Mastering electrical diagnostics with multimeter tips and tricks

In our January issue, Motor Age contributor Bernie Thompson presented very detailed information on the theory of and proper testing of voltage drops. Electrical problems such as voltage drops continue to be some of the most frequent and frustrating problems we face as automotive technicians today. This month we’ll be continuing our dive into electrical diagnostics with practical multimeter tips and tricks!

What’s your most cost-effective diagnostic tool — dollar for dollar?

The answer is the multimeter. The Digital Multi Meter (DMM), also referred to as a Digital Volt-Ohm Meter (DVOM) is to an automotive technician what a rifle is to a soldier. While even the most expensive DMM is only a fraction of your entire tool inventory investment, it often turns out to be the most valuable tool you have in your tool box if you know how to use it to its fullest potential. Most DMMs contain a digital voltmeter, digital ohmmeter and digital ammeter in one package. Many DMMs include a dedicated diode test and other useful features such as a digital thermometer display and tachometer function when coupled with the proper accessories. You most likely are comfortable with those terms like analog, digital and auto ranging but you might want to get on the road to mastering advanced DMM features such as 4 ½ digit mode, Peak-Hold/Relative Delta, Min-Max sampling rates and other practical multimeter tricks. To really know meters, we first take a quick look at why we have the kind of DMMs that we have today.

Figure 1 - If your shop does NOT have a quality analog voltmeter to connect to a charging system, you may want to hang on to an instrument like the one pictured here. Watching for a steady or flickering analog voltmeter does have it’s merits with some types of testing. However, when measuring solid state related circuits (modules, sensors, etc.) and circuits involving airbags, ALWAYS use a DMM rated with at least 10 MΩ impedance. This Simpson 260 meter has been in my dad’s tool box since the 1960’s. The same exact model is still in production today!

Meter evolution & input impedance

Without going back to the original galvanometers with moving coils, it’s not a bad idea to look back upon some equipment that still may be buried in some older technicians’ toolbox drawers – the analog multimeter. Like their modern digital counterparts, the quality and features of these instruments varied all over the place. Many of the lower to medium quality analog multimeters had internal impedances (resistance) that were low enough to load sensitive circuits like ECM 5-volt sensors to the point where a false reading occurred. Some could even damage the 0.45 voltage regulator inside the ECM that fed a bias voltage to the O2 sensor signal wire. A very cheap digital DMM has the potential to do the same thing. Not all analog meters are low impedance nor are all digital meters high impedance. For most circuits in the electronic systems you encounter on a vehicle, high input impedance meter will be essentially ‘invisible’ to the circuit and associated modules.

Shopping for a new meter?

If you are considering updating your current DMM, besides looking for a minimum of 10 MΩ input impedance, look for an instrument that is rugged and has a large easy to read display. A backlight is a must as is a kickstand to set the meter upright. High quality banana type universal leads are essential. I’ve owned meters that I loved but gave up on them due to unique non-universal leads that I hated!

Top 10 essential functions to look for in a DMM

Auto Off Function — keeps the meter’s battery from running down constantly
Auto Ranging Capabilities — reduces confusion when testing unfamiliar circuits
Min/Max/Average — for capturing intermittent problems
Hold Button — Allows user to quickly press a button to freeze whatever is on the display
Frequency and Duty Cycle — for pulsing DC square waves and AC sine waves
Fuses for Ammeter’s High and Low Current settings – prevents the meter from being ruined
Analog Bar Graph — for a visual quick look when testing
Diode test capabilities – to forward bias diodes while testing (superior to ohmmeter test)
Category III/1,000 Volt (or higher) rated — for working on hybrid / EV systems
True RMS rated — provides more accurate AC measurements

Many high-end DMMs sport lots of other features, which can be handy as well, but the 10 I’ve just listed are in my opinion are “must haves” for a daily use meter in the bay. Let’s go over a few of these features and cover some advanced tech tips as well.

Auto ranging or manual ranging

Math is most likely NOT the average tech’s favorite pastime. Even simple math is getting rusty to me sometimes due to my dependency on the calculator in my smart phone. Having to add digits in my head to determine if the reading on my meter is showing a good circuit/component or a bad one. Add to that the fact that when a manual ranging meter is used in diagnostics there will be lost time for most techs just trying to guess what range to start with (Figures 6 and 7).

Figure 6

Figure 7 Turn the manual meter to the range of volts, amps or ohms (Fig. 6) that your DUT (Device Under Test) is closest to. The problem is many times we don’t have a clue where to start unless the DUT is a 12-volt circuit, coil primary winding or some other obvious item. The chart in Fig 7 can help you get started if you’re not sure where to start. When in doubt, always go with the highest range and work your way down until you’re displaying a reading that’s applicable. Or better yet, update your meter to an auto ranging model that powers up in auto range mode and allows you to manually select a range if you so desire giving you the best of both worlds

Min/Max/Average

This feature is underutilized by many techs. Many DMMs will produce an audible beep when a new minimum or maximum measurement has hit. Most meters allow you to simply press the Min/Max button to begin. To use this extremely helpful function, set up your Device Under Test (DUT) or circuit and start measuring ohms, amps, volts, duty cycle, frequency just like always. If you suspect an intermittent connection in a circuit or module, ensure you have rock solid connections with your meter and go to town tapping, tugging, flexing, heating, cooling, road testing, etc. while the meter is in Min/Max mode. When you’re satisfied you’ve recreated all the gremlin producing maneuvers you can think of, go back to the DMM and press through the Min/Max button sequences to observe what the extremes were. On most DMMs with this feature, an average value will be one of the sequences in addition to the minimum and maximum value the meter recorded. Many DMMs on the market such as Fluke allow for the speed of the samples of values captured during Min/Max functions to be changed (Figures 8, 9). In most cases, faster is better.

