The Engine Coolant Temperature (ECT) sensor is located in a water jacket near the front of the intake manifold on 3.7L and 4.7L engines. On 5.7L engines it is located under the air conditioning compressor, into a water jacket near the front of the engine block. On 5.9L engines it is located in a water jacket behind the alternator.
The ECT sensor's resistance changes in response to engine coolant temperature. The sensor resistance decreases as the coolant temperature increases, and vice-versa. This provides a reference signal to the PCM, which indicates engine coolant temperature. The signal sent to the PCM by the ECT sensor helps the PCM to determine spark advance, EGR flow rate, and air/fuel ratio.
Some vehicles may have two ECTs fitted: one for input to the PCM and the other for input to the coolant temperature gauge or warning light.