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Learning the Digital Storage Oscilloscope

Wednesday, November 1, 2017 - 06:00
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Everyone is familiar with the saying “a picture is worth a thousand words.” It is thought the modern use of the phrase stems from an article by Fred R. Barnard. He claimed it was an old Chinese proverb to give it more legitimacy. The actual ancient Confucian saying is, “Hearing something 100 times isn't better than seeing it once." Both statements to me express that something that is illustrated or drawn out is more informative or reveals the greatest amount of information. The modern automotive Digital Storage Oscilloscope (DSO) reflects this concept in the world of diagnosing the modern vehicles that are in our service bays. 

There was a time in automotive repair in our past that we all embraced the value of an oscilloscope to diagnose ignition issues and “tune up” the cars in our service bays. It seemed almost every competent shop had some sort of “big box” scope or ignition analyzer. This was when cars were equipped with fairly simple point-style ignition systems that required adjustments and the performance of the car greatly depended on how well “in tune” or adjusted the system was. This was about the time that I entered the field, working at the gas station pumping gas and doing light repairs on the second shift. I still remember the daytime mechanic who was a local stock car driver “power tuning” his racecar on the old Sun 1015 big box analyzer. All the cylinders were in a “raster” pattern, stacked on top of one another and how they jumped to life on the blue cathode screen with each aggressive snap of throttle combined with sound and smell of the open headers. It had me at “Hello.” I knew this was something I had to learn.

I was able to capture the cause of the intermittent stall using the deep record capability of my scope.

The DSO – A game changer

Scopes have been around for years. An oscilloscope, simply put, measures electric signals and displays them in a linear fashion of voltage over time. Many of us that have been around a while learned the value of the scope for diagnosing ignition systems issues. We all had some knowledge of how to operate the analyzer much like a modern tech is familiar with operating an A/C recharge station. Today, most techs in shops I frequent seem either to struggle with or are reluctant to use a scope to diagnose modern car problems. What changed? 

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Well first off, think about all the advancements we have had in our modern scantools and the dependable improvement in the vehicle’s ignition system. As the scantool and the scan data got better, our dependence on the once very expensive and space consuming big box analyzer waned away and so did our scope skills. Well if you haven’t noticed, the scope has undergone some paradigm shifting advancements. First and foremost is the advent of the DSO. The drawback to the big box machine, aside from the size and cost, as I always saw it, was you had to catch the issue or “glitch” in real time. In other words, you had to be looking at the screen when the issue occurred.

The DSO was a game changer in this regard! Now we have the ability to capture and store large amounts of data in a “buffer” that we can stop, save and “rewind” after the failure event occurs. I like to think of it being akin to the modern TV DVR that affords us the ability to do something other than being in front of the screen watching in real time. The same way we can archive our favorite shows; we can save the digital files or scope captures in a database that we can go back and review at a later date for comparison or print as an educational tool to enlighten our customers. The price of this technology is considerably cheaper than the big box units that could be north of 25k “back in the day.” Depending on your needs a modern DSO can be very affordable starting as little as $154 for a cell-phone sized, single channel uScope from to a couple of thousand dollars for a very powerful Pico unit. Snap-On incorporates their very powerful scope into their Modis Ultra and Verus platforms as well as a handheld Vantage Ultra. My point of this whole article is not to sell you on a brand scope but to sell you on the idea of the practicality of using one to assist you in diagnosing today’s modern vehicles.

Adding the IC trigger circuit from the PCM to the ICM proved the PCM was at fault.

I like to think of its use as leveraging technology to my advantage. I am only limited by my creativity as to its application. If you think about it, we put a lot of credence in what the scan tool tells us. Let’s just think about how the scantool comes to get its information. First the vehicle’s sensors generate an electrical signal of some kind and sends it through the wiring harness to the module. The module then processes these signals and converts them into bytes of information that is transmitted over the data bus to the DLC. The scan tool (often with aftermarket software) converts the data to PIDs that we review. I think you can see there are a lot of places where things could go wrong. I think of that old social experiment where you line a group of people up and share a list of things with one person and tell them to share the information with the next person in line. Oftentimes, by the time they reach the end of line, the original information has somehow changed. If you suspect there is an issue with the information you are getting at the end of the line, the person to talk to is the first one in line. In this analogy if you suspect an issue with scan data or have a no code driveability issue the place to investigate is where the signals arrive at the PCM. The DSO is a very powerful tool in this regard. I can see if the signals are reaching the module, if their signatures or waveforms are correct for the vehicle I am working on and often times in the case of CM-CKP correlation, whether the engine is mechanically in time. Let’s try to keep an open mind and investigate some broken cars and how the modern DSO helped accurately and quickly diagnose the concern.

This capture from the ailing Equinox may have the answer I'm looking for, but I'll need a "known good" to know for sure.

The “no code” intermittent

A “no code” intermittent stall that several people have had their hands in can be possibly the worst ticket a tech or shop can take on. We never seem to get the complete story as to what has been done to it and especially if the problem is intermittent or only happens at a certain times like cold start up, catching it can be difficult at best. Leveraging the technology of the DSO I can setup my experiment to figure out whether the issue is on the input or output side of things. I can differentiate whether it’s spark or fuel. Moreover, I can control what I want to see and how I setup the scope to capture it. I am only limited by my creativity when developing a logical plan of attack (POA) and when “designing the experiment” to prove out my diagnosis.

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