Engine Coolant Temperature Sensor

Engine Coolant Temperature Sensor

The sensor is a thermistor, a solid-state variable resistor that changes resistance in response to a rise or fall in temperature. It is mounted in the engine block coolant system. The sensor is supplied with a reference voltage through a fixed resistor. As the current passes through the thermistor resistance, the ECM measures the voltage drop across the fixed resistor and translates this into a temperature using a pre-programmed table of values.

High/Low Input Failure
These are continuous monitors. The voltage from the sensor is compared to a failure threshold defined in the software.

If the voltage is below the low threshold, then a timer starts to increment. Once this timer exceeds another threshold, then a failure flag is set and a DTC is stored.

If the voltage is over the high threshold defined in the software, then a timer starts to increment. Once this timer exceeds a threshold, then a failure flag is set and a DTC is stored.

Range/Performance Failure
There are two parts to the range/performance monitor. The first part ensures that the ECT reaches the level required for closed loop fuelling. The second part ensures that the ECT reaches 80�C. Both parts of the monitor operate with the same strategy, are one shot monitors and each part has its own calibration values. If the ECT and intake air temperature are above the required level for each monitor part then the following strategy will be carried out otherwise the counters for that monitor part are reset.

There are two counters associated with each monitor - the load conditions met counter increments when the engine speed and load are above the required level - the load conditions not met counter increments when any of those conditions is not met.

A normal judgment is made if the ECT reaches the required level before the load conditions met counter reaches the value held in the judgment table.

A failure judgment is made if the load conditions met counter reaches the value held in the judgment table and the ECT has not yet reached the required level.

The judgment table holds the values that the load conditions met counter must reach, mapped against minimum ECT (and minimum intake air temperature for the range/performance monitor), for a failure judgment to be made.

The load conditions not met counter has a value associated with it which if exceeded will reset both the load conditions met counter and the load conditions not met counter.