Temperature sensors

Temperature sensors use temperature-variable resistors (thermistors). There are two types of temperature variable resistors: negative temperature coefficient (NTC) and positive temperature coefficient (PTC). With NTC, the resistance decreases with increasing temperature and vice versa. With PTC, it increases with increasing temperature and vice versa. On vehicles, NTC resistors are most often used as temperature sensors.

NTC resistor

There are various examples of the use of temperature variable resistors as temperature sensors on vehicles. They are most often used to measure temperature:
– engine coolants
– intake air
– ambient air
– air in the cabin
– fuel
– exhaust gas
– engine oil
– gearbox oil
– air conditioner evaporator, etc.

Temperature sensors measure the current temperature and convert it into a corresponding resistance, i.e. into a voltage. They are connected to control units to which they send data on the current temperature of the monitored area in the form of an electrical signal. The temperature sensors are supplied from the ECU with a constant DC voltage of 5 V through a series-connected resistor of constant resistance R. Together they form a voltage divider through which the ECU receives a voltage signal equivalent to the measured temperature.

Circuit of temperature sensor and ECU

Possible malfunctions in the operation of the state sensor are:
– defective temperature sensor
– open circuit between the sensor and ECU
– interruption of the power supply from the ECU

Testing of the NTC temperature sensor is carried out:
– ECU diagnostics
– by checking the power supply of the sensor
– by checking the resistance or output voltage of the sensor at different temperatures
– by checking for open (A) and short in the circuit
– by recording the oscillogram during the operation

Testing the temperature sensor by checking the resistance

The ECU constantly checks the correctness of the temperature sensor and can recognize certain irregularities, such as an open and a short circuit. The ECU stores errors as fault codes. However, when the temperature sensor’s output characteristic is disturbed, it begins to “lie” and display the wrong temperature. The ECU cannot recognize the change in sensor output characteristics and incorrect measurement. By testing the sensor, we can establish the deviation from the actual value. That is why it is necessary to read the actual values from the information system to check the correctness of the temperature sensor.

The procedure for testing the temperature sensor is as follows:
– we use the diagnostic device to read fault codes and actual values from the temperature sensor
– from the information system, we check the place of installation, the actual values for testing, and the circuit diagram
– we remove the socket from the temperature sensor. When the starting switch is on, we measure the supply voltage from the ECU on the socket connections. When there is no power, we check the condition of the cable for breaks and short circuits. If the cable is correct, we measure the supply voltage at the ECU connectors themselves, which tests the correctness of the control unit itself
– we check the correctness of the temperature sensor by measuring the resistance or the output voltage at different temperatures. We compare the obtained measurement results with the actual values from the information system. Likewise, we can establish the correctness of the sensor by recording the oscillogram of the output signal during operation.

The control unit monitors the temperature sensor only in the electrical correctness part. The ECU can recognize open circuit plus line, open circuit minus line, and short-circuit. When the temperature sensor is working, the output signal is between 0.5 and 4.5 V. When there is an open circuit in the plus or minus line, the output voltage is 5 V, and when there is a short circuit, the output signal is 0 V. In this way, the ECU recognizes the failure and stores the fault.