AKRON (Feb. 28, 2005) — A tire dealership that offers automotive services should have all three basic diagnostic tools in house. These include a voltmeter, a scan tool and an oscilloscope.
Owners, managers and shop foremen often tell me they're confused by the array of diagnostic equipment on the market. Understandably, they feel besieged by tool dealers and equipment sales people who are trying to promote a variety of diagnostic gadgets and goodies. “What do we need, what do we need?” they ask me.
My diagnostic experience—not to mention that of the best diagnostic specialists I know—suggests the basic triumvirate I mentioned. One of these does not replace the other. Rather, one device complements the other! What's more, you needn't be a rocket scientist to understand the function of each device and its place in your service department.
For all intents and purposes, electricity is like water in a garden hose. The water flows through that hose at a particular pressure. Furthermore, a certain volume of water passes through that hose. In electricity, voltage (volts) represents electrical pressure; current (amps) represents electrical volume.
To grossly simplify the issue, all these three devices do is measure voltage or electrical pressure. Since the late 1970s, the digital voltmeter has been the most fundamental and essential voltage-measuring device. A typical professional-grade digital voltmeter is extremely safe and accurate. Where necessary, a digital voltmeter can measure values down in the range of hundredths or thousandths of a volt. Safe means it won't damage a circuit or, equally important, influence the operation of a circuit you are testing.
The voltmeter always will be a universally required tool. No matter how sophisticated vehicles become, we'll always have to verify basic voltage values during routine diagnosis.
Some people still believe that a test light is an alternative to a digital voltmeter. However, a test light is a very crude, guesswork device. The age-old question for test-light users is: “How bright is a bright light and dim is a dim light?” In some situations, using the wrong test light can damage a circuit or alter circuit operation.
The biggest shortcoming of a digital voltmeter is that it doesn't sample or sense quickly enough to monitor rapidly changing voltages accurately. A classic example of a rapidly changing signal would be those produced by vehicle speed sensors or anti-lock brake system (ABS) wheel speed sensors.
Basically, the scan tool is a variation of the digital voltmeter. The scan tool connects to a data access port under the dashboard and monitors voltage signals going into and out of the vehicle's on-board computer(s). This tester saves a great deal of time by displaying a multitude of voltage signals simultaneously. Plus, it speeds up diagnosis by enabling a tech to evaluate the relationship between two signals or among several signals. Diagnostically speaking, these relationships are crucial.
An oscilloscope, commonly called a 'scope, is another variation of the voltmeter. A 'scope is an extremely powerful, universal diagnostic tool because it shows voltage as well as time on the same display screen. This gives the tech a highly detailed picture of a voltage signal that he couldn't see otherwise. That finely detailed picture often shows signal failures—especially intermittent ones—that the digital voltmeter and scan tool simply can't discern.
Digital oscilloscopes, which are commonly called lab 'scopes, can display extraordinarily detailed pictures of vital signals thanks to their powerful sampling ability. When you connect a digital voltmeter to a circuit, the meter only samples the circuit voltage about five to 10 times per second. However, a digital 'scope samples a signal anywhere from 250,000 to several million times per second! This phenomenal sampling rate enables a 'scope to monitor or track rapidly changing signals very, very accurately.
Therefore, the 'scope's detailed picture can reveal very small hiccups or aberrations in voltage signals that a voltmeter or scan tool simply cannot catch.
So the voltmeter, scan tool and 'scope are all important to your shop in one test situation or another. Well-trained techs know when to move up the diagnostic tier from one to another, so don't short-change the training aspect of the business.