1.10.1.1 Monitoring function

The coolant thermostat monitoring is done to detect a slow warm-up due to heat losses through thermostat and radiator. It is based on the comparison of the measured ECT sensor signal and the calculated ECT model (TCO_SUB).

The ECT model calculation is depending on the speed of the water pump, engine load and ambient temperature.

A malfunctioning coolant thermostat is detected, if the calculated ECT model (TCO_SUB) has exceeded the threshold 1 (thermostat regulation temperature) and the measured ECT sensor signal remains below threshold 2 (fault detection criteria).

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.

Example of Monitoring Method: 

Fig 1: Thermostat Temperature And Voltage Graph

Courtesy of BMW OF NORTH AMERICA, INC.

A comparison between the measured engine coolant temperature (ECT) and the fault detection criteria temperature is done after a specific time interval. The interval itself is based on the engine coolant temperature model.

If the measured engine coolant temperature (ECT) is higher than fault detection criteria (thermostat regulation temperature -11°K); the thermostat is concluded as normal thermostat.

If the measured coolant temperature (ECT) is lower than fault detection criteria (thermostat regulation temperature -11°K), the thermostat is concluded as opened stuck thermostat.

The thermostat regulation (opening) temperature is determined by the hardware. It is 99°C. The fault detection criteria for the thermost at would be therefore 88°C.