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

Coolant Temperature Sensor:

The coolant temperature sensor, located on the rear of the cylinder head, is an electrical thermistor with negative temperature coefficient. Negative temperature coefficient means that the change in resistance is inversely proportional to the change in temperature (as temperature increases, resistance decreases). The ECU measures the resistance through the sensor and calculates the engine temperature. The ECU uses this information to adjust the air/fuel ratio and ignition timing to meet the specific demands of the engine, at that temperature. The ECU also enables certain other functions only when the engine coolant temperature is above a certain value, such as knock sensor control (for ignition timing), oxygen sensor feedback (closed loop operation), deceleration fuel cut-off, and EGR control.

SELF DIAGNOSIS/SUBSTITUTE SIGNAL VALUE

The ECU can recognize an open or shorted sensor or sensor circuit. If a fault is detected, the ECU will store a fault code and substitute a signal value so the engine will run reasonably well. If there is a sensor failure, the ECU assumes an engine temperature of 68°F (20°C) when the engine is first started. Once the engine is started, the ECU then assumes a temperature increase of 18°F (10°C) for each minute of engine operation thereafter, until a maximum assumed temperature of 185°F (85°C) is reached.