A signal generator has a Hall-effect sensor 14 and a magnet 13 which are disposed inside a distributor housing. A vane 10 which is secured to a distributor shaft passes through a gap between the Hall-effect sensor and the magnet when the shaft rotates. An ignition coil 9 is disposed inside the distributor housing in a location such that the magnetic field which it generates combines subtractively with the magnetic field generated by the magnet in the vicinity of the Hall-effect sensor. Magnetic by-passes 17, 18 are disposed inside the housing to cause leakage flux from the ignition coil to by-pass the Hall-effect sensor.