Scope & Scan: The Z Graph: Every picture tells a story

Jan. 1, 2020
In two previous installments of Scope & Scan, we discussed how to format our data to get the most information from our scan tool captures. This month, we will explore different techniques to ensure that we capture the data we really want to begin

In two previous installments of Scope & Scan, we discussed how to format our data to get the most information from our scan tool captures. This month, we will explore different techniques to ensure that we capture the data we really want to begin with.

All scanners today have what is known as a "snapshot" capability, which captures data over a period of time and stores it in a movie-type data file for later analysis. The "trigger" we use to start recording can make a difference in the value of the data captured. Choosing the right trigger can also save you lots of time by not having to scroll through reams of irrelevant data.

Let's take a look at using a Diagnostic Trouble Code (DTC) to trigger a data snapshot. Scanners today have the ability to trigger a snapshot when a DTC sets in the Engine Control Unit (ECU) or any of several other ECUs. Until the DTC is set, the scanner is merely keeping the user-requested data in a rolling memory buffer of a certain time length (different tools have different buffer sizes). As new data fills the buffer, older data is overwritten and lost.

The scanner also regularly asks the PCM whether any codes are set. When the PCM confirms that a code has indeed been set, the scanner then begins making a permanent recording in the memory buffer. The data recording stops when the buffer is full.

When using a DTC to trigger a snapshot, you have to clear existing codes and recreate the fault. That means you must read the "Monitor Entry Criteria" and the "Code Set Criteria" in the service manual so that you can operate the vehicle in the proper DTC monitoring range. You also must understand how far and how long a particular data value must be out of range to set a code, and you must know whether multiple trips are required.

One of the most useful things about a triggered snapshot is that we can set the trigger event anywhere we want within the buffer. Notice that in Figure 1, I set up the trigger on a Vetronix MTS3100 scan tool to be in about the very middle of the snapshot record. The trigger event is shown by the "5-seconds" time marker. Data to the left of the 5-seconds marker was recorded before the trigger event, and data to the right of the marker is recorded after. A failing catalyst monitor caused the ECU to set a code P0420 (Catalyst efficiency low). You can see the Catalyst monitor completion Parameter Identification (PID) in purple immediately before the DTC sets.
Another method of triggering a snapshot is to use one or more PID data value levels to initiate a recording. The Snap-On scanner has the ability to set PID value trigger levels on several data channels, both high and low, to trigger a snapshot. In Figure 2, I am using the MAF sensor PID data value to trigger a snapshot. Notice the black dotted lines on the MAF graph (bottom of the screen shot). When the MAF data PID value goes either under or over the limits marked by the dotted lines, the snapshot will be initiated. In this case, when the MAF signal went below the lower limit that I selected, the recording started.

There are also times when you may need to manually trigger a snapshot yourself when the defect occurs. This method may have to be used if we do not have DTCs to trigger from that correspond to our defect. Other times we just may not have a clear idea what we're looking for in the PID data values that may be causing the defect. In these circumstances, we will simply wait till the stated problem occurs, then hit "enter" to trigger the snapshot.

In Figure 3, the defect was an intermittent stall and a temporary no-start. I simply hit "enter" when the vehicle stalled, then graphed the different PID data values to see which values might indicate the cause of the stall. As you can see in the graphs, the cause was an Engine Coolant Temperature (ECT) sensor defect, which increased injector pulse width so much that the engine flooded and stalled.

Now that you understand the various types of snapshot triggers, find out which ones are available in your scan tool. Then next time you are faced with a particular diagnostic dilemma, you can choose the best and most efficient method of data acquisition to suit your purposes. Happy Hunting!

Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.

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