Copyright Goodheart-Willcox Co., Inc. Chapter 10 ∣ Ignition Systems 205 Older cylinder coils have primary terminals marked with a (+) and (–). The plus sign indicates positive and the minus indicates negative. The wiring harness connectors of newer coils are designed so they cannot be installed incorrectly. The coil must be installed in the primary circuit according to the way the battery is grounded. This alignment of the positive and negative terminals is called polarity. If the battery’s negative terminal is grounded, the negative terminal of the coil must be connected through the ignition control module or distributor to ground as applicable. This is done to ensure the correct polarity at the spark plug. Actual Coil Output Even though the voltage output of some coils can exceed 100,000 volts, the coil will only build up enough voltage to produce a spark. This may be as low as 2000 volts at idle on an older vehicle without emission controls or as high as 60,000 volts on a new vehicle with the engine operating under a load and using the leanest possible air-fuel mixture. Methods of Current Interruption To cause the coil’s magnetic field to collapse, the current flow through the primary wind- ings must be interrupted instantly and cleanly with no flashover (current jumping or arcing across space) at the point of disconnection. Older vehicles interrupted current flow with mechanically operated contact points. All modern ignition systems interrupt coil current flow with electronic components. Simple Electronic Ignition The schematic in Figure 10-5 is a simple electronic ignition circuit. The ignition control module contains electronic components that control current flow through the coil’s pri- mary windings. Ignition control modules process inputs from other ignition components. Almost all modern ignition control modules provide a speed and crankshaft and camshaft position signal to the engine electronic control module (ECM). On many vehicles, the ignition control module is part of the ECM. In some cases, the ignition control module is installed on the engine firewall or inner fender to protect it from excessive engine heat. Other modules are located in the distributor, installed outside on the distributor body, or constructed as part of the coil assembly. Iron core Secondary winding (voltage) Induced current flow Battery Current flow from battery Primary winding (voltage) Magnetic field Magnetic field has collapsed Primary connections A B Primary Winding Operation Secondary Winding Operation 1 2 Deere & Co. Goodheart-Willcox Publisher Figure 10-4. A—Wiring schematic illustrating how the coil induces current flow in a secondary coil. B—Ignition coil operation. 1–Primary winding. Current flow through the primary winding builds up magnetic field. 2–Secondary winding. High-voltage current developed when magnetic field collapses. Current leaves the coil on its way to the spark plugs.