Chapter 8 Ignition System Fundamentals 149 Copyright by Goodheart-Willcox Co., Inc. Photoelectric Sensor. Some electronic ignition systems generate a triggering pulse by the use of a photoelectric sensor, Figure 8-13. This sensor consists of three major parts: • LED, sometimes called an emitter, that produces a light signal. • Photodiode, sometimes called a detector, that receives the light signal from the LED. • Rotating shutter assembly that breaks the light beam between the LED and photodiode. The LED produces light whenever the ignition is on. The shutter is attached to the rotating shaft and turns with it. A series of openings, or slits, in the shutter make and break the light beam from the LED to the photodiode. The photodiode converts the light pulses into an electrical signal that is read by the ECM. The slits in the shutter assembly are arranged so that the ECM can determine crankshaft position, engine speed, and which cylinder is coming up on compression. On most systems, at least one slit is dedicated to each cylinder with a master slit to determine when the firing sequence begins again. Many shutter assemblies are made with 360 slits, one for each degree of rotation, to ensure the fastest response to changes in engine speed. On vehicles equipped with OBD II, these speed inputs are monitored by the ECM for misfiring. This process is explained later in this chapter. Distributor-Type Ignition Systems The distributor-type ignition system uses an engine- driven device to send high-voltage electricity to the spark plugs at the proper time. The main components of the dis- tributor ignition system are: • Distributor body. This is an aluminum housing that encloses the other distributor parts. To keep ignition timing from changing, the housing is firmly attached to the engine. Bushings or bearings in the housing allow the distributor shaft to freely rotate. • Distributor shaft. This is a steel shaft connected to the camshaft through a gear or to the timing belt through a sprocket. The shaft always turns at one-half of the Figure 8-13. A typical photoelectric triggering device in a distributor. Note the relative positions of the LEDs, photodiodes, and shut- ter assembly. The position of the slots in the shutter control the reception by the photodiode of light from the LED. The photodiode converts the light to an electrical signal that is sent to the ECM. (Chrysler) Cap Cover Rotating disc Optical unit Timing slots High-data-rate slots Photodiodes Rotor LEDs Figure 8-12. 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 the Hall-effect sensor, the magnetic field is interrupted. The ignition coil fires whenever the magnetic field is interrupted. (Bosch) Distributor shaft Magnet Magnet Hall- effect sensor Hall- effect sensor Metal tab Metal tab A B