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Text Box: Diagnostics is a fundamental part of an automotive technician’s work. As automotive systems become increasingly complex there is a greater need for good diagnostic skills and tools such as scanners, multimeters and oscilloscopes. A scan tool can read fault codes as well as voltages and other signals by interpreting the serial data from the ECU. The drawback of using a scan tool to diagnose sensor problems and other faults is that serial data is only the ECU's interpretation of those signals which may not necessarily be what is really happening in its input and output circuits. Many problems won't even set a fault code, so diagnosis may be difficult when confronted with a drivability or intermittent issue. A common mistake is a fault finding procedure based on "not-present" or "sporadic" fault codes, which often leads to a inconclusive diagnosis. A multimeter used to take measurements from the vehicles electrical system is limited due to the increasing use of modulated signals, even on analogue systems. An example is a duty cycle to actuate a valve such as an idle air valve or an inductive sensor output as a sine wave, as seen on ABS wheel sensors.                                         

Text Box:              An oscilloscope  translates  electronic signals into a pattern or waveform on a screen. As the waveform is traced across the screen, it creates an image of the signal's characteristics, revealing powerful diagnostic information to the technician.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        

Text Box:  Fundamentally  the oscilloscope displays voltage vertically and time horizontally. This gives us the ability to visualize the five basic types of electronic signal: direct current (DC), alternating current (AC), fixed pulse width (variable frequency), pulse width modulated and serial data. Of course skill and experience are required to interpret these waveforms. However modern digital oscilloscopes (DSO), such as incorporated into Texa’s Axone range, have a much wider range of functions, making for faster and more accurate measurements. An example would be the waveform library, allowing the technician to compare signals with known correct samples or the ability to slow and freeze-frame a measurement. Texa’s Axone includes many guided tests which will make setup and comparisons automatically according to the vehicle and system or component being tested. Selecting a component from the wiring diagram switches the user to the oscilloscope function where it is correctly configured for testing that particular component and the waveform  then analysed for faults. In this way, the technician is able to work in a structured diagnostic procedure, working from fault codes, through live data to signal checking.         

Text Box:         Fig. 2  Wheel speed  sensor signal displayed on the Axone 3

Text Box:              Using ABS wheel sensors as an example, we can see from fig. 2 a typical sine wave sensor output signal. As the teeth pass the sensor, voltage is induced into the sensor  and can be seen as a change in the vertical axis (voltage) and the frequency of the teeth (wheel speed) can be seen in the horizontal axis (time). The undulating peak voltage is characteristic of a bent toothed wheel or target as voltage is dependant on the distance from sensor to toothed wheel (air-gap). A damaged tooth on the target will show as a missing part of the waveform.              In the case of intermittent faults, the sampled waveform can be studied whilst checking wiring and connectors either in situ or on road test (using Texa’s GPRS module, available soon) . Poor connections, short circuits etc. are indicated as disruptions in the wave pattern which may happen too quickly to register in live data or set a fault code. Electrical interference or “spiking” from other electrical systems such as charging and ignition, can be seen as “noise” in the signal. This can often lead to misinterpretation by the ECU or fault code setting due to an implausible signal.              In all these cases, simple fault code checking or data capture may not have highlighted the fault or possibly lead to incorrect diagnosis/replacement. The oscilloscope is an indispensible tool to  the technician when faced with those “problem” jobs.   

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Last modified: 06/14/08