(57) A system comprising:
a first sensor (107) for detecting the teeth (134) and a gap in the same on a pulley
(108) fitted to the drive shaft (111);
a second sensor (112) for detecting interruptions (135 and 136), a first of which
differs from the others, on a disk (109) rotating at half the speed of, but in time
with, the drive shaft (111);
first means (156), which, upon startup of the engine, compute and memorise the actual
relative angular position of the pulley (108) and the disk (109);
second means (152) for computing the absolute angular position of the drive shaft
(111), for enabling the electronic ignition and injection systems; and
third means (153) which, in the event of failure of the first sensor (107), and on
the basis of the second sensor (112) and the signal processed by the first means (156),
compute a presumed angular position of the drive shaft (111) for enabling the electronic
ignition and injection systems.
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