4.7.2 Description
This monitor operates once per drive cycle. It calculates the difference between the measured engine coolant temperature and an estimated temperature, derived from a model. When the estimated temperature reaches a calibrated threshold, the error between the two temperatures is accumulated. The model has look-up tables that use a number of engine and vehicle parameters (engine speed, engine airflow, vehicle speed and the difference between intake air temperature and engine coolant temperature) to derive compensation values. These are added or subtracted from the estimated engine coolant temperature as appropriate.
An after start counter is also included. The estimated engine coolant temperature is taken as the measured engine coolant temperature for a calibratable time following engine start (this time is dependent upon the starting engine coolant temperature) to overcome second order effects which introduce inaccuracy into the estimate of engine coolant temperature.
A normal judgement is made if the measured engine coolant temperature reaches 80 °C and the accumulated error is not above the failure threshold. A failure judgement is made if the accumulated error equals or exceeds a calibratable fault threshold before the measured engine coolant temperature reaches 80 °C.