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

PURPOSE
The Powertrain Control Module (PCM) uses Engine Coolant Temperature (ECT) sensor input to calculate injector pulse width, idle speed, and ignition spark advance.

OPERATION
The sensor element extends into the coolant passage, where it maintains constant contact with the engine coolant.

The ECT sensor is a Negative Temperature Coefficient (NTC) thermistor. (As temperature increases, sensor resistance decreases.)

The PCM supplies a reference voltage of approximately 5 volts to the sensor, see THEORY below.

As sensor resistance changes, the voltage drop across the sensor changes.

The PCM translates the voltage drop across the sensor into engine coolant temperature.

THEORY
As the engine warms up the ECT sensor becomes less accurate. To keep sensor readings accurate at all times, the 5 PCM routes the reference voltage through either a 10,000 ohm resistor (cold operation), or a 909 ohm resistor (warm operation).

The resistance circuits are inside the PCM.

For accurate readings at cold temperatures (below 125° F.), the 5 volt reference is fed through the 10,000 ohm resistor.

When the engine reaches approximately 125° F the PCM switches the 5 volt reference through the 909 ohm resistor circuit, making the ECT sensor more accurate at higher temperatures.


NOTE: The following information is intended to be used in conjunction with the electrical diagrams.

CIRCUIT OPERATION
The Engine Coolant Temperature (ECT) sensor provides an input to the Powertrain Control Module (PCM) on circuit K2. From circuit K2, the engine coolant temperature sensor draws up to 5 volts from the PCM. The sensor is a variable resistor. As coolant temperature changes, the resistance in the sensor changes, causing a change in current draw. The K2 circuit connects to cavity 2 of the PCM.

The PCM provides a ground for the engine coolant temperature sensor signal (circuit K2) through circuit K4. Circuit K4 connects to cavity 4 of the PCM connector and cavity 2 of the sensor connector.