The Digital Multimeter (DMM), often referred to as the digital volt-ohm meter (DVOM), is one of the most common tools found in a technician’s toolbox. Using a DMM, alongside an accurate wiring schematic, and just about any electrical issue can be solved. This, of course, assumes that the technician is comfortable with electrical diagnostics, understands the measurement results, and respects the limitations of the DMM. This article focuses on common usages, limitations, and some tips for using a DMM. In Part 1 of this article, I will focus on the Voltmeter function.
PARALLEL INSTEAD OF SERIES
Ideally, voltage checks should be made in parallel to the circuit, not in series. This means that the circuit is connected and intact, and the technician is backprobing the connection to gather the measurement. In this fashion, the circuit is trying to operate, and the DMM is sampling the voltage at specific points in the circuit. Refer to Figure 1, which shows a meter monitoring a circuit by connecting in parallel.
A less accurate and sometimes misleading method involves disconnecting the circuit and checking for the presence of voltage by placing the meter in series with the circuit. This is also referred to as an “open circuit voltage (OCV)” measurement. We’re all guilty of doing OCV measurements with a DMM, and depending on the circuit tested, there’s really nothing wrong with this method, so long as you understand the limitations of the results – more on that later in the article. At times, even manufacturers have the technician measure OCV. For example, when following the diagnostics for DTC P0717: Input Speed Sensor No Signal on a 10L-equipped vehicle, the service routine has the technician disconnect the transmission harness and check for 9V between the sensor power circuit and ground (Figure 2). That process places the meter in series, and for this type of circuit, that’s perfectly acceptable. The speed sensor is such a low-power, low-current device that checking for power by placing the meter in series on this type of circuit is likely fine. Why does it make a difference? Isn’t a DMM just an expensive test light that displays a value?
You might have heard DMMs referred to as Megaohm meters, which means that they have over 1 million ohms of resistance across the leads when set in the voltage scale. They are also referred to as “high impedance” meters, which basically means the meter offers high resistance to AC voltage. Most meters have an internal resistance of 10 megaohms or greater in the DC voltage scale, as shown in Figure 3, which includes various meters and their internal resistance. Having this high of resistance is great when checking sensor and communication circuits that have very little current flow, because the high meter resistance prevents the meter from influencing the circuit voltage. If the meter resistance was low and you placed the meter across a circuit, you are essentially making a parallel path for current to follow (one path through the load and one path through the meter), which could affect the circuit voltage and give you a misleading result. This was a big deal decades ago when technicians were first testing oxygen sensors. The oxygen sensor produces such a low current that when checking its output with a low-resistance analog meter, it would indicate low voltage output. This was because a low-resistance meter completed a parallel path to ground, which pulled the low-output oxygen sensor voltage to ground. A 10M ohm DMM places virtually no load on the circuit because of its very high internal resistance. This is great for checking sensitive electrical circuits, but it can be a drawback when performing OCV checks because there’s practically no current flowing through the circuit.
Now, going back to the “don’t place the meter in series” dilemma – since this meter has 10M ohms of resistance, it doesn’t take much of a connection for the meter to read a voltage, even when the circuit itself has unwanted high resistance. Refer to Figure 4, which shows me touching a 9V battery and measuring the voltage available at my fingertips. This is placing the meter in series, and it appears that almost all 9V is available! 
Obviously, my body doesn’t have low enough resistance to deliver enough current to operate anything, so this lesson should show you that you’re much better off doing a voltage drop test on an intact circuit that’s trying to operate. This is very important for circuits that require current flow to operate, such as solenoids, injectors, lights, and actuators. All it takes is continuity, even through a poor connection, to pass an OCV test, but when performing a voltage drop check on a complete circuit, any excessive resistance will show up as an unwanted voltage drop. In summary, when possible, use your voltmeter in parallel to a circuit and not in series.
DIFFERENCE BETWEEN THE LEADS
When measuring voltage, the DMMs display the difference between the leads. The black lead doesn’t have to be on the ground, and the red lead doesn’t have to be on a powered circuit. For example, in Figure 5, the solenoid circuit that is ground-controlled by a transistor in the TCM will have battery voltage on the complete circuit, even on the ground wires, when the transistor is commanded “off.” If the technician is checking the ground circuit by placing one meter lead at the solenoid ground and the other lead at the computer, the meter will read close to zero volts. 
This is because the positive lead is measuring battery voltage, and the negative lead is also measuring battery voltage, so the meter display will read close to zero volts, which is the difference between the two leads. Bonus tip: In this example, if you see a little bit of voltage displayed, even millivolts, there’s likely current flowing in the circuit, because the meter is measuring the normal voltage drop in the conductor. If it reads all zeros, there’s no power and/or no current flowing in the circuit.
If there was a break in the ground wire (Figure 6) and the transistor was commanded “on,” the meter would read battery voltage, because there would now be a difference between the positive and negative leads equal to battery voltage. 
This concept might seem simple or trivial to some, but once again, it’s all about gaining an understanding of the circuit and the functions and limitations of the DMM.
