Air conditioning not functioning properly:
When there are complaints about the poor performance of the air conditioning, we try to determine the specifics of the complaint. We also attempt to gather more information about when the air conditioning was last serviced.
- Check the outflow temperature from the open air vents with the air conditioning turned on (preferably in recirculation or MAX mode, where recirculation is automatically engaged);
- If the air is insufficiently cooled: check whether 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 monitor the temperatures of the components. This will be discussed in the following sections.
System pressure in the air conditioning system:
With 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 the nipples are tightened, the refrigerant will flow from the air conditioning to the gauges. If the system is empty, the needles will rotate to indicate the system pressure. The image below shows such a pressure tester. The gauges are also present on a service station (air conditioning refill machine).
The gauge in the image has two needles and three hoses.
- Blue is low pressure;
- Red is high pressure;
- The yellow hose in the gauge is used to add nitrogen to the system for leak detection.
If the air conditioning has been off for some time, the gauges will indicate 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 increases, the temperature also rises. And vice versa.
- The low pressure drops due to the temperature reduction of the refrigerant after it flows from the evaporator;
- The high pressure rises because the liquid refrigerant warms up after leaving the condenser.
The pressure will stabilize after a few minutes. The evaporator does not cool further than a few degrees above freezing, and the fan draws a constant outside air temperature through the condenser.
When the air conditioning is not functioning correctly, in addition to reading the error memory (possible pressure sensor error), we can measure the temperature and read the pressures with the gauges to diagnose. The pressure level indicates the system’s condition.
The pressures seen indicate a well-functioning system. The blue meter shows the low pressure (2 bar), and the red shows the high pressure (18 bar). The pressures are heavily dependent on temperature: changes in the temperature of the outside air, evaporator, or other components are directly 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 swash plate types:
- Fixed stroke: the low pressure (suction pressure) varies between 1 and 1.5 bar. The magnetic clutch switches the compressor on and off;
- Variable stroke with constant output: the swash plate is mechanically adjusted. The low pressure remains constant at 2 bar, regardless of compressor speed. A magnetic clutch provides the drive;
- Variable stroke with controlled output: the swash plate is electrically controlled. The suction pressure varies between 2 and 5 bar and depends on ECU control. This type of compressor does not have a magnetic clutch.
Diagnosing based on system pressures:
In the previous section, we saw the system pressures of a well-functioning system. In the event of a fault, this can often be reflected in the pressures. Whether we are dealing with a leak resulting in insufficient refrigerant, or an overfill during service, reading the pressures will reveal it. In this section, we address the possible causes of too high or too low pressure in the high or low-pressure circuit. Pay attention to the compressor version!
Low pressure and high pressure 0 bar
- The refrigerant pressure is 0 bar, indicating no pressure in the system. The system is empty and should be checked for leaks before it is refilled.
Low pressure and high pressure equal
- The pressure does not change when the air conditioning is on or off: the air conditioning compressor is not functioning. The compressor likely does not engage (due to an ECU enabling condition), or the magnetic clutch is defective.
High low pressure, normal high pressure
- Open expansion valve;
- Defective heater valve in the heater core allowing warm air from the heater into the evaporator. Pinch the coolant hose to the heater to see if this affects the low pressure.
High low pressure, high high pressure
- Too much refrigerant (measure and calculate the superheat);
- The condenser is overheating due to a restriction (perhaps visible damage?) or the cooling fan is not working;
- Too much oil in the system: perhaps the system has recently been topped up with too much oil;
- Air in the system.
High low pressure, low high pressure
- The expansion valve has an excessive opening or remains open;
- Defective compressor. Try rotating the compressor manually and check the resistance;
- Defective control valve for variable compressor output.
Low low pressure, low high pressure
- Insufficient refrigerant (measure and calculate the superheat);
- Defective compressor. Verify if the pressure is correct with the compressor off, but shows these pressures when on;
- Partially clogged high-pressure side (pressure when the system is off should be correct).