Figure 8

Figure 9 Some DMMs have a fixed rate for capturing glitches. This Fluke model 87 III series allows the user to enter the MIN / MAX mode and select the speed for updating the MIN / MAX recording samples by pressing the button marked “PEAK MIN MAX” to change from the default of a 100 ms capture rate (Fig. 8) to a 1 ms capture rate. (Fig. 9) The “HOLD” button is featured on a variety of quality DMMs on the market. It is a very useful feature if you are trying to catch Intermittents. When your meter’s display changes to a value indicating an intermittent problem ‘may’ have occurred, simply press the “HOLD” button to freeze whatever is on the screen to allow you to scrutinize the value more thoroughly. “Button ology” varies greatly from DMM brand to brand and even within a single brand from model to model. Make sure you can access the user manual for your DMM to determine what advanced features it has and how them may be accessed to help you get more from your meter!

REL ∆ or Relative Delta

More and more high-end DMMs are sporting this feature. On some meters, the abbreviation “DEL” next to a triangle symbol is the button for this function (Figure 10). On other meters, the button is simply marked “ZERO.” The triangle symbol represents the Greek letter we call “Delta.” Sometimes Delta refers to a rate of change. In the case of DMMs, “rate of change” is the most descriptive term for this feature. Oftentimes techs spot this button, clip their leads together and turn the meter on to Ohms. They know that when they are measuring a small bit of resistance in their leads they can press that button and the ohmmeter will go from 0.2 ohms (for example) to 0.0 ohms. So it zeros the ohmmeter, right? Well, kind of.

Figure 10 - This meter is connect to a 5 volt reference circuit. Why is it showing 00.04 volts? Is it shorted to ground? The answer is no! Notice the triangle shape next to the meter’s reading? That’s the indication you’ve activated REL ∆ mode. Whatever reading your meter was displaying prior to entering this mode becomes the new “0”. Changes in my 5 volt ref circuit in this case are minor. The circuit is good.

Relative Delta actually takes whatever the meter was displaying and sets that as the “new zero” reading. Any changes in the measurement from that point forward then registers on the meter. This function is not only useful in watching for a change in the resistance of a good connection when you wiggle and tug on wiring, it can be used for virtually any other function of the meter. Say for example you are measuring a 5-volt reference circuit suspected of having an intermittent drop out. Connect your DMM between ref low and 5-volt ref with the meter set to DC volts. Now press the Relative Delta button. The meter will immediately display “0.” If any loss of voltage or ref low (a.k.a. signal ground) in the circuit occurs while you have the meter connected, the meter will show you that loss. It’s an alternative to looking at the meter and thinking “did this circuit just lose 0.2 volts or was it at 4.8 volts all along?” Relative Delta will tell you exactly how much was lost (or gained). This advanced DMM feature is picking up in popularity on newer meters but should be used only on circuits that are steady. Steady voltage, resistance, frequency, etc. but steady. You wouldn't use REL ∆ to sweep a TPS signal. The “0” you are looking to maintain will quickly begin to vary even though there is nothing wrong with the circuit!

More digits — 4½ digit mode

More digits can give you more resolution when it comes to very low values in a measurement. When measuring something like a power ground, it’s not that important whether the resistance in that ground circuit is 0.3 or 0.2 ohms. If that ground is for a radio antenna, CAN bus shield wire, etc., the hair splitting can matter. Meters vary in how they allow you to enter this higher resolution mode but the ones I’ve had with the feature have a button marked “4½ Digits” that you hold down (Figure 11) while you turn the meter from off to your desired measurement category. Instead of one decimal point to the right of the whole number (ohms) there are now two. Now that RFI shield ground for that new Radar/Smart camera you’re trying to diagnose can been accurately measured down to the hundredth of an ohm. This type of feature activation is called a “Power Up” setting and is used in meters ranging from Snap On to OTC to Fluke. It’s the new “OFF and Warmer” trick or if you’re with the younger generation, a “CTRL/ALT/DEL” for meters!

Figure 11 - Do you notice another digit to the right of the decimal point? Meters like the one pictured have a feature called 4 ½ digit mode to give you the resolution needed to measure exacting circuits where a few hundredths of an ohm can make a difference.

These are just a few tricks and tips on mastering multimeters. Do a little research on the multimeter in your toolbox and start using the meter tricks that help you with your diagnostics!