CHAPTER 3 T EMPERATURE S ENSORS John Turner 1. INTRODUCTION AND OVERVIEW Temperature is an important parameter for automotive engineers. The first temperature sensors to be applied to motor vehicles were probably those based on bimetallic strips or mercury thermometers and used to monitor engine coolant (see figure 3.1). Thermistor-based temperature sensors have been used in automotive applications since the late 1940s to send signals to dashboard gauges, and later to electronic control modules. In modern vehicles temperature transducers are used in connection with electronic fuel control systems, and to measure the temperature of inlet air and exhaust gas. They are also used for environmental control, to regulate heating or air conditioning in the passenger compart- ment, and for ice warning systems. The majority of automotive temperature sensors are thermistors, though other forms of transducer are increasingly used. The two primary users of temperature data on a vehicle are the engine/powertrain management system, and the heating, ventilation and air conditioning (HVAC) system. Additionally, many elec- tronic modules require temperature compensation. This is normally provided by the use of a board- mounted thermistor. The engine/powertrain management system uses a number of temperature inputs to enhance the performance of the engine, control emissions, and optimize efficiency. The most common applications are as follows: · · · · Coolant temperature sensing ntake air temperature sensing ransmission oil temperature sensing Cylinder head temperature sensing The coolant temperature sensor measures the temperature of the coolant and interfaces with the electronic engine control module (ECM). This sensor provides feedback to the ECM regarding the 85