Every month, we produce short (15-30 minute) videos on a variety of technical topics. Our June video focused on basic uses of the digital storage oscilloscope (DSO) in an attempt to encourage those of you who have one gathering dust in a corner of your shop to break it out and use it. Judging by the response, many of you are doing just that and have asked us for more scope-related information. Rest assured, we will be adding more material to our video library soon.
In the meantime, I thought it appropriate to offer a print primer on scopes and their use as a diagnostic tool. I’m no power user, just an average guy like the rest of you, and if I can make use of this time saving diagnostic tool, I know you can too.
Let’s start with some basic stuff.
Why Use a Scope?
The very first scope I ever used was one of those big box engine analyzers you might recall from the mid-1970s. It was an analog scope, using a cathode-ray tube display and the neon green trace you saw on its screen was in real-time with no capacity to store the information for later viewing or analysis. It typically was used to check secondary ignition patterns and had some neat features. You could show all eight cylinders at the same time in series (the parade pattern), stack them on top of one another (the“raster pattern), or look at one cylinder at a time. This made spotting anomalies in the patterns easy to do and techs with even a rudimentary amount of training could pick out problems in the ignition system after a few minutes of viewing.
In the early 1980s, the digital storage oscilloscope (DSO) was born. This new style of scope used an analog to digital converter (ADC) to convert the real-time analog input signal to a digital form that could be stored indefinitely with no degradation in the quality of the signal attributes and later displayed on a variety of screens. Fluke was (and still is) a popular choice for early automotive techs that began experimenting with the use of these scopes as a diagnostic tool.
Some of the distinct advantages offered by the early DSOs were the ability to freeze a capture on the screen, to store that image in a permanent library either in the tool or on a personal PC, and the ability to share that image with other techs. And with the birth of the International Automotive Technicians Forum on something new called the “Internet”, experimenters were able to share their notes with others around the world and not just with those in their hometown.
The majority of the DSOs offered for sale as automotive tools today are more than capable enough of capturing the signals you want and display them with a resolution that almost mirrors on the older analog displays. And with the accessories that have been adapted and developed for automotive diagnostics, our scopes can rise to nearly any diagnostic challenge you care to throw at it. Want to monitor current in a circuit over time, looking for that intermittent cause of the battery drain you’ve been trying to nail down for the last month? A scope can do that. Want to be sure an input sensor signal is reliable? A scope can do that. Want to see if transmission line pressure changes in synch with the variable commands received by the linear solenoid controlling it? A scope can do that. All that, and a whole lot more.
Basic Settings
Today’s DSOs are available in a variety of platforms: handheld independent units, as an accessory module built in to your scan tool platform, and as PC-based platforms that use a USB interface. No matter the platform, they all perform a similar task and differ primarily in how the basic settings are adjusted.
Scopes take an input signal and trace that signal over time, producing a graph of that changing signal that allows you to see much more than you can using a multimeter or scan tool. There are two axis, one vertical and one horizontal, the DSO uses to plot that signal for viewing, and you can adjust both of them to suit the type of signal you are trying to view. Each axis is divided into units, the value of which is determined by the user. Just like the graphs we made in junior high math class.
For the sake of remaining on the same page, let’s start using the lingo of the scope user. When two scope users are comparing notes, they refer to the signal they’ve established on the screen as a capture. If that capture is displayed to show just the portion relevant to the system we are monitoring, the screen view is called a waveform or pattern. For example, I can capture several seconds of a cam sensor signal on my screen. A complete cycle of the cam sensor signal displayed, though, would be the sensor’s waveform.
The vertical axis is the voltage scale and the horizontal axis is the time scale, each having divisions of measurement. The total time displayed on the screen is referred to as a sweep. For example, if you’ve set the time divisions to one second each and there are ten divisions on your scope’s screen, scope users would call that a single ten-second sweep. Your scope’s settings control for both of the axis may be based on divisions or sweep or both.
The voltage scale typically should be set to allow your captured signal to take up most, but not all, of the screen. If you’re measuring a Throttle Position Sensor (TPS) signal, for example, with a range of 0.0 to 5.0 volts, adjust your scope to 0.10 volts per division (with the typical 10 divisions that gives you a range of 0 to 10 volts) or choose the sweep that best fits (-10 to +10 volts on my Pico).
The time scale is where the real power of a scope comes into play. With many of the newer DSOs, it is possible to set up sweeps that last minutes, even hours. The user then has the option to use the Zoom features on the tool to expand the capture and look at the same data in microseconds with almost no loss in resolution. Personally, I’ve never come close to exceeding the capabilities of a scope and I tend to prefer to adjust my time settings to allow me to view the signal I’m trying to capture in real-time. I’ll start with a sweep larger than I expect I’ll need and then make adjustments to the time divisions until the waveform is set the way I want.
Another setting you’ll need to master on your scope is the trigger setting. The trigger is the set point you establish that tells the scope when to start tracing the waveform on the screen. Some adjustments you can make to your trigger point include the time it starts on the screen, the voltage level that needs to be achieved, and whether that voltage level should be detected as the signal voltage rises or falls. Using a trigger allows you to set up the scope to capture a signal anomaly (scope users call that a glitch) or stabilize a running pattern on the screen for easier viewing and comparison.
