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Why do I need a DMM and a DSO?

There are little tricks to mastering both, but by having both tools in your lineup, there are problems you can solve quickly.
Friday, January 3, 2014 - 08:00
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The Digital Multimeter (DMM) and Digital Storage Oscilloscope (DSO) both can be used to measure voltage, but the way each displays that voltage reading, and what you can learn from it, is very different. If you can use a multimeter, you can use a scope, so hold on and let’s see where you might want to use the scope over the meter during your troubleshooting.

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The computer doesn’t know that the sensor signal dropped out for a millisecond or two. It only knows it didn’t see what it needed to see.

Try taking your DMM and your DSO and connecting both of them to the battery post until you see a straight line of voltage. Now you get to see the number from the meter and the straight line of voltage on the scope, so what’s the difference? While both are use digital processors to sample the voltage being measured, the meter shows only the average of the samples while the scope shows voltage over time. Additionally, most DMMs sample the input voltage at a significantly lower rate than even the cheapest scopes do. Today, many handheld and PC-based scopes are checking the input signal millions of times per second. That means that even momentary losses of the voltage signal will be seen and displayed in a form that is easy for the user to recognize.

Scopes measure time horizontally and voltage vertically. So you might be wondering what good does this do you, and why would you want to use a scope. Hang in there and see where it would be beneficial using a scope. Let’s take a look at a common sensor that produces a square wave like a Hall Effect sensor. Now you will get to see the difference between the DMM and DSO first hand. Take a look at the square wave voltage on the voltmeter. Do you notice that the voltage is about 2.5 volts? Now take a look at the scope reading that you measured from the zero line noting that each line is 2 volts per division going up from zero. Now you are looking at the true voltage of the sensor that is being displayed.

The other difference between the scope and meter is clearer when we are trying to find out why the engine is dropping out. Since a meter can only update 40 samples a second depending on the meter while a scope can typical sample 25 million samples a second. When you are trying to locate a drop out in a sensor you need a tool that is fast enough. So with all the theory out of the way let’s take a look at the example. The problem here is not that hard to see if you look at the cam signal to the left that is good and the one to the right that is bad. The top of the waveform was dropping down in the middle of the square wave causing a drivability problem. The fix for this vehicle was to replace the defective sensor.  

Any scope platform (like the one in the Snap-on Modis shown here) can be used to perform this relative compression test right at the DLC.

Relative Compression from the Driver’s Seat
This easy concept came to me as I was explaining relative compression to a class that I was teaching in Massachusetts. I was showing and explaining relative compression with the scope connected to the battery B+ and Neg - post while cranking the engine over. I continued on to explain that this test can also be performed using an amp clamp attached to one of the battery cables as well. Then one of my students asked the following question. 

“Since you said this was testing a voltage drop that we were observing with the scope, how about if we connected the scope directly to the Diagnostic Link Connector (DLC) using the breakout box you showed us?” I thought about it and responded back to Jeffery Allan Clark, the tech who asked the question that I believed he was on to something. I had never really thought about performing the test using a DLC breakout box. In fact, I have never seen anything about this in print or knew of anyone performing this test. I always say in all my classes and seminars that there is no such thing as a stupid question, and everyone knows something that we don't know yet. If we share our ideas we all can learn, and we did.

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