Abstract: An interlocking proximity switching device operated by the reciprocating member assembly of a hydraulic slave cylinder controlling the operation of the release mechanism of a motor vehicle mechanical clutch, the slave cylinder being hydraulically connected to a master cylinder having an input member operable by the motor vehicle clutch pedal. The switching device is adapted to close the motor vehicle starter relay circuit for a position of the slave cylinder output member, piston and/or push-rod, corresponding to full release of the clutch, such that the clutch must be fully released in order to allow starting the motor vehicle engine. Preferably, the switching device takes the form of a proximity transducer and of an appropriate circuitry for detecting full release of the clutch mechanism, irrespective of the amount of wear of the clutch.