Engine Coolant Temperature Sensor: Description
This sensor is a temperature dependent resistor (thermistor), which is a Negative Temperature Co-efficient (NTC) type, i.e. resistance decreases with increasing temperature. The sensor forms part of a voltage divider chain with a pull up resistor within the ECM. The change in resistance relates to change in the ECT.
The sensor is vital to the correct running of the engine as a richer mixture is required at lower block temperatures for good quality starts and smooth running, leaning off as the temperature rises to maintain emissions and performance. Should the sensor fail there is a software ECT warm-up model which will supply a changing default value during the warm up stage of the engine, based upon IAT. After the software model reaches 60°C (140°F) ECT, a fixed default value of 85°C (185°F) is used. The model also forms part of the diagnostics for the ECT sensor, in conjunction with open and short circuit tests. See Fig 1 .
A fault condition is recognized if the ECM is powered up and the ECT sensor resistance exceeds a minimum or maximum threshold, or the difference between the ECT model and the temperature indicated by the ECT sensor is greater than a threshold.