Air conditioning not working well:
When there are complaints about poor operation of the air conditioning, we try to find out what the complaint exactly entails. We also try to gather more information about when the air conditioning was last serviced.
- Check the air outlet temperature from the open air vents with the air conditioning on (preferably on recirculation or MAX mode, where recirculation is automatically activated);
- If the air is not sufficiently cooled: check if the air conditioning has not been serviced for more than four years. In that case, check if there is enough refrigerant in the system;
- Check the pressures with the air conditioning system on and off and check the temperatures of the components. The following sections cover these topics.
System pressure in the air conditioning system:
With pressure gauges, we can check the pressure in the air conditioning system. The hoses should be connected to the service ports of the air conditioning system. When tightening the nipples, the refrigerant will flow from the air conditioning to the gauges. If the system is empty, the needles will rotate and indicate the pressure of the system. An image of such a pressure tester is shown below. The pressure gauges are also present on a service station (air conditioning filling device).
The pressure gauge in the image contains two needles and three hoses.
- Blue is low pressure;
- Red is high pressure;
- The yellow hose in the pressure gauge is used to add nitrogen to the system for leak detection.
If the air conditioning has been off for some time, the meters will show approximately the same pressure after connection. After starting the engine, the low pressure will drop and the high pressure will rise. The pressure is related to the temperature: when the pressure rises, the temperature also increases, and vice versa.
- The low pressure drops due to the temperature reduction of the refrigerant after it exits the evaporator;
- The high pressure rises because the liquid refrigerant is heated after leaving the condenser.
The pressure will stabilize after a few minutes. The evaporator does not cool further than a few degrees above freezing point, and the fan draws a constant outdoor air temperature through the condenser.
When the air conditioning is no longer functioning properly, we can, besides reading the fault memory (there may be a fault from a pressure sensor), also measure the temperature and read the pressures with the gauges to make a diagnosis. The level of pressure indicates the condition of the system.
The pressures shown are for a well-functioning system. The blue gauge indicates the low pressure (2 bar) and the red indicates the high pressure (18 bar). The pressures are highly dependent on the temperature: as soon as the temperature of the outside air, evaporator, or other components changes, we also immediately see this reflected in the pressure.
The colored sections on the dials indicate the working pressures:
- Low pressure: between 0.5 and 3.5 bar;
- High pressure: between 9.5 and 25 bar.
In cars, we find compressors of the following variable displacement types:
- Fixed stroke: the low pressure (suction pressure) varies between 1 and 1.5 bar. The magnetic clutch engages and disengages the compressor;
- Variable stroke with continuous output: the swash plate is mechanically adjusted. The low pressure is constantly 2 bar, regardless of the compressor speed. A magnetic clutch provides the drive;
- Variable stroke with regulated output: the swash plate is electrically controlled. The suction pressure varies between 2 and 5 bar, depending on the ECU control. This type of compressor does not have a magnetic clutch.
Diagnosing based on system pressures:
In the previous paragraph, we saw the system pressure of a well-functioning system. In the case of a malfunction, we can often see this reflected in the pressures. Whether we are dealing with a leak resulting in too little refrigerant, or when there is too much filled during servicing, reading the pressures will reveal it. In this paragraph, we address the possible causes of too high or too low pressure in the high or low-pressure circuit. Note the compressor configuration!
Low and high pressure 0 bar
- Refrigerant pressure is 0 bar, so there is no pressure in the system. The system is empty and must be checked for leaks before refilling.
Low and high pressure equal
- The pressure with the air conditioning on or off does not change: the air conditioning compressor is not functioning. The compressor likely does not engage (due to an ECU activation condition) or the magnetic clutch is defective.
Low pressure high, high pressure normal
- Open expansion valve;
- Defective heater valve in the heater housing, causing warm air from the heating to enter the evaporator. Pinch the coolant hose to the heater core to see if this affects the low pressure.
Low pressure high, high pressure high
- Too much refrigerant (measure and calculate the superheat);
- The condenser overheats due to a restriction (perhaps a visible damage?) or the cooling fan is not working;
- Too much oil in the system: perhaps the system was recently filled with too much oil;
- Air in the system.
Low pressure high, high pressure low
- Expansion valve has too large a passage or remains open;
- Compressor defective. Try to manually rotate the compressor and check the resistance;
- Regulating valve for variable output of the compressor defective.
