Coolant Temp Sensor Mercruiser 805218T 

Coolant Temp Sensor
Replaces: Mercruiser 805218T / Volvo 3850397
Fits: Used with Temp Gauge MCM/MIE EFI Models
  
The coolant temperature sensor is used to measure the temperature of the engine coolant of an internal combustion engine. The readings from this sensor are then fed back to the Engine control unit (ECU). This data from the sensor is then used to adjust the fuel injection and ignition timing.