Why is this necessary? Are we duplicating tests?
So far all the information to base our diagnoses on has been obtained via our scan tool, which in turn displays data interpreted and surrendered by the ABS control unit. This introduces a number of variables in the interpretation of data with no verification.
The ABS control unit collects speed signal information, interprets there is an error with the signal and then transmits the error to the scan tool. The scan tool introduces further variables based upon system compatibility and update speeds.
Based on the action plan, we connected the PicoScope kit in order to monitor the speed signal from the right rear wheel speed sensor in real time.
Results
The wheel speed sensors fitted to this vehicle fall under the active design, presenting a measurement challenge to technicians, given their output varies from as little as 6 mA up to 15 mA approximately. With such a low output, the test procedure for these sensors ideally requires the use of a milliamp clamp.
Unfortunately, a milliamp clamp is very expensive (over £400), and given the resolution required at such low output it is unlikely a return on investment would be seen for some time given the minimal use of such clamps. All is not lost though, as there are numerous alternatives with which to measure these wheel speed signals.
Using either the Pico Technology TA018 20 A / 60 A (low amps) current clamp (or better still the Pico TA234 30 A [low amps] current clamp), in conjunction with a current multiplier and relevant Pico breakout lead, we can measure these low currents.
Using Pico breakout leads we extend the right rear wheel speed sensor wires and wind the signal wire through the jaws of the current clamp ten times, amplifying the speed signal by a factor of ten. The amplified current levels that will now passing through the jaws of our clamp will now fall within the 60 mA to 150 mA range, and be easily detected.
Using the filtering feature of the PicoScope software we are able to detect and measure the switching current of the wheel speed sensor with either of the above current clamps. Activating Lowpass Filtering under the Channel Options button of the channel we wish to filter (checking the box adjacent to Activate Filtering) applied a 1 kHz lowpass filter by default to our selected channel. This setting tells the software to reject all frequencies above 1 kHz and only display lower frequencies (this technique removes unwanted noise from the captured data).
For more information on filtering options, the following tutorial will help with the application of a variety of filters within PicoScope: https://www.picoauto.com/library/training/filtering.
The image below demonstrates the principle of a current multiplier where 5 mA is fed through a coil passing through the jaws of a current clamp 10 times.
The measured current is therefore 5 mA X 10 = 50 mA.
