* Thermistor as the Core: The heart of the sensor is a Negative Temperature Coefficient (NTC) thermistor. This means its resistance *decreases* as its temperature increases. The opposite, a Positive Temperature Coefficient (PTC) thermistor, exists but is less common in this application.
* Resistance Change: When the engine is cold, the thermistor has high electrical resistance. As the engine warms up and the coolant temperature rises, the thermistor's resistance drops significantly. This change in resistance is directly proportional to the coolant's temperature.
* Signal to the Engine Control Unit (ECU): The ECU (or PCM – Powertrain Control Module) monitors this changing resistance. The ECU uses a voltage divider circuit to interpret the resistance value. The sensor is wired into this circuit, and the voltage measured by the ECU is directly related to the resistance (and therefore the temperature) of the thermistor.
* ECU Interpretation: The ECU uses the voltage signal from the sensor to determine the engine's coolant temperature. This information is crucial for various functions:
* Fuel Injection/Mixture Control: The ECU adjusts the air-fuel mixture based on the engine temperature. A cold engine needs a richer mixture (more fuel) to start and warm up efficiently.
* Ignition Timing: Ignition timing is also adjusted based on temperature. Cold engines require slightly retarded timing.
* Engine Fan Control: The ECU activates the cooling fan when the coolant temperature exceeds a certain threshold.
* Choke Operation (in older carbureted engines): The temperature sensor helps control the choke, restricting airflow to the carburetor when the engine is cold.
* Dashboard Gauge: The signal is also used to drive the coolant temperature gauge on the instrument panel.
* Types of Sensors: While thermistors are the most common, other technologies exist, although less frequently used in modern vehicles.
In short, the ECT sensor passively measures the coolant temperature by monitoring the change in resistance of a thermistor. This information is then used by the ECU to precisely control various engine systems for optimal performance and efficiency. A faulty ECT sensor can lead to poor fuel economy, hard starting, overheating, or even engine damage.
