Chapter 12 Electronic Control Systems 273 Copyright by Goodheart-Willcox Co., Inc. Crankshaft position sensor Bolt Crankshaft sensor harness Figure 12-11. This speed and crankshaft position sensor is installed on the front timing cover. Every modern vehicle has at least one of these sensors. They may be installed in several places on the engine, depending on the vehicle manufacturer and the type of engine. (General Motors) MAF sensor MAF sensor Air temperature sensor Figure 12-12. The mass airflow sensor, or MAF, is usually installed on the air duct leading to the throttle plate. A—This sensor is attached to the air duct and can be removed without removing the ductwork. B—This sensor is part of an assembly that can only be removed by removing the duct. Note the air temperature sensor in the duct immediately after the MAF sensor. A B Barometric Pressure Sensors A barometric pressure sensor, or BARO sensor, mea- sures the pressure of the surrounding air and converts it to an electrical signal. This sensor is very similar to and oper- ates in the same manner as a MAP sensor. Some vehicles have both a MAP sensor and a BARO sensor. Engine Speed Sensors Some engines make use of an engine speed sensor. Most late-model systems use sensors mounted on the camshaft or crankshaft, while the distributor provides the engine speed signal for some systems. The operating principle of the engine speed sensor is identical to that of the vehicle speed sensor. The toothed wheel may be turned by the crankshaft or camshaft, Figure 12-11, or it may be part of the distribu- tor pickup assembly. Many transmission control designs use the engine speed sensor input instead of having a separate transmission input shaft sensor. Mass Airflow Sensors Some vehicles use a mass airflow sensor, or MAF sensor, installed in the air intake system. See Figure 12-12. This sensor measures the amount of air entering the engine. Most MAF sensors use a heated wire located in the intake air stream. The ECM sends a reference voltage through the wire, causing it to become hot and increasing its resistance to current flow. Airflow into the engine cools the wire. This reduces the wire’s resistance and allows more current to flow. Changes in current flow change the voltage in the circuit. The ECM interprets this voltage change as a change in airflow. Other MAF sensors use a mechanical valve called a vane, which is moved by airflow. The valve is connected to a variable resistor using a sliding contact. Some vehicles use a MAF sensor that operates from turbulence created by a calibrated restriction in the air intake passage. These are called Karmann vortex sensors. Oxygen Sensors The oxygen sensor, or O2 sensor, is a vital part of all modern engine emission controls. Input from the oxygen sensor may also be used to control some transmission func- tions, especially when a rich condition occurs. In most com- mon designs, the oxygen sensor produces electricity through