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

Engine Coolant Temperature Sensor:

DESCRIPTION
The Engine Coolant Temperature (ECT) sensor (1) contains a semiconductor device which changes the resistance based on temperature (a thermistor). The ECT sensor is located in the coolant crossover pipe at the center rear of the engine. The ECT sensor has a signal circuit and a ground circuit. The ECM applies a voltage (about 5.0 volts) on the signal circuit to the sensor. The ECM monitors changes in this voltage that are caused by changes in the resistance of the sensor in order to determine the engine coolant temperature.

OPERATION
When the engine coolant is cold, the sensor (thermistor) resistance is high, and the ECM's signal voltage is only pulled down a small amount through the sensor to ground. Therefore, the ECM will sense a high signal voltage (low temperature). When the engine coolant is warm, the sensor resistance is low, and the signal voltage is pulled down a greater amount. This causes the ECM to sense a low signal voltage (high temperature).

SCAN TOOL DIAGNOSIS
The scan tool displays engine coolant temperature in degrees. After engine start-up, the temperature should rise steadily to about 90°C (194°F) then stabilize when thermostat opens. If the engine has not been run for several hours (overnight), the engine coolant temperature and intake air temperature displays should be close to each other. When the ECM detects a malfunction in the ECT sensor circuit, the following DTC(s) will set:

^ DTC P0115 - ECT Sensor Circuit
^ DTC P0116 - ECT Sensor Circuit Performance