The engine coolant temperature sensor (ECT) senses the temperature of the coolant. The sensor is a negative temperature coefficient (NTC) resistor, which means that its resistance decreases as coolant temperature increases. The signal from the temperature sensor enables the ECM to calculate the necessary correction values for fuel enrichment, ignition timing, idle stabilization, and evaporative emission operation during starting and engine warm-up operations. If the engine coolant temperature sensor fails, a substitute value of 176°F (80°C) is stored in the ECM memory.
There are two coolant temperature sensors mounted in the upper radiator hose-to-engine flange. The black connector is for the gauge on the instrument panel. The blue connector is for the ECU.
There is also an air temperature sensor at the inlet end of the air flow sensor. Both temperature sensors operate with the same resistance values.
The Engine Coolant Temperature (ECT) sensor is mounted in the water outlet on the back side of the head. If you vehicle still has a original VW part it will be black in color with a four terminal connector. This device signals the engine management system with the current coolant temperature, the ECU takes this information and uses it to control other such circuits as the idle speed, knock control system, oxygen sensor circuit, exhaust gas recalculation, and fuel tank venting. It is also important to note that if at any time ECU does not receive a signal from the ECT it will substitute a value of 176°F (80°C).
The Engine Coolant Temperature sensor is mounted in the thermostat housing. The body of the sensor will be blue and the sensor will be blue and have two terminals if it is an original factory VW part.