Low pressure low, high pressure low
- Too little refrigerant (measure and calculate the superheat);
- Compressor defective. Check whether the pressure is good with the compressor off but shows these pressures when on;
- High-pressure side partially blocked (pressure should be good with the system off).
Low pressure low, high pressure normal
- Warm air is in the evaporator or the interior due to a possible issue with the recirculation mode or heater valves/air vents;
- The heater continues to provide warm air. Possibly due to a stuck heating valve;
- The evaporator freezes due to a possible defect in the anti-ice switch or interior fan.
Low pressure low, high pressure high
- Too much refrigerant in combination with another issue;
- Restriction in the high-pressure side, e.g., due to a bent pipe as a result of a collision;
- Clogged thermostatic expansion valve, due to a mechanical defect or ice formation.
In the latest pressure measurement, there is a low low pressure and a high high pressure. In the event of a restriction or blockage in the system, the low pressure can drop to 0 bar as the compressor pulls a vacuum on the low-pressure side. The low pressure may also slowly recover: after turning off the air conditioning, the low pressure rises noticeably slowly to the original pressure. A temperature measurement can detect a potential restriction (as a result of a bent pipe). The temperature measurement is discussed in the next paragraph.
Diagnosing based on pressure and temperature (overheating and post cooling):
As described in the first paragraph, modern air conditioning compressors with a variable stroke with continuous output adjust the pressure to the conditions. The low pressure (suction side) is constantly 2 bar, regardless of engine speed. When we measure 2 bar, it doesn’t say much about the system’s operation. With temperature measurements, we can diagnose this.
The prescribed temperatures for a well-functioning system are described in the table below. The temperatures are guideline values of an air conditioning system that has been running for at least 10 minutes and at room temperature. At extremely high outdoor temperatures, the temperatures and pressures in the air conditioning system may vary.
- A good diagnosis can be made with a temperature measurement;
- The compressor temperature must not exceed 90 °C: the oil can boil;
- A temperature difference of 30 °C between the inlet and outlet of the condenser is acceptable. A lower temperature may be due to poor passage in the condenser, causing it to work less effectively.
The following images show a low pressure of 2 bar, high pressure of 18 bar, and a temperature of 6 °C on the suction line after the evaporator (evaporator outlet to the compressor).
In the evaporator, the refrigerant transitions from saturated vapor (vapor-liquid) to fully gaseous. The temperature of the refrigerant rises from 2-5 °C (from the expansion valve) to 6-8 °C at the condenser outlet.
Overheating:
With the measured pressure and temperature, we can calculate the superheat. The superheat is the difference between the suction line temperature and the evaporation temperature of the refrigerant.
- In a well-functioning system, the superheat is about 5 to 6 °C
- Superheat more than 6 °C: the system’s charge level is too low. When emptying the system, for example, 200 grams will be recovered from the system, while the maximum charge level is 800 grams;
- Superheat less than 5 °C: the system’s charge level is too high. There is (much) more refrigerant in the system than prescribed by the manufacturer.
To calculate the superheat, we need the table next to here to find the evaporation temperature at a certain pressure. In the table, we see that at a pressure of 2.03 bar, the refrigerant evaporates at a temperature of 1 °C.
When the air conditioning is not functioning properly, we can use this data to determine the cause. The three examples below show well-functioning and non-functioning systems.
Example 1: Calculating superheat for a well-functioning air conditioning:
Pressures and temperatures with the engine off:
- LP: 6 bar, 20 °C (outdoor temperature)
- HP: 6 bar, 20 °C (outdoor temperature)
Pressures and temperatures measured with the engine running at 2000 rpm and 15 minutes of air conditioning operation:
- 12 bar, 85 °C
- 12 bar, 82 °C
- 12 bar, 50 °C
- 12 bar, 42 °C
- 2 bar, 1 °C
- 2 bar, 6 °C
- 2 bar, 7 °C
- 2 bar, 9 °C
Conclusion example 1:
In the event of complaints about a poorly functioning air conditioning, we can go through the following five steps to determine the state of the air conditioning:
- With the air conditioning off, both the high and low pressures are 6 bar. This is fine;
- With the air conditioning on, the low pressure drops to 2 bar. This pressure is regulated by the variable compressor. The high pressure depends on the condenser temperature: here we measure 12 bar.
