Comparing a Scope to a Digital Multimeter
11-02 Choose between an oscilloscope and a digital multimeter..
TECHNICIAN TIPAs signal frequencies have continued to increase on modern vehicles, the scope has been the only tool that can accurately measure, record, and display them for diagnosis.
Both DMMs and scopes measure voltage but display and measure it differently: One displays numbers (DMM); the other displays pictures (scope). A digital multimeter measures a variety of signals, whereas scopes measure voltage only, but in much more detail than a meter. DMMs precisely measure discrete signals in the form of voltage, amperage, or frequency. A DMM is equal to the scope when measuring a constant (discrete signal) or slowly changing voltage. The primary advantages of a scope over a DMM are their ability to measure complex, rapidly changing signals and their ability to catch intermittent faults. Glitch detection is better due to the scope’s high sampling rate (the number of measurements/samples collected in a given period) and its presenting the fault as a visible pattern on the screen. Also, most scopes have multiple channels, allowing more than one signal to be measured at a time. For example, voltage and amperage can be measured simultaneously (FIGURE 11-8).
FIGURE 11-8 Using the correct tool to diagnosis an electrical fault minimizes the time it takes to identify the part that has failed.
DMMs present two problems that require for a scope, both of which involve the meter’s update rate. Most DMMs update once every two seconds. The best and fastest DMMs update four times a second—or put another way, once every 250 milliseconds (ms). Anything that happens four times a second sounds fast. However, most intermittent glitches occur about 10 times faster than the 250 milliseconds update rate of a DMM, resulting in an unmeasured event by the meter. In comparison, scopes update 25 million times per second, easily capturing sporadic faults. Additionally, digital meters do not display live data. Instead they “average” the signals they are measuring. The speed and shape of a switching waveform, combined with the update rate of the meter, determine the DMM’s displayed measurement.
Pictures versus Numbers
The phrase that scope users use for the graphical display of a scope compared to the digital readout of a DMM is “pictures versus numbers.” Thus, the primary difference between a DSO and a meter is that the scope draws a picture to show the shape of the waveform. Using the scope’s buffer (internal storage capacity), transient events that occur infrequently or only once can be seen. A DMM, on the other hand, records an instantaneous voltage, making it difficult to use and see a changing quantity, especially in fast-moving signals. DSOs record sequential values of changing voltage.
TECHNICIAN TIPScopes display a number and draw a picture of a signal. DMMs display only a number.
The PCM uses rapidly changing signals sent by speed sensors that monitor the rate of rotation of a camshaft, crankshaft, transmission input, output shaft, wheel speed, or any other rotating object part. Many of these sensors produce a toggling on/off, digital signal, which a DMM displays as an average voltage of the two. The rate at which these changes or pulses occur is frequency. For example, most sensors use a 5-volt reference signal that is either on (high voltage) or off (low voltage) and switches at a fixed frequency. Since the signal is high (5 volts) and low (0 volts) 50% of the time, the DMM averages this signal and display 2.5 volts. DMMs do not show the voltage toggling, just the average voltage level of the signal. DMMs update the screen’s display too slowly to show the switching voltage. The voltmeter will also not show the 5-volt high or 0-volt low signals. The Peak Min/Max function of the meter is needed to capture the high and low voltage (FIGURE 11-9).
FIGURE 11-9 Choosing whether to use either a DMM or an oscilloscope requires the technician to understand what they are trying to view and how they should view it. Using the wrong tool will not tell the technician what they are trying to understand.
TECHNICIAN TIPPulse-width–modulated (PWM) signals are best measured using a lab scope. The scope shows the actual operating voltage of the signal, the amount of on and off time, any intermittent circuit glitches or abnormalities that a DMM or graphing scan tool cannot, due to update speeds.
Unlike the DMM that displays the average voltage, a scope handles this measurement in another way. The scope captures the toggling event, transfers it as a picture to the screen, and then displays the shape of the waveform as well as the high and low voltage levels. Using the scope’s cursors can also help to determine how long the voltage stayed at the different levels. The scope provides much more information than the DMM when measuring fast-moving signals.
Oscilloscopes add a wealth of information to the numeric value of a DMM. Oscilloscopes visually illustrate a signal’s shape, strength, and instantaneous value in a waveform. The pattern shows the signal’s amplitude (voltage) and frequency without having to change the screen. A DMM requires changing the function and can only display one measurement at a time. DMMs do not decipher the type of signal; they just provide an average voltage of whatever is being measured. Scopes display the signal waveform to distinguish between a sine wave, a square wave, a damaged reluctor, or a signature pattern from a sensor such as a crankshaft position (CKP) sensor that uses a missing tooth to identify Cylinder 1 (FIGURE 11-10). Scopes also show how these signals change over time. Use like signals to compare several channels by plotting both on the screen at the same time. For example, measure an input and output signal simultaneously to monitor the changes in the circuit in real time.
