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Scoping Out Your Options for Equipment

Adapted from Brian Manley’s article in Tech Shop

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Looking back on why I purchased my first oscilloscope, I recalled it was the one recommended by our instructor to aid in diagnosing emissions failures. I learned to scrutinize O2 sensors, TPS waveforms, injector operation and crankshaft sensors for flaws that could lead to high emissions and driveability issues.

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One day, you too, will be leaving school and entering the automotive industry as an eager repair technician. And when faced with more choices of scopes and probes than ever before, how will you decide which one is the best one for you? The first question you need to ask yourself, even before cost, is: What will I need to use my scope for?

Most scopes, regardless of price or features, are capable of analyzing most basic automotive electrical signals, but if you want to own a scope that will serve as a powerful diagnostic tool, you may want to add high and low current probes and ignition system adapters. If you are uncomfortable with lab scope set-up, you may want to choose an instrument that has presets for automotive use, and perhaps one with a built-in automotive database. Do you want a recording feature? All meters can save at least one screen, but having a scope that can record a signal over time can be extremely useful when pursuing an intermittent “glitch.” What platform do you want? Do you want a hand-held unit or would a PC-based scope be sufficient?

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Do You Need a Lab Scope?
A while back, this question was posted on International Automotive Technicians Network (www.iATN.net) and it raises the most basic of points; can you successfully diagnose and repair vehicles without the use of a scope? The consensus seems to be that, although modern OBD II vehicles are easier to diagnose using a scan tool than their OBD I ancestors, there are times when you must monitor a voltage pattern or current waveform. One tech has even been using his scope to help determine enabling criteria on late-model systems.

Looking for an Oscilloscope or a Power-Graphing Meter?
A digital storage oscilloscope is ideal for analyzing the dynamics of any type of electrical signal, including primary and secondary ignition. The DSO provides you with a waveform, which represents the actual characteristics of an electrical signal. These are the signals that we all grew up viewing while trying to catch a waveform breaking down.

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A power-graphing meter provides a graph of a particular aspect of the electrical signal. They can graph many different characteristics of a signal, such as voltage, frequency, duty cycle, pulse width, etc. Several scopes on the market today have both capabilities.

Which Platform Will You Need?
When buying a DSO, you have several options: handheld, PC plug-in, PC external or bench-top. Each platform has special requirements. Handheld DSOs must be small and capable of withstanding abuse (you know what I’m talking about!). These are the most prolific offered today, and the primary feature is portability. Designers of PC plug-in cards and PC externals must shrink measurement circuits, and the cards need to operate within the noisy PC environment. In addition, if you want to match the portability of a hand-held, you must use a laptop. Many techs like the larger computer screen on a bench-top to analyze waveforms and prefer saving captured data right to their hard drive.

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High Current Probes
We’re all familiar with using a starting/charging analyzer to look at starter draw and alternator output. You’ve probably found many bad starter motors and other problems this way. Today’s lab scopes and graphing meters offer a whole new level of detail compared to traditional equipment when used for this type of testing.

To measure high current, you will need a high current probe. This type of probe is built to measure high current levels, usually in the hundreds of amps. Low current probes on the other hand, are designed to measure extremely low levels of current, down to only a few milli-amps. In fact, low current probes usually aren’t even physically large enough to clamp around a high current conductor, such as a battery cable.

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The current probe you select should also be of the type that is intended for use with a multimeter or lab scope. Most high current probes use a calibration of 1 mV per amp. This means that every milli-volt displayed on the meter represents one amp.

In addition to starting/charging systems, relative compression tests can be performed using your high-current probe to determine if compression is the root cause of a weak cylinder.

Low Current Probes
Using a low current probe in conjunction with a lab scope or graphing multimeter gives you the ability to look into a window where diagnosis becomes fast and non-intrusive. Current ramping has become a hot topic out there and waveform libraries are becoming filled with known-good and bad patterns.

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Think of the possibilities: A no-start is dropped at your door and you suspect the fuel pump, but the vehicle starts right up. No problem – a current waveform can reveal a worn-out pump even when it has satisfactory flow and volume. With this probe, you can analyze ignition coils, fuel injectors, and even test for parasitic drain from the vehicle’s battery without disconnecting, and powering down, all of the vehicle’s modules.

Looking For A Package Deal
Scope manufacturers cater to all types of electronics technicians, and many of them cater to techs who don’t work on cars. But, many companies also cater to the automotive technician by packaging clamps, leads and probes that we need for working in the harsh underhood environment. This includes shielded cables to reduce noise that skew signal interpretation.

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Secondary Ignition Pickups – Capacitive pickups clamp around the insulation of a secondary ignition component without subjecting the meter to damaging voltages. Many of these are shielded and are suitable for conventional ignition systems, as well as HEI and DIS systems. Many adapters can turn an oscilloscope into a full-blown ignition analyzer, capable of raster, display and single cylinder functions.

Current Clamps – Some automotive “kits” come with a high current clamp (up to 500 amps) and a low current clamp (10 ma to 60 amps) for testing everything from starter motors to parasitic drains.

Automotive Test Leads – Some test leads are up to 10′ long so a scope can sit on a bench while you probe in the engine compartment. These shielded leads come with a wide array of clips, probes and gators for back probing. Some manufacturers also make automotive break-out leads that will clip into the vehicle’s engine harness and sensor, then readings are taken from two remote leads.

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Attenuators – Scope manufacturers sell these in 10:1 (10 volt signal appears as a 1 volt signal on the scope) and 20:1 versions.

Oscilloscope Probes – Some probes are sold that are x10, meaning that the measured input to the probe can be extended to 200 volts. This can be helpful when viewing injector signals and primary signals that may have inductive kicks as high as 100 volts or more.

Combining these leads can give you the abilities that those old “big boxes” had: Power balance, cranking rpm and cylinder contribution.

Waveform Libraries
Some scope manufacturers make waveform libraries available to techs who purchase their product. Although some libraries already exist on the Web (iATN has a large one for paid members) many scope makers begin their own libraries to get their customers familiar with what signals will look like with that particular unit. Plus, if you are observing what you believe to be a faulty signal, then you have known-good, and known-bad, patterns to compare it to.

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Along with the libraries, manufacturers are making sections on their websites that are designed to help you get the most out of their product. Tips, tricks and caveats are listed along with links to associate information on different topics.

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