Chapter 12 Electronic Control Systems 271 Copyright by Goodheart-Willcox Co., Inc. Terminals Spring Terminal Switch body Diaphragm Fluid Contacts Contacts Diaphragm Fluid B A Figure 12-5. Pressure switches transmit information to the ECM, usually telling it what gear has been selected by the driver or the hydraulic control system. A—This switch is a pass-through type that connects the battery with another electrical device. B—This switch grounds against the case to complete the cir- cuit. (General Motors) Mounting bracket Diaphragm Contacts Terminals Fluid Thermal element Switch body Figure 12-6. This temperature sensor is an on-off switch used to signal the ECM when the transmission fluid reaches a cer- tain temperature. A bimetallic strip completes the circuit at a set temperature. (General Motors) Resistor temperature sensor Figure 12-7. This temperature sensor changes its resistance in response to temperature changes. Resistance changes affect the amount of current flowing in the wire, which in turn affects the voltage sent to the ECM as a signal. (Ford) open or normally closed. When the transmission fluid reaches a certain temperature, the switch position changes and signals the ECM. These switches were widely used on older transmissions to warn of overheating. Other temperature switches prevented converter lockup clutch application until the transmission reached normal operat- ing temperature. Some temperature sensors are resistor-type units that send a varying electrical signal based on transmission fluid temperature changes to the ECM. Figure 12-7 shows a resistor temperature sensor. In some designs, current flows through the ECM, through the sensor, and then to ground. In other designs, a reference voltage is sent to the sensor and grounds through the ECM. In either case, any change in fluid temperature causes a change in the resis-tance of the sensor. Changes in sensor resistance affect the amount of current flow and, therefore, the voltage in the sensor circuit. The ECM reads this change in voltage as a temperature signal. A resistor sensor allows the ECM to monitor transmis- sion temperature and, therefore, fluid viscosity. Monitoring changes in viscosity helps the ECM modify shifts for maximum smoothness and durability. In addition, the sen- sor input allows the ECM to compensate for transmission overheating. If the transmission fluid begins to overheat, the ECM can increase pressures to prevent holding member damage. Either type of temperature sensor may be used to signal the ECM that the transmission fluid is overheating. If the ECM receives an excessive temperature signal over a certain period of time, it will illuminate the dashboard warning light. Figure 12-8 shows an assembly that includes both the pressure switches and a temperature sensor. Input Speed Sensors Some transmissions and transaxles have an input speed sensor, sometimes called a turbine speed sensor. This sensor tells the ECM how fast the transmission input shaft is turning. Input speed sensors operate in the same manner as vehicle speed sensors. The toothed wheel is mounted on the input shaft and the magnetic pickup is mounted to the case. Figure 12-9 shows a typical input speed sensor.