In this issue, we’ll cover some of your questions on the topics of diagnostics, inspection tools and scopes. This area probably represents the greatest frequency of change, making it the most difficult to keep up with. Technology doesn’t stand still and neither should you in your quest to do your job effectively, efficiently, and accurately—the first time. Let’s get started.
Q. I see lab scopes mentioned every now and then for diagnosing electronic circuits. How is a lab scope different from older types of oscilloscopes?
A. First of all, a lab scope is a lot like the ignition scope you’re probably already familiar with. The lab scope also displays voltage on the vertical axis and time on the horizontal axis. The lab scope differs in that it measures more minute quantities of voltage and time than the ignition scope. The lab scope also allows you to select various voltage and time thresholds, giving you complete control over the waveform. Consequently, a lab scope's sensitivity, speed and control let you see problems as they happen in what's called a real-time format. This is priceless when tracking down a glitch in a system that may not appear on other types of diagnostic equipment.
Q. What exactly can I see in vehicle electronic systems with a lab scope?
A. A lab scope really shines when checking for the presence and condition of signals used in various onboard electronic systems. The scope also has several other uses including checking for noise on a signal line, but you will mainly check the signal waveform integrity of the two most common types of electronic signals. Analog signals generally come from variable-reluctance, magnetic-based position sensors, such as wheel speed sensors on antilock brake systems and some engine position sensors. The output of this signal is a variable, AC voltage that continuously changes in polarity. The strength of an analog signal depends on the rotating speed of the sensor's trigger wheel. The faster the speed the trigger wheel goes, the stronger the signal.
Digital signals, on the other hand, appear as a series of squares known as a square wave. Unlike the analog signal, the digital signal represents voltage turning on and off. Digital signals are typical of Hall-effect sensors such as those used in cam and crank sensors. The best way to get a feel for the type and importance of the signals used on different cars and systems is by experimenting with your scope. If possible, sample different cars that roll into your shop to get an idea of the different signal “signatures.”
Q. What are some tips for choosing the right lab scope?
A. Whatever you do, don't be hasty and buy the first thing that comes along. Talk with as many manufacturers’ reps as possible and ask a barrage of questions. Then, get as much hands-on time with different scopes and note your likes and dislikes. If possible, sign up for a training clinic or take a class at a local technical college on using a lab scope. Lab scopes with multiple channels allow you to compare two signals and compare their timing to make sure that events are properly synchronized. You can also use a two-channel scope to check cause-and-effect relationships, like checking the input signal to a certain component and seeing if it makes the proper response. Make sure the manufacturer recommends and stands behind a given scope in the arena of automotive testing. Since there's a lot more radio frequency interference (RFI) present in your shop than in a laboratory environment, the right scope will need some healthy shielding.
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