Chapter 12 Electronic Control Systems 269 Copyright by Goodheart-Willcox Co., Inc. + – Sliding contact Resistor construction + – High resistance + – Low resistance Material with high electrical resistance Current must travel a long distance through the high resistance material Current travels a short distance through the high resistance material Variable Resistor Operation Figure 12-1. Most throttle position sensors are variable resistors. Changes in the position of the sliding contact as the throttle is pressed vary the amount of voltage reaching the ECM. The throt- tle position sensor is always located at the throttle valve. is used by the computer to control engine operating systems and to control the transmission. Both types of sensors are discussed in the following sections. Transmission Shift Control Sensors On older transmissions, the governor and throttle valve operated the shift valves to change gear ratios. In an electronic transmission, throttle position sensors and vehicle speed sensors are used in place of these valves. The following sections explain the operation of these and other sensors used to help control shifts. Throttle Position Sensor The throttle position sensor, or TPS, is mounted on the throttle body and is connected to the throttle linkage or the throttle shaft. Throttle position sensor input tells the ECM how hard the driver is accelerating. There are two types of throttle position sensors. The most common is the variable resistor type, Figure 12-1. This sensor con- sists of a sliding contact that moves along a resistor wire. Current flows through the wire and the siding contact connection. Moving the throttle plate moves the contact. As the contact moves, the amount of the resistor wire and, therefore, the amount of resistance the current must flow through changes. This change in the amount of resistance in the circuit changes the current flow and voltage through the circuit. The ECM uses this voltage change to calculate the throttle position. Some throttle position sensors are transducers. A transducer is a coil of wire wrapped around a movable iron core. See Figure 12-2. A small current flows through the coil, producing a magnetic field. The iron core is connected to the throttle linkage. Moving the throttle linkage causes the iron core to move inside the coil, affecting the magnetic field. Affecting the magnetic field causes the amount of cur- rent and voltage in the coil circuit to change. The ECM uses this change to compute throttle position. Vehicle Speed Sensor The vehicle speed sensor is mounted on the output shaft of the transmission or transaxle and tells the ECM how fast the vehicle is traveling. Speed sensors consist of a toothed wheel and a magnetic pickup. The wheel is attached to the output shaft and rotates with it, or it is driven by a gear on the output shaft. The pickup is mounted to the transmission case. Figure 12-3 illustrates the relationship of the wheel and pickup when the wheel is installed directly on the output shaft. As the toothed wheel turns, it induces an alternating current (ac) in the magnetic pickup. Since alternating current changes direction, current flow in the pickup is reversed every time one of the wheel’s teeth passes it. The faster the wheel is moving, the more often the current changes direction. The speed, or rate, of the change in cur- rent flow is called the frequency. The ECM measures this frequency and uses it to calculate input shaft speed. Manual Valve Position Sensor The manual valve position sensor provides the sim- plest input to the ECM. This sensor, which is sometimes called the shift lever position sensor, is a set of on-off switches. Each gear shift lever position selected by the driver closes a particular switch in the manual valve position sen- sor, telling the ECM which transmission operating position has been selected. Some manual valve position sensors also operate the vehicle’s backup lights and serve as neutral safety switches. This eliminates the need for separate switches. The sensor may be installed on the valve body or the external manual valve linkage. Figure 12-4 shows a typical manual valve position sensor installed in the valve body. Some manual valve position sensors are installed on the shifter assembly. This type of shift position sensor is called an inductive sensor. The inductive sensor has four wire coils called inductors. Current flows through the inductors when the vehicle is operating. A damping element, similar to the iron core shown in Figure 12-3, is attached to the shift lever