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The DVOM versus the DSO

Making the right choice can save you time and headache.
Tuesday, May 28, 2013 - 13:13
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Almost every automotive or truck technician has worked in a flat rate pay environment at one time or another. I am no exception. The idea behind a flat rate pay plan incites us to be the fastest we can be. On that plan, the faster we get the job done, the more money we stand to make. It is this type of remuneration that prompts us to constantly think ahead to what our next move is (while still performing in the present) or suffer the consequences of lower pay for production that’s less than optimal. Increasing our income is one good incentive for us to constantly improve our skills so that there is a minimal amount of time wasted when we perform repairs.

Over time, smart technicians learn lots of ways how to save time. For example, we might choose one tool over another, like using an air ratchet instead of a non-powered tool, to decrease the time it takes us to do the job. It is for the very same reason that I’ll reach for a Digital Storage Oscilloscope (DSO) instead of a Digital Volt & Ohm Meter (DVOM) when I have to perform certain electrical diagnostic routines. In those cases, the DSO delivers more information to me in a shorter period of time than a DVOM will, thereby enabling me to decrease my diagnostic time and to increase my diagnostic accuracy and productivity.

A DVOM can tell you if a WSS is outputting a signal, but it would never catch this defective signal (tone ring is damaged). This defect can be the cause of unwanted ABS activation.

Now, I won’t argue there are times when a DVOM is all I need, and that using a scope during those instances would be more time consuming, be more cumbersome, etc. I admit, there are tasks where employing the use of a scope would be like trying to kill mosquitoes with a cannon. What I will insist though is that I save time when I use a DSO if for example, I need to use more than just one part of Ohm’s law to find a problem, or need to analyze a particular part of a circuit in detail, or want to compare multiple signals at the same time (the list can go on). Let’s explore some examples of how a DSO might save us some time.

First, consider a simple one-speed radiator cooling fan circuit that’s comprised of a PCM-controlled relay, the fan itself and of course all the wiring that connects the components. The relay has four electrical connections: two for the control side and two for the load side. When the relay is activated, what tests can we perform?

If we were to choose a DVOM, we could read the average of both the control-side voltages (Supply & Ground) and both of the load-side voltages (Supply & Feed) on each “leg” of the relay and we could read the average amperage flowing through each side of the relay if we plug in a current probe. Notice I said the average of those readings; that is important to remember. We can also perform voltage drop tests on all four circuits one at a time. If instead, we chose to use a scope on that radiator fan circuit, we could do all of the same tests. Naturally, the time it would take to perform those tests, whether we used a DVOM or a DSO, would depend on many factors but would be fairly equal.

Now here’s the exciting part (at least to me it is). If we did decide to use a DSO, we could perform even more tests and not take much more time to do them. For example, if while we are looking at the average amperage of the cooling fan motor and then changed some settings, we could zoom in to inspect the integrity of the armature windings in the motor itself. If one or more of the armature windings was shorted to ground, shorted to itself or if it had an open circuit, we would be able to see that on the display. But any of those situations could be averaged into the DVOM readout, and you might never know they existed. We can say averaging the readings will mask the problem.

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