Engine Performance: Sensors and Actuators

The (ECT) or engine coolant temperature sensor is a variable resistor type sensor. It's an (NTC) or negative temperature coefficient thermistor because, as the coolant’s temperature increases, its resistance decreases. It’s vital in maintaining an engine’s proper operating temperature. The sensor’s signal changes according to the temperature of its environment; the engine coolant stream. It's tip is located in the coolant stream and the sensor is usually on or around the thermostat housing or the intake manifold. It's constantly sensing the temperature of the engine’s coolant. The sensor lowers resistance as the engine warms to operating temperature.

The (EGR) or exhaust gas recirculation valve allows a regulated amount of exhaust into the combustion chamber. This cools the chamber and reduces the formation of harmful (NOx) oxides of nitrogen. A stuck open EGR valve will result in a rough idle and a stuck closed EGR valve will result in high cylinder temperatures, NOx production, and engine knock. The EGR valve is an actuator not a sensor.

The (IAT) or intake air temperature sensor is a thermistor that changes its signal according to intake air temperature. The PCM will provide a richer air fuel ratio as cool air passes the sensor. The is because cool air is denser and contains more oxygen than warm air does.

The (TPI) throttle position sensor is a potentiometer that indicates throttle angle to the PCM. The PCM uses this sensor's information along with several others to control injector pulse width and timing. It indicates throttle plate angle and engine load to the PCM. It's attached to the throttle plate shaft and changes the voltage signal sent to the PCM as the throttle plate rotates. Output voltage should be low, around 0.45 volts when the throttle is closed and high or 4.5 volts at (WOT) wide open throttle. The fuel injector pulse width and ignition timing are adjusted by the PCM as the throttle plate is moved. If the driver suddenly accelerates their vehicle to WOT, the PCM would read this as a 4.5 volt signal.