Case Study: Clutching at straws
24 September 2014 
13 plate Audi 1s don’t often go wrong but one was recently brought into the workshop which was a dual control driving instructor’s vehicle, so time was in short supply.
Clutch and brake pedal switches have been around for some time and their purpose has expanded for several reasons. There are advantages of short stroke over square engines, with the piston travelling a shorter distance, especially with diesel-powered turbo vehicles producing very high lowdown torque outputs. The need to smooth out gear change stress on the transmission is helped with torque reversal strategies. The clutch switch provides an input request, reducing fuelling and re-introducing fuelling by smoothing out fuelling quantity. This can be achieved in several subtle ways of phasing injector control.
Vehicle starting safety has also introduced the need for clutch pedal operation monitoring, leading to Stop/Start requirements. Simple stepped voltage change has been replaced with Hall sensors.
Our particular problem vehicle was compounded by the need of a block booking. The symptoms were straightforward enough – no crank start, correctly attended and predicted to be a clutch pedal sensor by the AA. I say predicted because there was no DTC to suggest any problems!
The hunt for clues
The next, dare I say, sensible action would be to test its output with our Pico but David decided upon some lateral thinking. Based on the principles of Hall sensor operation, a change in the magnetic field across the Hall IC leads to a change in the applied voltage. Enquiring as to the contents of my tool box, I produced a small rectangular magnet.
With the sensor removed from the clutch master cylinder, it was possible to extend the wiring so that the magnet could be passed across it whilst monitoring live data.The result confirmed a binary change from 0 to 1. Conclusion – the sensor was OK, the problem was an operational environment issue within the hydraulic cylinder.
In view of the urgency, a VOR order was placed for a replacement hydraulic cylinder, see Figure 2. Unusually for TPS, the wrong part arrived so more joint lateral thinking, why not leave the magnet in a binary 1 position allowing the starter to be engaged? So, it was a temporary repair allowing use of the vehicle. The only down side was that a DTC was present preventing operation of the stop/start function, not a bad swap though. The correct part was in short supply, eventually when fitted all DTCs cleared and full operation was restored.