Copyright Goodheart-Willcox Co., Inc. Auto Fundamentals 208 rotor plate turns, it interrupts the light beam from the LED to the photosensitive diode. When the photosensi- tive diode does not detect light, it sends a voltage signal to the ignition control module, prompting it to fire the coil. Distributor Ignition System The distributor ignition system has an engine-driven distributor to distribute high voltage, Figure 10-9. Most distributors also have components to create high voltage. The distributor is usually gear driven from the camshaft. A few distributors are driven by the camshaft timing belt. The last engine with a factory-installed distributor ignition system was manufactured in 2008, but many distributor-equipped vehicles are still on the road. The main components of a distributor ignition system are as follows: • Distributor housing—An aluminum casting that encloses the other distributor parts. To keep ignition timing from changing, the housing is firmly attached to the engine. Bushings or bearings allow the distributor shaft to rotate freely in the housing. • Distributor shaft—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 engine speed. The rotor is installed on the top of the shaft, and the lower end of the shaft may drive the oil pump. The shaft contains the rotating part of the triggering device. • Triggering device—The triggering device can be a magnetic pickup sensor, Hall-effect switch, or optical sensor as discussed earlier in this chapter. The stationary parts of the device are mounted on a plate inside of the housing and the rotating part is attached or built in to the shaft. The rotating and stationary parts create a signal that is fed to the ignition control module or the ECM. • Distributor cap, rotor, and spark plug wires—These components work together to distribute secondary voltage to the plugs. They are discussed in more detail later in this chapter. On some engines, the distributor has no triggering device and is used only to distribute the high-voltage electricity to the plugs. Distributorless Ignition Systems As the name suggests, a distributorless ignition system has no distributor. It uses a crank- shaft speed and position sensor, which is a magnetic pickup sensor, a Hall-effect switch, or an optical sensor. The crankshaft sensor is mounted on or in the engine block. Some dis- tributorless systems have a second sensor on the camshaft. The crankshaft sensor performs the same job that a pickup coil, Hall-effect switch, or optical sensor does in a distributor, matching the firing of the spark plug to the piston compression stroke. The advantage of this system is the elimination of the distributor assembly, rotor, and distributor cap. An electrical signal is generated whenever the crankshaft is rotating. This signal is sent to the ignition control module and/or the ECM. The ignition control module or ECM uses this signal to determine the position of each piston in the engine. On systems with crankshaft and camshaft sensors, both sensor readings are used to determine piston position. The sensor input may also be used by the ECM to determine engine rpm and the amount of ignition timing advance. Distributor housing Distributor shaft Rotor Distributor cap Goodheart-Willcox Publisher Figure 10-9. An exploded view showing distributor components. The distributor shaft is driven by the camshaft through the gear shown on the bottom. The rotor is pressed into the top of the shaft. The triggering device is installed under the plastic cover in the housing.