The engine coolant temperature sensor resistance changes in response to engine coolant temperature. The sensor resistance decreases as the coolant temperature increases. This provides a reference signal to the PCM, which indicates engine coolant temperature.
On 3.0L engines, the ECT is mounted at the top of the water outlet housing on the lower intake manifold. The second sensor (green) mounted to the lower intake manifold is the coolant temperature sender for the coolant gauge on the instrument panel.
On 3.8L engines, the ECT is mounted to the side of the water outlet housing on the lower intake manifold.