Engine - Coolant Temperature Sensor/Switch: Description and Operation

Left-hand illustration
2-pin connector

Right-hand illustration
4-pin connector

Task
Detecting coolant temperature and supplying information to engine control module

Design
Metal housing with an NTC resistor.
2-pin connector.
Contact assignment at connector:
contacts 1 and 2 engine control module

Metal housing with two NTC resistors.
4-pin connector.
Contact assignment at connector:
contacts 1 and 2 engine control module contacts 3 and 4 air conditioning/automatic A/C

Function
The NTC resistor integrated in the coolant temperature sensor alters its electrical resistance in line with the coolant temperature. NTC means Negative Temperature Coefficient, i.e.the electrical resistance becomes less as the temperature rises.
Example:
coolant temperature resistance
20 °C 3100 ohm ± 5%
80 °C 315 ohm ± 5%