Coolant Temperature Sensor/Switch (For Computer): Description and Operation

Coolant Temperature Switch:

This switch is located behind the cylinder head, and tells the MCU when the coolant temperature has reached a certain temperature. Below 57 ~C (135 ~F), the switch is open, so if the MCU does not see a voltage returned from the coolant temperature switch, then it knows the engine is still cold. Once the temperature reaches 57 ~C (135 ~F), then the switch closes allowing voltage to return to the MCU. If the MCU sees voltage from the coolant temperature switch, then it knows that the engine is warmed to normal operating temperature.

The MCU uses this and other signals to help determine when it should switch from open loop to closed loop operation.