Lab scopes open a window of opportunity for sensor testing

When electronic engine controls first emerged on a large scale in the early '80s, some saw the new technology merely as a passing fad. Well, the fad never passed and electronics are now more entrenched in vehicle systems than ever. Powertrain control systems, brakes, climate control, steering and suspension, and cruise control serve as but a few examples.

When working normally, electronics contribute to levels of vehicle performance and efficiency never before thought possible. Despite their rosy track record, however, these systems can fail. When this happens, it's critical to root out the problem as quickly as possible.

When working with electronic controls, the biggest challenge you face is finding the culprit, not fixing it.

To help pinpoint electronic woes, an increasing number of technicians have discovered the merits of using a laboratory oscilloscope (frequently called a lab scope) when diagnosing electronic-related problems.

In this article, we'll give you some basic insight about lab scopes and how one could prove to be a welcomed addition to your diagnostic arsenal.

See more about lab scopes in Diagnstic Review.

WHAT IS A LAB SCOPE?

First of all, a lab scope is a lot like the ignition scope you probably already have in your shop. 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.

DON'T BE AFRAID

When you see a lab scope for the first time, you may feel intimidated by all the switches, dials and controls. You're not alone. To ease your anxiety, some manufacturers provide automatic setup routines that make the correct settings for you, based on the signal being measured.

Of course, different signals will require different settings, but that part will come easier as you gain experience performing tests. The main thing is that you become comfortable using a scope as soon as possible.

SIGNAL SENSE

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 here we will concentrate on sensor signals.

Analog signals:
This waveform is typical of variable reluctance magnetic sensors such as the crankshaft sensor on some distributorless ignition systems, wheel speed sensors on antilock brake systems and the pickup coil on some electronic ignition systems. 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 stronger the signal that's produced.

Digital signals:
This type of signal appears as a series of squares. 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 for some engines with distributorless ignition, and ignition pickups in some electronic ignition systems.

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 every car that rolls into your shop.

SHOPPING ADVICE

Lab scopes come in a number of different types including:
• Various speeds (specified in megahertz like 10, 20, 40, or 50).
• Digital or analog.
• Digital storage (DSO).
• Number of channels (one, two, three or four).
• Stand-alone lab scopes.
• Built-in lab scope functions inside console engine analyzers.

Which scope is best for your needs? Everyone has different needs, so that's a question you'll eventually have to answer on your own through some careful guidance. Of course, your financial resources will also play a major role in your buying decision.

Don't be hasty and buy the first thing that comes along. Talk with as many manufacturer's 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, take a class at a local technical college on using a lab scope.

The question often arises about whether to choose a single- or two-channel (sometimes called dual-trace) scope. Though a single-channel scope will do the trick in many cases, a two-channel does have some advantages. You can 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.

Though a 10-mHz scope is fast enough for automotive testing, manufacturers sometimes include other features with their faster scopes. It's like ordering air conditioning on a new car and getting tinted glass with the package. Don't shop for the sake of speed alone.

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.

Finally, make your decision on the information you've gathered and your experience. In the long run, you'll be happy you waited.

The real beauty of the lab scope is that it brings all cars — regardless of make, model and system — down to a common denominator.

A lab scope also needs no application- or system-specific software. And, since you're looking at signals in their native form, a lab scope won't become obsolete in the foreseeable future.

See more about lab scopes in Diagnstic Review.

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