Blow the Dust Off
There’s no better way to learn how to use a scope than to actually drag it out of your toolbox and use it on every “known good” car that comes into your bay. In our video, we show you how to use it to conduct a battery/starting/charging system test and that ‘s a good thing for a few reasons. One, it gives you the motivation to turn it on every morning when you come into work so it’s ready when you are. Two, performing this simple test will be of real value to both you and your customer so it’s not a waste of time. And three, it provides you with practice in setting up and using the controls on your particular scope. For more information on this test, check out the June video in the AutoPro Workshop or the Motor Age YouTube channel. You can also read a great article by Albin Moore on the topic online at MotorAge.com.
And while we aren’t going to go into that test in detail here, I do want you to start learning your scope by connecting channel one to the battery the same way you would if you were going to measure its voltage using your multimeter (DMM).
Wait a minute? What’s a channel?
Another powerful aspect of the scope is its ability to trace more than one signal at a time. Each input is a channel, and there are scopes offering anywhere from one to as many as eight channels.
For a long time, I relied on two but I must admit I like the fact that I have four now. The advantage is being able to compare apples to apples by monitoring multiple, but similar, signals at the same time. An ignition system immediately comes to mind. It can also be an advantage when you want to compare one signal to another or see if they are in synch as would be the case with a Crankshaft Position Sensor (CKP) and a Camshaft Position Sensor (CMP). A third use is for “if/then” testing. For example, if the Engine Control Module (ECM) completes the ground path for the fuel pump relay, does current flow through the pump circuit?
I’m sure you can see more diagnostic possibilities.
Back to the battery. Most voltage measurements you’ll make with your scope will have you connect in the same way you would if you had chosen to use your DMM instead. In fact, there will be cases where the DMM is enough to get the job done but if you make a habit of using your scope instead (at least during the first few months), you’ll see where using the scope allowed you to see something that you would have missed with the meter alone.
Set your voltage divisions based on the test you are making. The battery voltage shouldn’t exceed 20 volts, should it? Or go less than 0.0 volts? Set your scope accordingly.
Adjusting the time divisions is next. Adjust your scope to record a twenty second sweep. If you have to set your scope’s time scale by divisions, simply divide the number of divisions into twenty. For example, my Pico has ten divisions, so I’ll select a time base that will be close to two seconds each from its dropdown menu. The scope module built into the Snap-on Verus®, on the other hand, allows you to select the time scale base by total sweep time. As a quick side note, both have accessory menus that change the scaling on the voltage scale to correspond with a variety of accessories like pressure transducers and current clamps. If you’re scope doesn’t have this feature, or you’re using an accessory that doesn’t match up with the scale choice, just remember that all of the available accessories are converting their inputs to a voltage signal your scope can read and there should be a chart offered with that accessory telling the end user what the “xxxx” (pressure, temperature, current) to voltage relationship is.
For this battery test, I want to set the trigger to start capturing the battery voltage input after I’ve reached in and started the car and I want it to stop after the single sweep is completed. I do this on the Pico by selecting a single trigger starting one time division in from the beginning of the screen. Now to set the voltage level of the trigger.
I want the scope to begin its capture just after I turn the key on but before I actually start the car. What happens to battery voltage when you turn the key on? It drops, right? So I’m going to choose a voltage mark that is less than OCV but more than what I expect the voltage to drop to on start-up. Then I’m going to use the scope’s trigger settings to tell the scope to look at OCV and start its trace when it sees that level drop to the marker I’ve established.
Make sense?
Now I’ll start the scope and then start the car. Once enough time has passed for the scope to complete it’s capture, I’ll shut the car off and take a look at the pattern on the screen. From this first capture, I can check batter OCV, system in-rush voltage and charging system voltage. Do you have a similar pattern on your scope?
There is more to see!
Using the Zoom function, I’m going to take a closer look at the charging system voltage portion of the pattern. By zooming in from a two second per division time base to a ten millisecond per division time base, I’ll be able to see the condition of the alternator’s diodes. This waveform is the A/C ripple that you’ve probably measured with electronic testers in the past. If you can’t zoom in with your scope, simply readjust your time settings.
Add a high current amp clamp to a second channel to measure starter current draw and charging amperage. You’ll get good practice with your scope while providing a valuable service to your customer.
Additional Daily Tests
Any test you make with your digital voltmeter can be made with your scope. Lead placement is the same for any voltage test you want to make. The power of the scope is the ability to see the changes in voltage as the component operates in a way you’ll never see with a meter alone. This takes voltage drop testing to a whole new level. Imagine looking at the voltage drop over a groundside module driver? Or being able to actually measure how long it takes for a driver to turn on or off?
CAUTION: Be sure to know the voltage input limits of your scope! Some circuits (injector and primary ignition, for example) can produce flyback voltages that can harm your tool.
With accessories, you can expand your scope’s (and your own!) diagnostic capabilities. Check the pressure drop across the fuel injectors with a pressure transducer. Inspect the electrical health of a fuel pump in seconds with a low amp clamp. The uses of a scope are only limited by the imagination of its user. Yes, there is a learning curve to becoming proficient with it. Yes, it will take some investment of time and training.
Didn’t every skill you’ve mastered so far?
Subscribe to Motor Age and receive articles like this every month…absolutely free. Click here