FIGURE 11-10 Viewing the scope pattern of a CKP sensor will allow the technician to view the waveform and determine whether there is an issue with the sensor.
In addition, a DSO can store these waveforms for analysis. Captured waveforms can then be replayed and further scrutinized by using zooming features, allowing the well-versed scope user to gain insight into fluctuations in a signal that would typically go unnoticed when using a DMM. An oscilloscope also graphically shows any glitches, noises, or distortions that may be present in a signal; the DMM does not.
TECHNICIAN TIPScopes measure voltage only, not resistance or amperage. Measuring other signals with a scope requires using amp clamps, pressure transducers, and other accessories that convert a measured signal into voltages that the scope can display on the screen.
Differences between a Digital Multimeter and a Scope
Both the DMM and DSO are essential but different tools (TABLE 11-2).
Computer-Based Oscilloscopes versus Stand-Alone Oscilloscopes
There are many scopes to choose from, and many are automotive specific. Choices include desktop-based and laptop-based USB scopes, stand-alone handheld scopes, scanner and scope combos, and big-box lab scopes. Each one of these options has its benefits and drawbacks. Using a laptop-based scope allows the technician to have the laptop power to do a lot, though it is fragile and thus may not last long in a shop environment (FIGURE 11-11). Using a handheld purpose-built scope can endure a little more abuse so dropping it isn’t as serious a problem, but it doesn’t have online capability, which a laptop-based one has. A scope and scanner combo has its benefits: it usually has an Internet ability that can act as a guide for hooking up the scope and looking up diagnostic information. The downside is that they are all one tool, so if the combo breaks, both parts break. This section will explore the differences of each scope and how they work.
FIGURE 11-11 A laptop-based scope has the power of a laptop, which increases the effectiveness of the oscilloscope. Allowing the laptop to integrate with the oscilloscope, the hard drive can be used to save waveforms and increase the capabilities of the oscilloscope.
Handheld Lab Scopes
Handheld automotive-based scopes are portable, built to handle the automotive environment, and contain the power, speed, and features required for almost all diagnostic situations. The number of channels varies from two to as many as eight (FIGURE 11-12). Many handheld scopes also include DMM and the graphing multimeter (GMM) functions. Handheld scopes are available from a wide range of suppliers, including most major automotive tool providers. Used scopes are readily available, generally at substantial cost savings compared to new—a good option for a new scope user.
FIGURE 11-12 A handheld oscilloscope is built to operate within the environment of an automotive repair facility, so it can endure some abuse before it fails.
Combination Scanner/Lab Scopes
Combination tools combine a scan tool with a lab scope. Sharing the scanner body that includes the case, screen, navigation buttons, and other hardware reduces production costs. Including both tools in one package saves the technician from purchasing two individual devices.
A downfall of some of the early combination units is that only one function can be performed at a time, either the scanner or the scope. If a technician needs to control a component using the scanner’s bidirectional controls, the scope function will be unavailable. The evolution of combination tools is overcoming this drawback. Newer combination units are tablet based, running traditional computer operating software (FIGURE 11-13). The computer-based software allows the user to open multiple windows (modes) and move back and forth between the scan tool and scope functions at the same time.
FIGURE 11-13 A combination of a scan tool and scope as one unit is convenient for the technician, because they have both tools at their disposal at all times.
Computer-Based Lab Scopes
Computer-based scope usage is increasing rapidly as technology advances and costs are reduced. Computer-based scopes use an interface box connected by a USB cable to turn a laptop or desktop into a lab scope. All computer-based scopes include minimum operating requirements that must be met for correct operation. Any computer that matches the specifications, from a throwaway model too expensive ruggedized laptop, can be used. Modern computer-based scopes include several channels, some offering as many as eight, and large memory buffers for extended circuit monitoring. The resolution of the waveform is typically better on computers than on handheld or combination scopes, a result of the desktop/laptop screen.
Big-Box/Engine Analyzer Lab Scopes
While no longer produced, engine analyzers introduced lab scopes to the automotive industry in large quantities. These analyzers introduced lab scopes to technicians. Expensive and bulky, these rolling diagnostic machines use an analog oscilloscope. The analog scopes are accurate, delivering a surprisingly powerful display resolution. Engine analyzers have multiple faults outside of their size and lack of portability. Requiring alternating current (AC) power to operate them eliminates needing to use the scope during road tests (FIGURE 11-14). Big-box analyzers do not include the settings of current scopes, including trigger adjustments, the ability to zoom, and various other functions. While no longer available for purchase, many of these units remain in use today, functioning as they did when new.
FIGURE 11-14 The “original” lab scopes, known as engine analyzers, were found in shops everywhere. Big and bulky, these analog scopes were most technicians first experience with lab scopes. Many remain functional and in use today.