272 Automatic Transmissions and Transaxles Copyright by Goodheart-Willcox Co., Inc. Temperature sensor Pressure sensors Figure 12-8. This assembly contains both pressure sensors and temperature sensors. Input speed sensor Output speed sensor (vehicle speed signal) Figure 12-9. Two speed sensors installed on a transaxle case. The front sensor measures input shaft speed and the rear sen- sor measures output shaft speed. The ECM compares the two signals to determine the transaxle gear and detect any slip- page. (Chrysler) MAP sensor Filter Figure 12-10. Typical manifold absolute pressure, or MAP, sen- sor installation. Note the vacuum line and electrical connector. Most MAP sensors do not have a filter. Many MAP sensors are combined with the BARO sensor. (General Motors) Other Input Sensors: Engine Engine and body sensors also provide input to the ECM. Some of the more common sensors that affect the control of the transmission are discussed below. Engine Temperature Sensors Every electronic control system receives input from an engine temperature sensor. Engine coolant temperature is a good indicator of overall engine temperature. Therefore, every computer-control system has at least one engine coolant temperature sensor, which is threaded into an engine coolant passage. Most late-model engine controls also have an input to the ECM from an air temperature sensor, and a few vehicles have exhaust temperature sensors. These sensors are similar to those used to monitor transmission temperature. Resistance changes affect current and voltage in the sensor circuit. The ECM interprets changes in current and voltage as changes in temperature. Note: Some electronically controlled transmissions monitor the ambient (surrounding) air temperature. A few systems also monitor battery temperature. These sensors operate in the same manner as engine and transmission temperature sensors. Manifold Absolute Pressure Sensors To monitor the load placed on the engine, a manifold absolute pressure, or MAP, sensor is attached to intake manifold vacuum. This sensor measures the intake mani- fold vacuum and converts it to an electrical signal, which is sent to the ECM. The vacuum developed in the intake mani- fold can be thought of as a pressure difference between the air inside the manifold and the outside air. Therefore, these sensors are called pressure sensors, even though they mea- sure vacuum. MAP sensors contain a flexible diaphragm and a pressure-sensitive material called a piezoelectric crystal. The crystal produces an electrical current that is proportional to the pressure exerted upon it. Figure 12-10 shows a common MAP sensor. Changes in manifold vacuum cause a change in the pressure on the piezoelectric crystal. The pressure change creates a small voltage pulse in the crystal. The ECM reads this voltage pulse as manifold vacuum.