Engine Temperature Sensor: Description and Operation

Engine Coolant Temperature Sensor:

The coolant temperature sensor is a thermistor (a device whose resistance changes with temperature) mounted in the engine cooling system near the distributor. Low temperature produces high resistance while high temperature produces low resistance.

The ECM supplies a 5V signal to the sensor through a resistor in the ECM and measures voltage variation. By measuring this variation, the ECM knows coolant temperature. This input is a factor in calculations involving most ECM controlled systems.

The temperature/resistance relationship is as follows:

Coolant Temp. Resistance
-18°C (0°F) 25K ohms
-7°C (20°F) 13.5K ohms
4°C (40°F) 7.5K ohms
20°C (68°F) 3.4K ohms
38°C (100°F) 1.8K ohms
70°C (160°F) 450 ohms
100°C (212°F) 185 ohms