Circuit/System Description

The engine coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 volts to the ECT sensor signal circuit and a ground for the low reference circuit.

The purpose of this diagnostic is to analyze the performance of the ECT sensor and the thermostat by comparing the measured ECT to the following 2 distinct models:

• The first model will analyze the performance of the ECT sensor, by using the ECT sensor to determine if the temperature of the engine coolant increases at the correct rate, by comparing it to a modeled ECT under various operating conditions.
• The second model will analyze the performance of the thermostat, by using the ECT sensor to determine if the temperature of the engine coolant increases at the correct rate, and then maintains that temperature within a calibrated range of the modeled ECT under various operating conditions.

The ECM uses the start-up ECT and the start-up intake air temperature (IAT) to begin the diagnostic calculation. The air flow into the engine is accumulated, and vehicle speed, distance, and engine run time are also factored in to determine if the ECT does increase normally and then maintains that temperature within a calibrated range of the modeled ECT.