ABS Sensor Fault - Monday 13th October 2014

Hooking up the Picoscope
to ABS Sensor Input

Today we were assigned to the Practice ABS Fault Task which involved the use of a Picoscope, an Autel Diagnostics Scanner and a multimeter.  The task was somewhat accelerated, due to the fact that the vehicle we were working on was already suspended on axle stands at the front end.  The vehicle in question was a Volvo S40.  With all the relevant vehicle data required for the task gathered in advance, we then set about removing the 2 front road wheels in order to clearly access the 2 front wheel sensor input/output points.  Once located we then began the internal hunt for the EOBD connection port for the Autel Diagnostics Scanner, in order to clear all previous fault codes, re-run the engine and see what revised fault codes appeared.  The scanner worked well on this vehicle and we were able to view live sensor data and also retrieve a code (ABS-311 FL Sensor).  Isaac was out the passenger door like a whippet and immediately noticed the NSF ABS Sensor was disconnected.  With the connection re-established we duly cleared the codes and re-started the engine under a new search.  This came up as "No Codes Found" and the results duly recorded on our Job Cards.  We then used a multimeter to measure the resistance of the sensor itself and compared the results to the expected readings. The results were satisfactory and were also recorded.

Measuring Resistance of ABS Sensor
(Expected 1.734 KOhms)

Autel Diagnostics Scanner

Picoscope Pattern ABS Sensor
(Road Wheel Rotating)

Picoscope Pattern ABS Sensor
(Brakes Being Applied and Feathered)

We were then required to obtain Sensor output voltages which required the use of a Picoscope.  This is a very handy piece of kit and I am pleased to report that I am actually coming to grips with it! From the on-board examples contained within the software, we were able to ascertain that expected voltage readings should be in the region of between 1 and 2 Volts.  By using a marker "drag-down" facility on the live data graph, we were able to precisely pinpoint the relevant output Voltages at 1.829V NSF and 1.809V OSF respectively.  These fell within the expected parameters and were duly recorded on the Job Cards accordingly.  We were then required to obtain Oscilloscope patterns of a sensor with wheels rotating and also with brakes being applied.  This was initially hampered by a dodgy BNC test lead, however once swapped the patterns were duly obtained and, with due regard for the welfare of trees and printer ink consumption, they were diligently reproduced on lined paper with a pen, pencil and ruler for inclusion within our portfolio's.  The results obtained from the OSF also threw up a surprise, as under a "Zoom-In" facility, we were able to identify a damaged tooth on the respective Reluctor Ring!  With all of our results recorded and compared to manufacturers specifications, we were then able to be signed off as complete.  The remaining time was spent reattaching the road wheels to specified torque and tidying and accounting for our tools and equipment.  A very interesting and enjoyable task indeed.