Copyright Goodheart-Willcox Co., Inc. Chapter 10 ∣ Ignition Systems 207 When a tooth of the rotating reluctor aligns with the stationary pickup coil, a small alternating current is generated in the pickup. The alternating current is an analog signal that is converted to a digital input to the ECM. The current produced is small (about 250 millivolts), but the pulse can be read easily by the ignition control module circuits. When the ignition control module circuitry reads the pulse, it turns the power transistor off and interrupts primary current to the ignition coil, causing it to fire. An air gap between the rotating and stationary pickup coil prevents physical contact and eliminates wear. The air gap is critical on all magnetic sensors and must be set to specifications. Hall-Effect Switch The Hall-effect switch can be mounted in the distributor or at the crankshaft, Figure 10-7. The Hall-effect sensor is a thin wafer of semiconductor material with voltage applied to it constantly. A mag- net is located opposite the sensor. There is an air gap between the sensor and magnet. The magnetic field acts on the sensor until a metal tab, usually called a shutter, is placed between the sensor and magnet. This tab does not touch the magnet or the sensor. When the tab interrupts the magnetic field acting on the sensor, the sensor’s output voltage is reduced. This signals the ignition control module to turn the power transistor off. This interrupts primary current to the ignition coil, causing it to fire. Optical Sensor An optical sensor is used in some ignition systems, as shown in Figure 10-8A. This type of sensor consists of a rotor plate, a light-emitting diode, and a photosensitive diode. The rotor plate, Figure 10-8B, has many slits in it through which light passes from the light-emitting diode (LED) to the photosensitive diode (light-receiving diode). As the Distributor shaft Magnet Hall- effect sensor Hall- effect sensor Metal tab Metal tab A B Magnet Bosch Figure 10-7. A—A magnetic field can act on a Hall-effect sensor. B—When the metal tab attached to the distributor shaft rotates between the magnet and Hall-effect sensor, the magnetic field is interrupted. The ignition coil sends high voltage to the distributor any time the magnetic field is interrupted. Rotor head Sealed cover Light-emitting diode Photo diode Wave- forming circuit Rotor plate A 120° signal slit for No. 1 cylinder 1° signal slit 120° signal slit B Nissan Figure 10-8. A—An optical sensor in this distributor uses an LED to send a beam of light to a photo diode through slits in a rotor plate. B—A rotor plate used with the optical sensor. Note the spacing of the slits.