What is the coolant temperature sensor for and how does it work?

The operation of the engine in the car is associated with a constant process of combustion of the fuel mixture. Because of which, the internal combustion engine (ICE) can overheat and fail. To prevent such incidents, the internal combustion engine is forcibly cooled by circulating a special fluid. But its condition is monitored by a coolant temperature sensor (DTOZH).

Appointment

Such a sensor is designed to monitor the condition of a car engine by recording temperature changes in the cooling fluid. For this purpose, it is placed in antifreeze, where there is a direct interaction of the sensing element and the coolant layer.

The sensor transmits measurement data to the control unit for further adjustment of the system operation. The logical block decides whether to continue operating the car in the same mode or to decrease the parameter that affects the heating factor.

In addition to electronic models, there are mechanical sensors that are not intended to interact with a logical unit, but to output information to a thermometer in the cabin. In the case of mechanical models, the driver himself decides to change the driving mode or completely stop the unit.

Depending on the machine model, the sensor is designed to perform the following functions:

• Temperature control at a specific point in time for the cooling system.
• Influence on the choice of the operating mode, depending on the current situation.
• Giving a signal for emergency switching on or off of the motor, with a sharp rise or fall in temperature.
• Ignition advance or lag control - allows you to adjust the intensity of exhaust emissions and the load on the piston system.
• Signaling for enrichment of the fuel mixture in the event of an impermissible decrease in the coolant temperature.

Device and principle of operation

Unlike older models, modern temperature control devices rely on a thermistor. In accordance with clause 22 of GOST 21414-75, this is such a non-linear resistor that changes the value of its own ohmic resistance, depending on the degree of heating or cooling.

For the coolant temperature sensor, NTC resistive elements are used. This means that, unlike classical conductive materials, where ohmic resistance increases with heating, an increase in the sensor temperature leads to a decrease in resistance.

For example, when measuring readings at +20 ºС, the thermistor resistance will be 3.5 kOhm. When the antifreeze is heated to +90 ºС, the resistance of the sensor will drop to 0.24 kOhm. But, there are exceptions, for example, for Renault cars the sensor has a positive temperature coefficient.

The principle of operation of the coolant temperature sensor is based on the following diagram:

1. When the engine is at rest, the coolant will have a temperature comparable to the ambient temperature. The resistance of the thermistor of the sensor Rt will remain at the maximum mark and the applied voltage will practically not deliver current to the indication circuit of the logic block.
2. When the V contacts in the ignition switch are closed, voltage from battery A will be applied to the temperature sensor when the engine starts. As the speed rises, the resistance of the thermistor, Rt, will decrease according to its characteristic.
3. If the permissible temperature limit is exceeded, Rt will go into conduction mode. According to Ohm's law, the amount of current flowing through the thermistor will increase. The signal will come to the logic block and a command will be given to reduce the volume of fuel injected, or decrease the number of revolutions of the crankshaft.
4. With a decrease in engine speed and power, over time, the combustion chamber will cool down and the internal combustion engine will come to the standard temperature. The coolant will cool down and the resistance of the Rt thermistor will increase again. The value of the current in the indication circuit of the logic block will decrease again and the vehicle will return to normal operation.

Depending on the magnitude of the voltage drop across the thermistor of the Rt sensor, the current temperature will be evaluated. In this example, we considered the electrical method of measurement, but some types of sensors can also use a mechanical one, which works due to thermal expansion.