DMMS DISPLAY AN AVERAGE
The actual voltage displayed on the DMM is an average value. This is important to realize when diagnosing Pulse Width Modulated (PWM) circuits and circuits that might have noise present. PWM circuits will appear as a lower voltage depending on the duty cycle (also referred to as the “on” time) and the overall pulsed voltage values. 
There’s no doubt that a digital storage oscilloscope (DSO) does a better job at evaluating PWM or noisy circuits, but not all technicians have a DSO. The DMM can still evaluate a PWM circuit – so long as you understand the DMM’s limitations.
For example, for a simple EVAP purge control circuit, the PCM pulse width modulates the ground path for the solenoid and changes the duty cycle to alter the amount of current flowing through the solenoid (Figure 7). This solenoid will open and close to allow engine vacuum to draw fuel vapor from the fuel tank and charcoal canister into the engine’s intake, reducing hydrocarbon emissions. As you can see in Figure 8, the DMM shows 3.82 volts, but the solenoid is pulsed between zero and 12 volts (with a 50-volt inductive kick when the transistor opens). 
The meter averages this voltage pulse and displays the result. It’s interesting to note that not all DMMs average the voltage the same way. For example, refer to Figure 9, which shows common DMMs measuring the same purge solenoid on a GM 5.3L-equipped truck. Notice how each DMM displayed a different average voltage. This might be due to the inductive kick and how the meter handles it in the calculation. Regardless, this characteristic makes measuring a PWM circuit with a DMM less reliable, so we need to consider this as a limitation of the DMM.
Also, keep in mind the current/voltage relationship of ground-controlled solenoids. When measuring 12V on the ground side of the solenoid (like in the EVAP solenoid examples), that measurement result indicates that the solenoid is electrically “OFF.” 
This is because when the transistor turns OFF (no current flow), the voltage will build in the complete circuit up to the point of the open (the transistor in the PCM), and therefore the DMM will read 12V if measuring between the ground wire and battery ground. When the transistor turns ON, the voltage will drop through the solenoid, and the measured voltage on the ground side of the circuit will be close to zero.
If you want to ensure the circuit is switching between zero and 12V, many meters offer a recording or min/max function. Figure 10 shows the results of an EVAP vent solenoid displaying the maximum (top image) and minimum (bottom image) of the pulsing solenoid. The min/max feature of the DMM is very handy, but be aware that many DMMs record at a low sample rate and might not show the whole picture. For example, this is the same EVAP solenoid example used earlier, which shows on the scope with a max of 50V and a min of zero volts, but, as you can see with the DMMs, the max is way off, and except for the Pico and the Fluke, the minimum voltage doesn’t come anywhere close to zero volts.
If the min/max feature is important to you, look for meters that offer “PEAK” recording, because they sample at a higher frequency. Figure 11 shows the PEAK min/max recording feature of the Pico and Fluke, which were the only meters I had on hand with a PEAK setting. The Fluke 87 did a great job of picking up the actual min and max circuit values. The Pico didn’t do well with the max voltage in this scenario, but it did fine with the minimum voltage. Once again, a scope is really the tool of choice when evaluating PWM circuits, and the variability in these experiments really proves that statement.
On some circuits, you pretty much need a scope to fully evaluate the circuit operation. Let’s take, for example, the ZF 9HP Dog Clutch A Position Sensor found in a 2022 Nissan Pathfinder. This circuit uses a Hall-Effect sensor to output a low-voltage PWM signal that indicates the position of the Dog Clutch A found on the transmission’s input shaft. Refer to Figure 12, which shows a scope pattern displaying the voltage and duty cycle of Dog Clutch A and a DMM showing the voltage when it’s engaged, and compare it to the image with a scope displaying the voltage and duty cycle with Dog Clutch A disengaged (Figure 13). On the scope, you can see the voltage changed between approximately 0.6V and 1V, and the duty cycle changed from 30% (engaged) to 70% (disengaged). 
The DMM couldn’t detect the duty cycle change, likely because the signal voltage never dropped close to zero volts. It did, however, show an average voltage change between 0.768V when the dog clutch was engaged to 0.919V when the dog clutch was disengaged. Without the use of a scope, it would be difficult to determine if this is an acceptable signal and measurement. Many speed and position sensors use advanced strategies that will be hard to fully diagnose with a DMM. In these situations, a scope would be the tool of choice to fully evaluate these advanced sensor circuits.
CONCLUSION:
Key points to remember include:
- The high resistance of the DMM makes parallel measurements more practical and accurate when diagnosing electrical circuits.
- DMMs display the difference between the leads.
- With an open circuit, voltage in the circuit builds to the point of the open.
- DMMs average the voltage values.
- The Min/Max results can vary greatly between DMMs.
- Use meters that offer PEAK Min/Max for greater accuracy when testing switching circuits.
There’s more to learn when using a DMM in the voltage setting, such as frequency (Hz) and additional duty cycle measurements. Hopefully, this article gets you thinking about your meter, its capabilities, and its limitations. In part two, we’ll dive into the DMMs’ resistance measurements and share how these tests are, in many cases, preferred by manufacturers’ diagnostic routines, but at the same time, offer the least reliable results.
