exactly what you would expect.

 

At 5 and 10 ms, the square waves shrink even more and more are visible on the screen. At 10 ms, there are 10 times as many yellow square waves visible as there were at 1 ms....easy enough...makes sense. At 20 ms, the yellow signal is not really usable. Even though there is little if any aliasing going on, the sweep time is just too slow to read the pattern. So what happens if we slow it down even more? At 50 ms, the yellow pattern returns to a semi normal looking square wave. This is NOT an accurate display of what is happening. Even though the screen shows about 20 square waves, we know that it SHOULD be closer to 500 square waves shown. Of course, with that many square waves in such a small space it would look like a solid block of yellow on the screen, but that is not what is displayed. The scope can't handle it, so the image "aliases".

 

 

When viewing an pattern on a scope, make sure the sweep is fast enough to prevent any aliasing. If a pattern appears erratic with signal points dropping out, decrease the sweep time. If the patterns stays the same, then there is a legitimate problem. If the pattern turns normal with a faster sweep, then the scope was aliasing.

 

With a little practice, you can quickly become proficient with setting up a scope to properly view patterns. Take the time to play with the various settings when viewing a signal and see how it affects the patterns displayed. Once you have mastered setting up the scope, you will be ready to begin learning to use this new tool to accurately diagnose drivability and electrical problems. Dont forget though, this tool is just like any other in that it will not do the work for you and is no better than the technician using it, so study and practice as time permits.

 

 

Rick Seagle

ASE Certified Master Technician

Advanced Engine Performance Specialist