Coolant Temperature Sensor/Switch (For Computer): Description and Operation

LOCATION

The Coolant Temperature Sensor is threaded into the intake manifold next to thermostat housing.


PURPOSE

The sensor element extends into the coolant passage where it maintains constant contact with the engine coolant. The engine controller uses this sensor to measure the temperature of engine coolant.


THEORY/OPERATION

The sensor is used to calculate injector pulse width when the engine is cold. Input from the sensor will also affect IAC motor position, and ignition spark advance.

The sensor is a NTC Thermistor-type sensor (Resistance varies opposite temperature). At cold temperatures, resistance is high. As temperature increases, resistance will decrease.

The engine controller sends out a reference voltage of approximately 5 volts to the sensor. As coolant temperature changes, the internal resistance of the sensor will change, causing a varying voltage drop across the sensor. The engine controller reads the voltage drop across the sensor as engine coolant temperature.

To make the sensor more accurate, the 5 volt signal goes through a 10,000 ohm resistor or a 909 ohm resistor circuit in side the engine controller. If the engine is cold (below 125° F.), the 5 volts is fed through the 10,000 ohm resistor. If the engine is warm (above 125° F.), the 5 volts is fed through the 909 ohm circuit.

The 10,000 ohm resistor makes the sensor very accurate at cold temperatures. Once the engine warms up the sensor becomes less accurate. The engine controller then switches the 5 volt feed through the 909 ohm resistor circuit. This make the sensor very accurate at higher temperatures. The engine controller changes scales to keep sensor readings accurate at all times.