148 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. A triggering device signals the ignition control mod- ule. When it is time to collapse the coil’s magnetic field, the module applies a voltage signal to the power transistor. The power transistor becomes an insulator. Since current cannot flow through an insulator, current flow is stopped through the coil primary windings. When current flow stops, the magnetic field around the coil windings collapses, Figure 8-8B. Triggering Device To cause the coil’s magnetic field to collapse, the cur- rent flow through the primary windings must be instanta- neously interrupted. Current flow is broken by the ignition control module based on a signal from a triggering device. Three types of triggering devices are commonly used in electronic ignition systems: pickup coil, Hall-effect switch, and photoelectric sensor. Magnetic pickup sensor. A magnetic pickup sensor consists of a stationary pickup coil and a rotating toothed assembly called a trigger wheel, or reluctor. On most engines with distributorless ignition, the pickup coil is mounted on the engine block and the trigger wheel is located on the crankshaft. See Figure 8-9. On some vehicles, the trigger wheel is mounted on the flywheel, Figure 8-10. In this case, the pickup coil is mounted on the transmission or transaxle bell housing. On older engines, the magnetic pickup coil is mounted in the distributor and reacts to distributor speed, which is one-half of crankshaft speed. The relationship of the pickup coil and the trigger wheel is illustrated in Figure 8-11. As the trigger wheel rotates, each tooth on the wheel passes the pickup coil, cre- ating a magnetic field. As the shaft rotates and each tooth of the wheel passes the pickup coil, a magnetic field is created. This produces a voltage signal that is sent to the ignition control module. The module then interrupts current flow in the ignition coil’s primary windings, causing the coil to fire. Hall-effect Switch. In some electronic ignition sys- tems, a magnetic field is created by current flowing through an electromagnet with two separate poles. The electro- magnet is called a Hall-effect switch. In Figure 8-12, it is mounted on the stationary distributor plate. Current to the Hall-effect switch flows through the ignition control module. The switch assembly also has a shutter assembly, sometimes called a chopper. Some shutter assemblies rotate with the distributor shaft. In many cases, the shutter assembly is part of the distributor rotor. The magnetic field is interrupted every time a shutter blade passes between the two poles of the Hall-effect switch. This causes a voltage fluctuation, which is read by the igni- tion control module. The module then shuts off current to the coil’s primary windings, causing the coil to fire. Note: Some engines have a Hall-effect switch and shutter installed on the front or rear of the crankshaft. The Hall-effect principle is the same, but the switch may need to be adjusted with a special tool during reassembly. Figure 8-10. This trigger wheel is installed on the flywheel. As the flywheel rotates, the holes in the wheel create a magnetic field in the sensor pickup. Trigger wheel Figure 8-9. A magnetic pickup sensor. The pickup coil is mounted in the engine block and the trigger wheel is attached to the crankshaft. (Goodheart-Willcox Publisher) Reluctor Pickup coil Figure 8-11. The relationship between the pickup coil and trig- ger wheel is shown here. When the trigger wheel passes near the pickup coil, a magnetic field is created in the wire coil. This produces a voltage that serves as a signal to the ignition control module. Ignition control module S N