Thermostat Monitor

Thermostat
The monitor operates once per trip and is not a continuous monitor. Every 1024 msec the monitor compares the actual coolant temperature with an estimated temperature, which is derived from a model and accumulates the error between the two temperatures. The model to calculate the estimated coolant temperature has look-up tables, which use various engine and vehicle parameters to derive compensation values by which the estimated coolant temperature is increased or decreased. These look-up tables take into account engine speed, engine airflow, vehicle speed and temperature difference between intake air temperature and coolant temperature.

A judgement of whether the thermostat is behaving normally or not is made when the estimated coolant temperature reaches a judgement level which is 35 °C above starting coolant temperature or 80 °C, whichever is reached first. The monitor has the ability to make one of three judgements once the judgement point is reached. The judgement made can either be "Normal", "Fail" or "Null". The normal judgement is made if the accumulated error is below the calibratable normal level and the actual coolant temperature has reached 80 °C at the judgement point. The failure judgment is made if the accumulated error equals or exceeds the calibratable failure level at the judgement point and a null judgement if the accumulated error is above the normal level and below the failure level at the judgement point. The null judgement is included to allow for the grey area which exists between normal and failed thermostats, as in extreme conditions a failed thermostat may resemble normal behaviour and a normal thermostat could resemble failed behaviour.