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Scoping out engine sensors

A guide to putting your scope to work for you.
Wednesday, August 27, 2014 - 07:00
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“How do I know where to put my test leads and what settings to use on the scope?’ That’s the most common question that comes up while I am teaching a scope class. Digital Storage Oscilloscopes (DSOs) are not much different than what you have used for years to test electrical circuits. If you can use a test light, logic probe or multimeter, you can use a DSO. Scopes take accurate measurements and provide a picture that really is worth a thousand words, providing information that the other testers miss.

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Remember the graphs you had to draw in school? That is essentially how DSOs work. They take millions of samples and plot them on an X and Y axis, plotting voltage over time. All DSOs measure voltage over time, whether they are a handheld or PC-based scope. Some DSOs have their graphs split up into eight grids of voltage and 10 grids of time, while other scopes might have a 10-by-10 grid display. It really does not matter, because you have control over the waveform. Some scopes allow you to zoom in and out, allowing the user to make the waveform as big or small as they like. If your scope does not have this feature, just adjust the voltage and time settings on your scope to achieve the same results.

The answer is different depending on what component we are going to test. You connect a scope the same way you would connect a multimeter to test for voltage. The difference is rather than looking at voltage numbers you are looking at a waveform.  

Setting Up For Success
And just like your meter, you first have to pick between AC and DC voltage. AC should be selected to measure any circuit or component producing an AC voltage signal. Some examples include some speed sensors (vehicle and/or wheel), some cam and crank sensors and, of course, when looking for excessive AC ripple in the charging system. Selecting AC also plays a role in some DC tests. By selecting AC, you remove the DC component of the signal and can focus on the changes in amplitude of the waveform itself. A great example is when testing relative compression using a current clamp.

But for most testing, you’ll be setting your scope to read DC voltage. And you’ll be adjusting the voltage scaling on the meter’s X axis in the same way you need to select your measurement scaling on your multimeter.

The first step to take when using a scope to take a simple voltage measurement is to make sure we have a good engine ground. Connect the black (negative) lead of the scope to a good ground (don’t forget to scrape the alligator clip to the ground), followed by connecting the scope positive lead to the battery positive post. With your scope set to read DC volts, you should see a straight line being displayed as the waveform on the scope equal to the battery’s Open Circuit Voltage (OCV).

Why don’t I suggest connecting the ground of the scope to the battery? Many times there is corrosion, interference or noise from the charging system. Another reason is that many lead sets only have about a six-inch ground lead that is connected to the main lead set and that’s not allowing the length needed to always ground to the battery. Use a ground close to the sensor or actuator. You can always verify the integrity of the ground path back to the battery by performing a voltage drop test with your multimeter or the scope.

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