Low low pressure, normal high pressure
- Warm air is present in the evaporator or 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 heater valve;
- The evaporator freezes due to a possible defect in the anti-ice switch or interior fan.
Low low pressure, high high pressure
- Excessive refrigerant combined with another problem;
- Restriction in the high-pressure side, e.g., due to a bent line resulting from a collision;
- Clogged thermostatic expansion valve, due to a mechanical defect or ice formation.
In the last pressure measurement, there is a condition of low low pressure and high high pressure. In the case of a restriction or blockage in the system, the low pressure can drop to 0 bar as the compressor draws a vacuum on the low-pressure side. In this case, the low pressure may also recover slowly: after turning off the air conditioning, the low pressure will rise noticeably slowly to the original pressure. A temperature measurement can detect a possible restriction (resulting from a bent line). The temperature measurement is discussed in the next section.
Diagnosing based on pressure and temperature (overheating and subcooling):
As described in the first section, modern air conditioning compressors with variable stroke and constant output adjust the pressure according to conditions. The low pressure (suction side) remains constant at 2 bar, regardless of engine speed. When we measure 2 bar, it doesn’t say much about the system’s performance. However, temperature measurements allow us to make a diagnosis.
The table below shows the prescribed temperatures for a properly functioning system. The temperatures are reference values of an air conditioning system that has been on for at least 10 minutes and at room temperature. In extremely high ambient temperatures, the temperatures and pressures in the air conditioning system may deviate.
- A temperature measurement can provide an accurate diagnosis;
- The temperature of the compressor should not exceed 90 6C: the oil could start boiling;
- A temperature difference of 30 6C between the inlet and outlet of the condenser is acceptable. A lower temperature may result from poor passage in the condenser, reducing its efficiency.
The following images show a low pressure of 2 bar, a high pressure of 18 bar, and a temperature of 6 6C on the suction line after the evaporator (from evaporator outlet to compressor).
In the evaporator, the refrigerant transitions from saturated vapor (vapor-liquid) to fully gaseous. The refrigerant temperature rises from 2-5 6C (from the expansion valve) to 6-8 6C at the condenser’s exit.
Overheating:
With the measured pressure and temperature, we can calculate the superheat. The superheat is the difference between the suction line temperature and the refrigerant’s evaporation temperature.
- In a well-functioning system, the superheat is around 5 to 6 6C
- Superheat over 6 6C: the system’s charge is too low. When evacuating the system, only 200 grams might be recovered, while the maximum charge is 800 grams;
- Superheat under 5 6C: the system’s charge is too high. There is (much) more refrigerant in the system than the manufacturer prescribes.
To calculate the superheat, we need the table next to us to check the evaporation temperature at a specific pressure. We see in the table that at 2.03 bar pressure, the refrigerant evaporates at a temperature of 1 6C.
When the air conditioning doesn’t function correctly, we can use this data to determine the cause. The three examples below show well-functioning and malfunctioning systems.
Example 1: Calculate superheat with a properly functioning air conditioning:
Pressures and temperatures with engine off:
- LP: 6 bar, 20 6C (outdoor temperature)
- HP: 6 bar, 20 6C (outdoor temperature)
Pressures and temperatures measured with engine running at 2000 rpm and air conditioning on for 15 mins:
- a012 bar, 85 6C
- a012 bar, 82 6C
- a012 bar, 50 6C
- a012 bar, 42 6C
- a02 bar, 1 6C
- a02 bar, 6 6C
- a02 bar, 7 6C
- a02 bar, 9 6C
Conclusion example 1:
When there are complaints about a poorly functioning air conditioning, we can follow these five steps to assess the state of the air conditioning:
- With the air conditioning off, both the high and low pressures are 6 bar. This is fine;
- When the air conditioning is 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: this is 6 6C;
- We look up the refrigerant’s evaporation temperature corresponding to the value measured on the low-pressure line: at 2 bar pressure, the evaporation temperature is 1 6C;
- We calculate the superheat by subtracting the evaporation temperature from the suction line temperature: (6 – 1) = 5 6C.
