I think that most technicians are comfortable using their scan tools when attempting to diagnose a customer's drivability concern. It's a tool we use on a daily basis to access stored DTCs (Diagnostic Trouble Codes), look at Freeze Frame and history data and, of course, look at current data - all to gather the information we need to fix the problem.
But when you look at data PIDs (Parameter Identifiers), do you look at a list of PIDs as the bounce around with each refresh of the scan tool's screen or do you look for the anomaly by graphing the data and comparing multiple PIDs to each other at one time?
I'm betting you've learned to graph the data, maybe even have learned to perform a drivability test drive that will allow you to record critical information in a repeatable process that speeds up your diagnostic time even further.
And that, my friends, is what it's all about. Using our time efficiently. After all, that's what we truly sell to our customers and the more efficiently we use that time, the more money we can make.
Getting more with less
A modern DSO is just another way to graph information over time. In this case, the scope graphs voltage over time but with the variety of scope accessories available, nearly any drivability parameter can be converted to a voltage input that can be read by the scope; voltage, current, pressure, vacuum, even vibration.
And using a scope is not overly complicated. Operation of the tool, I think, is less involved than learning the operation of a scan tool. It's understanding what the tool is sharing with you that is the challenge to mastering both!
So let's start with a little Scope 101. As I stated, a scope is a tool that graphs voltage over time. The scope screen is separated into equal sections on both the X-axis (horizontal line) and the Y-axis (vertical line). These are referred to as "divisions". Typically, scopes are divided into 10 divisions in both directions but there are some exceptions to the rule.
Time is plotted on the X-axis and the scope settings may require the user to specify the time range wanted by division or by the total time displayed on the screen. This is referred to as "sweep." Voltage is plotted on the Y-axis and can also be set by division or by the total voltage "range" displayed on the screen. To capture most automotive signals, you can use the "20/20" rule as your starting point. This means you should set your scope to read a total voltage range of 20 volts and your time per division to 20 milliseconds (that's the same as a 200 millisecond "sweep.")
You'll also need to set some kind of "trigger." A trigger is a combination of settings that tells the scope when to start its trace on the screen. You'll need to select the type of trigger (None, Normal, Auto or Single are common selections), the voltage level of the trigger, and whether the scope should wait for the selected voltage level to be exceeded (called a "rising slope") or dropped below (called a "falling slope"). You can also set the location of the trigger on the screen, relative to the X-axis.
With these basic settings made and your scope connected to the signal you want to see, you should have something visible on the screen. To fine tune it and make it usable as a diagnostic aide, you can alter the voltage and time settings to "zoom in" on the pattern.
Practical tests with a pocket scope
One of the least expensive scopes on the market today is the AESWave "uScope." The full Master Kit can be yours for under $500 and includes accessories that add a lot of functionality to the base scope. But even if you only own the basic kit, there is still a lot you can do with it.
One test I encourage every technician to perform on every car that comes in is a basic battery/charging system test. The uScope is a single-channel (that means it can only trace on input at a time as opposed to multi-channel scopes capable of tracing two, four or even eight inputs at a time) and is designed for quick, point-of-need use. To perform this test with the uScope, adjust your scope settings slightly by adding to the time range. Set the scope to 5 volts per division (or a 40-volt range) and the time to 500 milliseconds per division (or a 5 second sweep).
For the trigger settings, I find the "Single" option very helpful when working alone. A Single trigger will start the trace once the trigger level and slope parameters have been met and the scope will stop once the trace has completed its way across the screen, eliminating the need to stop the scope manually or having to look at multiple screens on those scopes that record their data for later review. In this case, I set the trigger to 12.4 volts on a falling slope so the pattern would start as soon as I turned the key on. You can see the resulting capture in Figure 1.
With the capture on the screen, I adjusted the voltage to 2 volts per division to zoom in a bit on the pattern. Take a look at Figure 2.
The point labeled "A" is called the "in rush" voltage and is comparable to the loaded voltage you're used to when performing a battery test. The difference is in the speed at which the scope acquires its data. Scopes are so fast, they are able to capture the millisecond moment when current first started flowing into the starter motor. It takes quite a bit to get that motor moving, especially when it's tied to the mass of the engine, so you'll see test results here lower than the 9.6 volt loaded maximum used when testing conventionally. In this test, any battery that stays above 8.6 volts is considered a "pass".
The area labeled as "B" is showing a lot going on. The first section shows a series of spikes that represent the change in voltage caused by the changes in current demand as the starter motor starts moving individual pistons up and down in the cylinders. Shortly thereafter, the spikes get closer and closer as engine speed increases until, finally, the engine starts. There, you see the gradual rise in voltage as the charging system replenishes the spent battery, ending at a steady charging voltage level you can compare to traditional specifications.
Want to take a closer look at that alternator? AC ripple is a great way to see if the diodes have failed and as a tool for checking for excessive AC bleedover into the electrical system. ECUs do not like to be confused and when too much AC voltage is riding around on top of the DC, it can confuse the heck out of them!
With the scope, checking for the presence of excess AC ripple is simple enough. By adjusting the time and voltage, and adding an AC filter (or selecting the "AC Coupling" option on our scope), we can zoom right in on it. Figure 3 is a great example of what AC ripple looks like.
Checking the engine's relative health
You've read several references in our magazine to the next test I want to share - relative compression testing. Typically, technicians use a high amp current probe to perform this test but if you're a tech on a budget, you can perform this test without one.
As we saw in the battery/charging test, every time a piston moves up on TDC of its compression stroke, it adds resistance to the starter motor. Increased resistance to turning also increases the current demand on the starter - and that extra current demand means more voltage drop on the battery. And that's exactly what you're seeing in those first few spikes on the battery test!
So, by disabling the engine's ability to start, we can perform the same test and see what happens to the spikes once the rpm has stabilized. That's what you're looking at in Figure 4. Each "drop" in the pattern is the voltage drop at the battery caused by the individual cylinders coming up on TDC. If one or more isn’t sealing, the drop will be less and that justifies performing a more detailed test. With the complexity in some engine designs, wouldn't you'd like to know that there is a definite indication of a weak cylinder prior to spending a few hours on a conventional compression test?
The last example is Figure 5. All I did here was "invert" the image to make it look more like the current-based relative compression tests you may be used to. Either way, this one pocket-sized scope has delivered a lot of information with just one connection at the battery!
Of course, adding additional horsepower to your diagnostic arsenal by investing in a good multi-channel scope will open up even more diagnostic possibilities to you. Possibilities that will improve your efficiency - and your paycheck!
