Thermostat Monitoring

Thermostat Monitoring

General Description
The coolant thermostat monitoring is done to detect a slow warm-up due to a blocked open thermostat.. It is based on the comparison of the measured ECT sensor signal and the calculated (modeled) ECT. The ECT model calculation is depending on engine load and ambient temperature.

A malfunctioning coolant thermostat is detected, if the calculated ECT model has exceeded the failure threshold (warmed-up temperature) and the measured ECT sensor signal remains below this threshold.

Before a malfunctioning coolant thermostat is entered into failure memory, the conditions concerning low load, coasting duration and IAT during the monitoring are checked. If the monitoring conditions are met, the coolant thermostat is entered into failure memory. Otherwise the coolant thermostat monitoring is inhibited for this driving cycle.

Flow chart: Thermostat monitoring






Example of Monitoring Method:






A comparison between the measured coolant temperature and the failure threshold is done after a specific time interval. The interval itself is based on the coolant temperature model.
As soon as the model temperature exceeds the failure threshold and all other monitoring conditions are fulfilled at the same time, a valid diagnosis occurs.
At that time, if the measured coolant temperature is higher than warmed-up temperature, the thermostat is concluded as faultless.
On the contrary, if the measured coolant temperature is lower than warmed-up temperature, the thermostat is concluded as blocked open thermostat.

With the measured coolant temperature exceeding the failure threshold for the first time, driving conditions concerning minimum vehicle speed and minimum engine load are checked.
If these conditions are fulfilled, the thermostat is concluded as faultless (fast end of diagnosis), if not, the monitor is inhibited for this driving cycle.
In every case RBM calculation is done only after the time interval based on calculated ECT.