In a well-functioning system, the superheat is around 5 to 6 6C, so this measurement indicates the air conditioning is in order.
Example 2: Calculate superheat with a malfunctioning air conditioning:
Pressures and temperatures with engine off:
- LP: 5 bar, 20 6C (outdoor temperature)
- HP: 5 bar, 20 6C (outdoor temperature)
Pressures and temperatures measured with engine running at 2000 rpm and air conditioning on for 15 mins:
- a012 bar, 98 6C
- a012 bar, 81 6C
- a012 bar, 55 6C
- a012 bar, 40 6C
- a02 bar, 5 6C
- a02 bar, 13.2 6C
- a02 bar, 14 6C
- a02 bar, 15 6C
Conclusion example 2:
When 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 adequately.
We measure a temperature of 13.2 6C at the evaporator outlet line with an infrared thermometer. This is noticeably higher than the 6 6C in example 1.
- The low pressure is again 2 bar, so the refrigerant’s evaporation temperature is 1 6C;
- The superheat amounts to: (13.2 – 1) = 12.2 6C.
Here, we observe a much higher temperature difference than in the example with a well-functioning air conditioning. This results in a smaller temperature difference with the flowing air. The interior air is therefore less effectively cooled. Passengers in the car notice this as a malfunctioning air conditioning. The cause is insufficient refrigerant charge. The system still operates with the available refrigerant quantity, but not as expected.
Example 3: Calculate superheat with a malfunctioning air conditioning:
Pressures and temperatures with engine off:
- LP: 6 bar, 22 6C (outdoor temperature)
- HP: 6 bar, 22 6C (outdoor temperature)
Pressures and temperatures measured with engine running at 2000 rpm and air conditioning on for 15 mins:
- a024 bar, 98 6C
- a024 bar, 81 6C
- a024 bar, 55 6C
- a024 bar, 40 6C
- a03.5 bar, 10 6C
- a03.5 bar, 6 6C
- a03.5 bar, 1 6C
- a03.5 bar, -2 6C
Conclusion example 3:
The pressure, and especially the temperature, at the compressor outlet is too high. Also, the low pressure is consistently on the high side across all measured points. Between the evaporator and compressor, the temperature continues to drop, indicating that evaporation is still occurring. If refrigerant continues to evaporate after the TEV, this indicates an excess of refrigerant in the system.
Besides reasoning out the cause, we can also mathematically demonstrate this by calculating the superheat. At 3.5 bar pressure, the evaporation temperature is 13 6C. The measured temperature is 6 6C. By subtracting the evaporation temperature from the measured temperature, we calculate the superheat: 6 6C – 13 6C = -7 6C. Hence, the superheat is -7 6C. With a superheat of less than 5 6C, the conclusion is: the system’s charge is too high.
Subcooling:
In addition to the temperature measurement at the condenser, subcooling can also be determined. Subcooling is the difference between the condensation temperature and the temperature at the condenser outlet. This helps establish whether the charge level is too high or too low, confirming that liquid is exiting the condenser. Subcooling is typically between 5 and 15 6C.
- No subcooling means insufficient refrigerant;
- Excessive subcooling results from too much refrigerant.
To determine subcooling, follow these steps:
- With the system on, determine the refrigerant’s condensation temperature in the condenser outlet line: in the table, we find for 12 bar a condensation temperature of 50 6C;
- We measure with the thermometer a temperature of 40 6C at the condenser outlet;
- Subcooling is calculated as: subcooling = condensation temperature – condenser outlet temperature, so (50 – 40) = 10 6C. This temperature is acceptable.
The software in the air conditioning ECU also determines the superheat and subcooling if the system is equipped with multiple pressure and temperature sensors. The ECU can thus ascertain low refrigerant in the system and log a fault code, even when pressures seem relatively fine.
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