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Diagnostic voyeurism

Tuesday, February 1, 2011 - 01:00
Most scopes today have the capacity to record several minutes or even hours on one screen, then allow the user to zoom in on the trace to see more detail.

Diagnosing drivability issues often starts with a check for diagnostic trouble codes (DTCs) and, if available, a review of any stored freeze frame information. This info is a snapshot the engine control module (ECM) took of the conditions present at the time of failure. What we don't know, however, is whether the car was being operated in a steady state just before the fault or if it was accelerating or decelerating at the time. It does give us a place to start, though.

With that information in hand, the next step might be a test drive of the car in an attempt to duplicate those same conditions. We call in the recording and/or graphing functions of the scan tool (if it has them) to track operating parameters before, during and after the problem. Instead of a snapshot, we now have a movie that we can review frame by frame looking for the problem's cause. The drawback here is in the time it takes to refresh the recorded data. The more parameter identifiers (PIDs) we want to monitor, the longer the delay in their updates, and the more likely we may be of missing the problem entirely.

If that's the case, there is another option: the digital storage oscilloscope, or DSO. Think of the DSO as a high-end slow motion video camera recording at 2,000 frames per second (fps) versus a more typical digital camera recording at 30 fps. The DSO will catch and display glitches that the scan tool might miss.

This cam/crank sensor capture shows the synch between the sensors, and the electrical health of the signals.

Coupled with a little imagination and a few accessories, a scope also can be used to test almost anything on the car. It can do it faster and more efficiently than using more traditional testing methods. Here are a few examples of how a scope can speed up and enhance your diagnostic process.

A Voltmeter Video Recorder
A scope is a voltmeter that traces voltage over time. This allows the user to display a moving image of whatever he is monitoring on the DSO's screen. Most DSOs today have the ability to store multiple screens, providing even more diagnostic power by allowing the user to capture the fault in the DSO’s memory and then reviewing the information to isolate the culprit. Any voltage input or output from the ECM can be monitored using a scope.

By monitoring key inputs/outputs on this Stratus secondary air injection system, I could test the system in operation, even though it only lasted a few seconds. The pump was on but no flow was being reported by the MAF sensor.

A common example is tracing the patterns produced by engine management sensors like the throttle position sensor (TPS). A scan tool reports only what the ECM is seeing or doing. If the sensor information is inaccurate, the problem still remains as to why it's inaccurate. Is it the sensor, the ECM or the wiring in between? Is the problem consistent or varying? It might be a result of electrical interference from the charging or ignition systems, or even be caused by a totally unrelated system. A scope will show you exactly what's happening and provide quick direction in locating the fault.


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