Digital MultiMeters: Navigating the Waters of Diagnostics with a DMM

Digital MultiMeters: Navigating the Waters of Diagnostics with a DMM

Diagnostic tools as a category have evolved more quickly and completely than almost any other group of tools and equipment. Driven by increased vehicle electronic controls, this group of tools easily represents one of the largest investments for you as a technician, and they account for a large chunk of aftermarket tool sales dollars every year. There are several layers or classes of tools for diagnosing and repairing auto electrical and electronics systems. I want to talk a little bit about these different layers and some decision points when buying the tools in each group.

In 1492, Christopher Columbus set out to discover a new trade route. He was equipped with three small ships, no real navigational equipment or maps, and the constant threat of disease, death, pirates and mayhem. His directions were simple, Queen Isabella promised him great wealth, respect and power if he succeeded. Of course if he failed, he risked imprisonment, torture and death. So to recap, he was given a nearly impossible task, with a high chance of failure and few tools to accomplish the assignment.

This sounds a little like modern-day diagnostics and driveability troubleshooting. As a professional technician, you are asked on a daily basis to diagnose and repair incredibly complex problems. While there are some similarities with your day-to-day jobs and the plight of Columbus, there are some big differences too.

There are some great diagnostic tools and equipment available to you today, to make your job a little less impossible. This article is about those tools. I will attempt to cover some of the major tools, some applications and some things to think about when shopping for these tools.

Diagnostic tools as a category have evolved more quickly and completely than almost any other group of tools and equipment. Driven by increased vehicle electronic controls, this group of tools easily represents one of the largest investments for you as a technician, and they account for a large chunk of aftermarket tool sales dollars every year.

There are several layers or classes of tools for diagnosing and repairing auto electrical and electronics systems. I want to talk a little bit about these different layers and some decision points when buying the tools in each group. Once upon a time, the one and only tool for a technician (back then the term was “mechanic”) was the time-tested and trusted DMM. The Diagnostic Multimeter was the go-to tool for all kinds of tests. Some of you are probably even old enough to remember learning how to interpret subtle needle fluctuations on analog meters. For those of you who have never worked with an analog meter, you are missing something!

Anyway, over the course of the last 20 years, the multimeter has gone from hero to zero and back at least a couple of times. About the same time that the last cars with points rolled off the line in Detroit, the multimeter starting spending more time in the box and less time on the fender. For all kinds of reasons, both good and bad, we all fell under the spell that demanded that we could only fix a car with a “scan tool.”

Now don’t get me wrong, I think that scan tools are indispensable for repairing many cars today. My problem is that we turned our back on one of the most versatile and capable tools available. Add to that the fact that DMMs are usually a fraction of the cost of full-blown scan tools, and it might just have you taking a second look at these work horses.

Just like that car commercial a few years ago, today’s meters … “aren’t your father’s multimeters!” Many of the meters available today have a host of functions incorporated that just a few years ago would have only been available on a scan tool costing many times the price.

Routes to Explore
When making a buying decision about a DMM, it is important to identify what kinds of things you want to be able to do with the tool. Before stepping on that tool truck or picking up a catalog, you need to be able to answer a bunch of questions about the meter you want.

Let’s start with automatic (auto ranging) versus manual ranging. The auto ranging meter has become more and more popular and is almost the default value for most automotive DMMs. While generally speaking, this is a good choice, there are situations, where you need to be able to set range on the meter. When working with milli-volts and extremely small values, you can get caught by an auto-ranging meter that picks the “wrong” range relative to what you are looking for. There have been some really great discussions on this recently out in the forums and blogs. Do some research to see what the discussion surrounding this issue is.

Current is the next thing to talk about. Do you want to be able to use the meter for higher amperage applications? If the answer is yes, look for a meter with a max range of at least 15a, if not 20a.

You need to know that the meter you are going to use is computer safe. The general rule of thumb is a 10 Meg Ohm rating. This is generally a low enough rating to be safe around computer systems and delicate electronics. This is a must-have; if the meter isn’t rated as such, I would pass on it.

DC voltage is a test range that varies with meters. Almost all meters are rated to 600V DC voltage. This does not meet the 1,000V CATIII rating, which has become more important with the advent of hybrid vehicles. Most suppliers are moving toward offering at least the CATIII rating.

Two areas of evaluating a meter are number of test ranges and number of test functions. While there is no standard for these, as our hero Tim Allen would say: “more power, arghhh, arghhh, arghhh!” Seriously, the more test functions and ranges, the more control you will have for all kinds of circuits and tests.

Two tests that are nice if you work on older equipment, or DC motors, are dwell and RPM. Both of these test functions will allow you to tune and diagnose small motors as well as check timing on older equipment. You may not use these functions a lot, but they are still nice to have.

Audible alarms for continuity is almost a must-have. Any meter that doesn’t have this should be looked at hard. This function has become so standard that a meter without is guilt by absence. An audible alarm allows you to concentrate on where the test probes are when searching for a ground or a circuit. You don’t have to be looking at the meter to know when you make the right connection, because the meter will beep.

Temperature is another one of those functions that, while not a deal breaker, is nice to have. With the right probe attachment, you can use the DMM to test A/C vent temps, engine oil temps, tire temps, the list goes on and on. The good news is that most mid-grade and higher meters include this feature.

Diode test. This is a nice feature that has many applications in the charging system, as well as motor controls and a host of other circuits. Again, this has become ­almost standard on all meters so no worries about searching for this function.
Graphing is a feature that until the last five years was either not available on a DMM or it was prohibitively expensive. This feature, while not universal, is more available and affordable. This is a very nice feature and I would look for it when making a buying decision.

Frequency is something that I think is a must-have. So many tests now are looking for a frequency range, that without this test, you are going to be limited in what you can do.

Pulse (MS Pulse Width) this is a test that is pretty important and speaks volumes about the meter’s speed and processor(s). This is still a relatively uncommon capability, but one you should decide whether you need it when shopping for a meter.

Be picky about which accessories you choose. Let’s start with the test leads. Check them for length and construction materials (heat-resistant cable covers, good hardware). Next, what adapters and/or probes are included? RPM probe? Temp probes? Clamps? Piercing adapters? Does the unit come with the batteries it needs?

Next is meter protection. I’m talking about a boot or protective over-cover. These are cool-looking and all, but they also serve a purpose. This is to protect the tool when it takes a header off the fender or off the workbench. Without the rubber casing, you can end up with a smashed case or worse. Also in the family of cosmetics is the carrying case. You want a good carrying case to help the tool when it is being stored and transported, and a good case also helps keep the accessories organized.

So, here we are some 518 years later, you have been commissioned to find that fault. The difference is, that you now have the knowledge to choose tools wisely to help you on your journey. If you choose wisely, you will be rewarded handsomely! Now get out there and start exploring Multimeters!  

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