Air Conditioning Temperature Sensor Detection Standard

Whether it is our home wall air conditioner, central air conditioner, or car air conditioner, the temperature sensor is a very critical component. Many soft faults of air conditioners are caused by damage to the temperature sensor.

Understanding the function and principle of the temperature sensor is very important for us to repair air conditioners.
The temperature sensor of the air conditioner is actually a thermistor made of NTC material with a negative temperature coefficient. The resistance is small when the temperature is high, and the resistance is large when the temperature is low.

NTC temperature sensor for air conditioner

Different brands of air conditioners use temperature sensors with different resistance values.

In ordinary household air conditioners, the commonly used resistance values of temperature sensors are 5K, 10K, 15K, 20K, 50K, etc. (these are nominal resistance values corresponding to 25 degrees Celsius).
Various commonly used sensors

Household air conditioner sensors are usually divided according to their functions: indoor ambient temperature sensor; indoor coil temperature sensor;

Temperature sensor for outdoor environment; temperature sensor for outdoor coil; temperature sensor for compressor exhaust.

Its function is roughly as follows –

1. The indoor ambient temperature sensor is located at the air outlet of the indoor unit.
Generally, when you remove the filter, you can see a black plastic head line.
Room temperature sensor location
room temperature sensor

Its function is to collect the indoor temperature and provide it to the CPU for comparison with the set temperature. This controls the start and stop of the compressor and monitors the running speed of the internal fan.

2. The indoor coil temperature sensor is usually a copper head and is fixed on the copper tube of the evaporator of the indoor unit. You can see it by removing the inner unit casing.
Pipe temperature sensor

Its function is to mainly detect the coil temperature of the internal unit.

Prevent cold wind during heating in winter. When the temperature is not high enough, control the indoor fan to avoid blowing out cold air.

During summer cooling, the internal unit is protected against freezing.

Prevent the refrigerant temperature from being too low and freezing the pipeline.

3. Outdoor ambient temperature sensor
Its location is on the outdoor switch (usually fixed on the fins on the back of the outdoor unit), and can be seen on the back of the outdoor unit.
External ambient temperature

Its function is to monitor the ambient temperature of the outdoor unit and control the operating speed of the outdoor unit during air conditioning cooling and heating.

4. Compressor exhaust temperature sensor, which is fixed on the exhaust pipe of the compressor.

Its main function is to monitor the exhaust temperature of the compressor to determine the opening degree of the expansion valve, thereby controlling the operating speed of the compressor and ensuring that the exhaust temperature is not too high.

5. The outdoor coil temperature sensor is fixed on the coil of the outdoor condenser.

Its main function is to prevent the system temperature from being too high and causing overheating protection.

Temperature sensor (thermistor) failure is relatively high. Once there is an open circuit, short circuit or poor characteristic curve, the air conditioner will display an abnormal code and will not work properly.

Measurement of the quality of the temperature sensor:

In actual maintenance, the following three methods are generally used to measure the quality of the temperature sensor:

1. Resistance measurement method
Since the sensor is a thermistor, we only need to measure its actual resistance at a certain temperature and compare it with the nominal resistance to know its quality.

Of course, in the open-circuit and short-circuit states, one of the resistance values is infinite and the other is zero. Everyone knows this.

2. Voltage measurement method
The voltage of the sensor (thermistor) in the circuit is divided by resistors connected in series, and the voltage changes are fed back to the CPU.

The voltage at both ends is basically this voltage, and the normal value is between 2 and 3 volts.

By testing the voltage at both ends and comparing it with the normal value, you can also know whether it is good or bad.

3. Comparative method
Generally, the resistance values used for air conditioning ambient temperature sensors and pipe temperature sensors are the same.

If you suspect that one of the sensors is damaged, test the resistance of the other sensor without knowing its resistance. Replacing the sensor with a sensor of the same specification will solve the problem.

Of course, when replacing the temperature sensor, there is another problem that needs to be paid attention to: in addition to the resistance value, the temperature sensor of the air conditioner also has a key parameter.

The temperature parameter refers to the rate of change of the resistance value with temperature when the temperature rises by one degree, which is called the “temperature coefficient”. The most commonly used air conditioner manufacturers are 3450, 3480, 3950, etc.

The same resistance value has different temperature coefficients. At different temperatures, the resistance value changes greatly.
Resistance values of ambient temperature and tube temperature sensors of various brands

In actual maintenance, if the resistance value and temperature coefficient are selected incorrectly, it may cause the air conditioner to work abnormally in a certain temperature area.

For example: the resistance of the indoor temperature sensor becomes larger, causing the air conditioner to start frequently.

If its resistance becomes smaller, it means that the temperature detected by the air conditioner has increased and cannot come down, so it will not stop.

And if the resistance of the indoor pipe temperature sensor becomes smaller, it will cause the air conditioner to malfunction soon after it is started.

If its resistance is zero, the whole machine will have no response when it is turned on remotely.

In short, changes in the performance of the air conditioner’s temperature sensor can cause the air conditioner to work abnormally, and there are various faults!

This is something that everyone must pay attention to during actual maintenance!