156 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. decrease the engine speed. To aid in reducing misdiagnoses, manufacturers add programming to the ECM to detect pat- terns consistent with rough roads. To further reduce misdi- agnoses, the ECM monitors the wheel speed sensors in the anti-lock brake system. Note: The ECM on some vehicles equipped with an overdrive automatic transmission will disable the torque converter clutch when a misfire is detected. Ion-Sensing Ignition System Some vehicles use an ion-sensing ignition system to determine if complete combustion has occurred, thus reducing false misfire problems. The main component of the ion sensor is a capacitor that sends a low-voltage sig- nal across the plug electrodes without producing a spark. Unburned hydrocarbons have less resistance than exhaust gases. If the gases in a cylinder have high carbon content, more current will flow across the plug electrodes than if the carbon content is low. The signal produced by this current flow is analyzed by the ECM and used to determine whether complete combustion has occurred. The presence of excessive unburned hydrocarbons is a good indication that a misfire has occurred. The ion sensor can also determine whether spark plug knock has occurred. An explosion (knock) in the cylinder produces more unburned hydrocarbons than normal combustion. Summary Fuel is ignited in a diesel engine by the heat of com- pression. A gasoline ignition system uses a high-voltage spark to ignite the air-fuel mixture. This ignition system is composed of primary and secondary circuits. The primary circuit creates the high-voltage electricity using a coil, igni- tion module, and triggering device. The secondary circuit distributes the high-voltage electricity created by the pri- mary circuit and creates the spark in the cylinders. Secondary components are made of materials that direct the high-voltage coil output to the spark plugs with- out allowing any of it to leak away to ground. Any part of the secondary system may contain resistors to reduce elec- tromagnetic interference. Spark plugs are designed with a gap between the center and ground electrodes. The high voltage developed by the ignition system allows current to jump this gap to create a spark that ignites the air-fuel mixture. Spark plug gaps vary according to engine designs. Spark plugs are made in many sizes, threads, and heat ranges. Most plugs contain an inter- nal resistor for suppression of electromagnetic interference. Ignition timing on distributorless ignitions is not adjustable, but can be checked with a scan tool as an aid to diagnosis. The spark is advanced or retarded by the ECM based on operating conditions. The OBD II protocol requires that the computer con- trol system monitor the engine for misfiring. The misfire monitor is an internal function of the ECM. It can detect a misfire within 200 crankshaft revolutions. Review Questions—Chapter 8 Answer the following questions using the information provided in this chapter. 1. Th e ignition coil converts _____-voltage electricity into _____-voltage electricity. 2. What is the function of the ignition control module? 3. A Hall-eff ect switch: (A) is an ignition system triggering device. (B) operates by generating a magnetic fi eld with a shutter. (C) modifi es a signal from the ignition control module (D) Both A and B. Figure 8-25. The OBD II misfire monitor separates crankshaft speed into even samples of 200 rpms. If a sample significantly drops, the ECM sets a type A trouble code and flashes the MIL. If the misfire is relatively minor, the ECM sets a type B code and illuminates the MIL. However, if the condition goes away, the MIL goes out, but the code remains stored in the ECM. Type A Misfire 14 12 18 0 200 200 200 200 200 200 200 200 Type B Misfire 195 193 200 200 200 200 200 200 200 200 200 200