Copyright Goodheart-Willcox Co., Inc. Auto Fundamentals 206 Current from the ignition switch enters the ignition control module after it passes through the coil. Within the module, the current enters a power transistor, as in Figure 10-5A. The power transistor acts like a conductor, allowing full current to flow in the circuit. This begins the buildup of the magnetic field in the coil. When the power transistor is signaled by the triggering device (explained later in this chapter) and other module circuitry, it becomes an insulator. Since current cannot flow through an insulator, current flow through the coil primary circuit stops, Figure 10-5B. When current flow stops, the magnetic field collapses, creating the high-voltage current in the secondary windings. After the coil collapse is complete, the process is repeated as cur- rent flow through the power transistor begins again. Electronic Triggering Devices Electronic triggering devices send a signal current to the ignition control module, which then breaks the primary circuit. The parts of the triggering device do not wear, which gives it a much longer life expectancy than contact points. Since the triggering device does not wear, engine timing does not change. This improves engine performance, emissions out- put, and reliability. There are three types of triggering devices currently in use: • Magnetic pickup sensors. • Hall-effect switches. • Optical sensors. Most triggering devices are installed in or on the engine block and are operated by the rotation of the crankshaft or camshaft. Some triggering devices are operated by rotation of the distributor shaft. Magnetic Pickup Sensors Magnetic pickup sensors consist of a stationary pickup coil and a rotat- ing toothed assembly called a reluctor, or trigger wheel. On engines with distributorless ignition, the pickup coil is installed in the engine block or the engine front cover, and the reluctor is part of the crankshaft or mounted on the engine flywheel. See Figure 10-6. The magnetic pickup sensor on older engines is mounted in the distributor and reacts to distributor speed, which is one half of crankshaft speed. Battery To spark plugs Ignition switch Coil magnetic field collapses, coil fires Ignition control module Power transistor off Signal to module Other module circuitry Triggering device (pickup coil or Hall-effect switch) No current flow Ignition switch Current flows in circuit Power transistor on Ignition control module Other module circuitry Triggering device (pickup coil or Hall-effect switch) Magnetic field builds up in coil To spark plugs Battery A B Goodheart-Willcox Publisher Figure 10-5. A—Ignition systems use a power transistor to control power flow through the coil(s). The power transistor is controlled by a low-voltage signal from the pickup coil. In this figure, the power transistor is on and current flows through the coil, building up the magnetic field. B—In this figure, the power transistor is off, stopping current flow to the coil. This causes the coil’s magnetic field to collapse and discharge through the secondary terminal. Reluctor Pickup coil Goodheart-Willcox Publisher Figure 10-6. A magnetic crankshaft position sensor and reluctor located at the crankshaft.