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.
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.
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.
PAGE 2Checking the relationships between various signals is done easily with a scope. For example, a common scope test is the inspection of the camshaft and crankshaft position sensors (CMP and CKP, respectively). With just a few minutes of preparation, the scope can show you the signal integrity (voltage level and ground side voltage drop) and signal synchronization. If incorrect, it also will show you if the synch is varying (as might occur when you have a worn timing chain or loose timing belt) or steady (an indication that the timing belt or chain is off a tooth or more).
A scope can be used to monitor several variables at once to make the single test yield the most results. For instance, I diagnosed a Dodge Stratus with a secondary air injection system code for "insufficient flow." The system only functions when the engine is cold, and then for only 20 to 30 seconds. It uses a dedicated mass airflow sensor (MAF) as the feedback to the ECM, and if the ECM does not see the airflow expected, it records the fault.
By using my scope to monitor the ECM command to the air pump relay, current flow through the pump itself, and the signal from the MAF back to the ECM, I quickly eliminated all electrical causes and isolated the fault to a restricted passageway in the 30 seconds I had to work with. Compare that to the time it would have taken to test all the individual elements separately. Even if I had disconnected the line at the combi-valve to see if I had airflow, I would not have known if the airflow was sufficient or whether or not the MAF feedback to the ECM was correct.
Engine Mechanical Condition
Troubleshooting misfires or erratic running is a problem we deal with routinely. A cylinder that isn't sealing properly or erratically can make this task a bit more complicated. That is, if we don't have a scope.
There are several methods for testing the engine's health using a DSO and the appropriate accessory. One accessory you might already own is a high amp clamp. This tool reacts to the magnetic field present in any conductor when current is flowing, and converts that into a voltage signal the DSO can trace. You
may have used it several times to check starter current draw or in performing charging system tests with your digital multimeter (DMM).
And you can use it to check for a low compression cylinder when you send that voltage signal to your scope instead. Disable the fuel and ignition systems to prevent the engine from starting, place your amp clamp around either the positive or negative battery cable. Then crank the engine. The resulting starter current draw is traced on the scope's screen and the time setting can be adjusted to allow you to see the current required by the starter to overcome each individual piston's resistance to movement. If compression is good, current demand will be higher and if it is low, it will be less.
PAGE 3Relative compression testing can also be done using intake manifold vacuum and/or by measuring the pressure pulses passing out of the tailpipe. These tests use specialized pressure transducers which have a variety of applications that I'll touch in a bit.
Checking compression on most four cylinder designs is relatively easy, but accessing the rear bank on many V6 designs can quickly eat into valuable shop time. This technique will quickly tell you if there is a compression related problem, and provide the evidence you can use to validate to your customer the need for additional teardown and/or testing.
Engine testing isn't limited to just cranking compression. Pressure transducers can also be used to measure the changes in cylinder pressure as the engine runs, painting a picture of the entire engine cycle that can be used to quickly pinpoint engine mechanical and breathing problems.
Fuel System Testing
One of the first tests I learned to make with a scope was the fuel pump "current ramp" test. This trace records the current passing through the fuel pump motor and can tell you a lot about the health of the fuel delivery system in one shot. Because the scope measures voltage over time, it can be set to view the current passing through each winding of any electric motor. Nice sharp peaks generally indicate good contacts, while cut off, flat peaks indicate wear in the brushes and commutator strips.
By taking a moment to note the repetitions in the capture, you can check the rpm of the running pump, and of course you can measure total current draw. These two numbers can provide clues to fuel flow through the system. For example, a pump spinning faster than normal at a low current level could indicate a pump that is sucking air while one that draws high current and is spinning slowly can indicate a restriction to flow in the system.
Remember those pressure transducers I mentioned earlier? They can be used to convert pressure to a voltage signal you can see on your scope. Of course, you can measure fuel pressure just as you would with your mechanical gauge (with a calibrated transducer). But you can also test injector flow by looking at the variances in the pattern of each individual injector. Add another channel to monitor the injector for cylinder No. 1, and you can even isolate which cylinder's injector has the abnormal trace by following the firing order.
If your scope has four channels, you can do all this at the same time. I don't know about you, but I'd rather spend my time sitting down with a cup of coffee reviewing the test results that spend it just making the tests.
PAGE 4Ignition System Testing
Ignition system testing with a scope is a staple in this business. After all, it's all electrical. A scope can test current flow through the coil, trace voltage through both the primary and secondary circuits and do it all on one screen. With a competent scope and a little training, a technician can take this information and quickly identify a number of ignition system problems. Not only that, but a really good technician trained in how to interpret the elements of the secondary ignition waveform can tell if there is a problem in an engine's mechanical health, it's fuel system or in it's ability to breathe.
There's a real art to interpreting these patterns, and Mac Vandenbrink has been sharing his expertise with our readers. The secondary pattern is comprised of certain basic elements. The firing line is a measure of the voltage required to ionize and prepare the spark plug gap for spark. Spark kV is a term used to describe the voltage required to overcome the secondary circuit's resistance and initiate a spark across the gap. The burn line is that portion of the pattern that occurs as the spark is crossing the gap. By carefully reviewing how these elements react under idle, load and snap throttle conditions, a technician can tell a lot about what is happening inside the combustion chamber. To learn more from Mac, look up the November 2010 issue on our website at www.motorage.com.
Imagine
Modern scopes are capable of recording hours of input with amazing accuracy. Zoom features allow a capable user to home in on even the smallest of glitches captured in the process. Add accessories that allow the scope to measure vacuum, pressure, high and low current to that user's arsenal and the scope's use as a diagnostic tool is limited only by the his imagination and creativity.
Look for more on how to use a scope to speed up your diagnostic process by joining me on the AutoPro Workshop, the Motor Age online community at http://workshop.search-autoparts.com. There you'll find features and video clips only available online. While you're there, sign up for our monthly Motor Age e-zine, How2. This primarily training/technical publication is delivered directly to your inbox and will help you add more to your paycheck with monthly training videos and other resources.
