Coolant temperature sensor

The coolant temperature sensor is bolted onto the housing of the coolant pump.

Fig 1: Identifying Coolant Temperature Sensor Components

Courtesy of BMW OF NORTH AMERICA, INC.

The coolant-temperature sensor converts the temperature of the coolant into an electrical value (resistance). To do this, a resistance with negative temperature coefficient (NTC) is used. The coolant temperature is one of the measured variables for the following calculations:

• Fuel injection rate
• Nominal idle speed.

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. The resistance is converted into a temperature 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 167 kΩ to 150 Ω depending on temperature, which corresponds to a temperature of -40 °C to 130 °C.

The electrical voltage at the resistor is dependent on the coolant temperature. There is a table stored in the Digital Engine Electronics (DME) that specifies the associated temperature for each voltage value. This compensates for the non-linear relationship between electrical voltage and temperature.

Fig 2: Hot Conductor Characteristic Graph

Courtesy of BMW OF NORTH AMERICA, INC.