Engine - Coolant Temperature Sensor/Switch: Description and Operation

Coolant Temperature Sensor

Coolant temperature sensor
The coolant temperature sensor is installed on the front of the cylinder head.
The coolant temperature sensor converts the temperature of the coolant and therefore of the engine oil into an electrical value (resistance). To do this, a resistance with negative temperature coefficient (NTC) is used. The coolant temperature is the measured variable for the following calculations, for example: Fuel injection rate and nominal idle speed.

Functional description
A temperature-dependent electrical resistor is used for temperature sensing. The circuit contains a voltage divider where the resistance can be measured depending on the temperature. A temperature is converted using a characteristic curve specific to the sensor. An NTC resistor (NTC) is installed in the coolant temperature sensor, whose resistance value drops when the temperature increases.







The resistance changes from approx. 76 kOhms to 41 Ohms temperature-dependent, which corresponds to a temperature of -40 °C to 150 °C.

Structure and inner electrical connection
The coolant temperature sensor is connected by a 2-pin plug connection.
The resistor is part of a circuit as a voltage divider that is supplied with 5 volts by the engine control.







Characteristic curve and setpoint values
The electrical voltage at the resistor is dependent on the coolant temperature. A table that specifies the temperature associated with each voltage value is stored in the engine control. This compensates for the nonlinear relationship between electrical voltage and temperature.







Please note the following setpoint values for the coolant temperature:

Size Value
Voltage range 4.9 to 5.1 V
Signal voltage approx. 0.2 to 3.3 V
Response time 15 s
Temperature resolution ±1 °C
Maximum output current 20 mA
Temperature range -40 °C to 150 °C

Diagnosis instructions

Failure of the component
If the coolant temperature sensor fails, the following behavior is to be expected:
- Fault entry in the engine control unit
- Emergency operation with substitute value

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