- We measure the temperature at the evaporator outlet with an infrared thermometer: it is 6 °C;
- We look up the evaporation temperature of the refrigerant that corresponds to the value measured on the low-pressure line: at a pressure of 2 bar, the evaporation temperature is 1 °C;
- We calculate the superheat by subtracting the evaporation temperature from the suction line temperature: (6 – 1) = 5 °C.
In a well-functioning system, the superheat is about 5 to 6 °C, so based on this measurement, we can conclude that the air conditioning is in order.
Example 2: Calculating superheat for a poorly functioning air conditioning:
Pressures and temperatures with the engine off:
- LP: 5 bar, 20 °C (outdoor temperature)
- HP: 5 bar, 20 °C (outdoor temperature)
Pressures and temperatures measured with the engine running at 2000 rpm and 15 minutes of air conditioning operation:
- 12 bar, 98 °C
- 12 bar, 81 °C
- 12 bar, 55 °C
- 12 bar, 40 °C
- 2 bar, 5 °C
- 2 bar, 13.2 °C
- 2 bar, 14 °C
- 2 bar, 15 °C
Conclusion example 2:
When turned off, the high and low pressures are both 5 bar. With the engine and air conditioning on, the low pressure drops to 2 bar and the high pressure rises to 12.0 bar. The pump engages, and the air conditioning should now cool effectively.
We measure a temperature of 13.2 °C on the outlet line of the evaporator with an infrared thermometer. This is significantly higher than the 6 °C in example 1.
- The low pressure is again 2 bar, so the evaporation temperature of the refrigerant is 1 °C;
- The superheat is: (13.2 – 1) = 12.2 °C.
We see a much higher temperature difference than in the example with a well-functioning air conditioning. Consequently, the temperature difference with the passing air is also smaller. Therefore, the interior air is less well-cooled. Passengers in the car experience this as a poorly functioning air conditioning. The cause is a too low charge level. The system still works with the current refrigerant amount but not as expected.
Example 3: Calculating superheat for a poorly functioning air conditioning:
Pressures and temperatures with the engine off:
- LP: 6 bar, 22 °C (outdoor temperature)
- HP: 6 bar, 22 °C (outdoor temperature)
Pressures and temperatures measured with the engine running at 2000 rpm and 15 minutes of air conditioning operation:
- 24 bar, 98 °C
- 24 bar, 81 °C
- 24 bar, 55 °C
- 24 bar, 40 °C
- 3.5 bar, 10 °C
- 3.5 bar, 6 °C
- 3.5 bar, 1 °C
- 3.5 bar, -2 °C
Conclusion example 3:
The pressure and especially the temperature at the compressor outlet are too high. Also, the low pressure is on the high side at all measured points. The temperature continues to drop between the evaporator and the compressor, indicating that evaporation is still occurring. If refrigerant is still evaporating after the TEV valve, this indicates too much refrigerant in the system.
In addition to reasoning the cause, we can also demonstrate this mathematically by calculating the superheat. At a pressure of 3.5 bar, the evaporation temperature is 13 °C. The measured temperature is 6 °C. By subtracting the evaporation temperature from the measured temperature, we can calculate the superheat:
6 °C – 13 °C = -7 °C. The superheat is thus -7 °C. When superheat is less than 5 °C, the conclusion is: the system’s charge level is too high.
Post cooling:
In addition to the temperature measurement at the condenser, the post cooling can also be determined. Post cooling refers to the difference in condensation temperature and the temperature at the condenser outlet. This can, among other things, help identify a too high or low charge level and ensure that liquid is coming from the condenser. The post cooling is usually between 5 and 15 °C.
- No post cooling indicates too little refrigerant;
- Too much post cooling results from too much refrigerant.
To determine the post cooling, follow these steps:
- When the system is on, determine the condensation temperature of the refrigerant at the condenser outlet: in the table, we find a condensation temperature of 50 °C at 12 bar;
- We measure a temperature of 40 °C at the condenser outlet with the thermometer;
- Calculate the post cooling as follows: post cooling = condensation temperature – temperature at condenser outlet, so (50 – 40) = 10 °C. This temperature is fine.
The software in the air conditioning ECU also determines the overheating and post cooling if the system is equipped with multiple pressure and temperature sensors. Thus, the ECU can determine that there is too little refrigerant in the system and store an error code for this, while the pressures still seem somewhat correct.
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