The air conditioning system in any car is one of the most finicky systems that you have. There can be a number of problems that crop up with air conditioning not working at all to working at a diminished capacity. And for a lot of drivers this is something they may let slide for a while since air conditioning is not required for the proper functioning of a vehicle. One of the problems that can pop up with your air conditioning is when the pressure on the low side of the compressor is running high, while the pressure at the high side of the compressor is running low.
The temperature that your air conditioning is able to produce is directly related to the pressure in the overall system. The refrigeration side of your air conditioner needs to be at the correct pressure in order to produce the temperature required to cool down the vehicle. You can even look up the recommended temperatures and refrigerant pressures that are required for the particular kind of air conditioner in your vehicle. Generally, these can be found online, and you may also be able to find the correct pressure listed in the owner's manual for your car.
How to Check your AC Pressure
Determining the pressure in your AC system is not entirely difficult to do. You just need to hook up a gauge set to the AC system and record the pressures on the low side and the high side. You can buy an AC pressure gauge on Amazon for about $30 or so if you want to do this yourself. Some of them can be quite a bit more expensive, but if all you're doing is simply checking the pressure that should get the job done for you at a fairly reasonable price. From that point all you need to do is compare it to the ambient temperature and the pressure is listed on the number chart for your particular air conditioning unit.
If the pressure on the high side and the low side match what is recorded for the ambient temperature around your air conditioner then you don't have a problem with the pressure of your coolant. If the pressure is too high than what the chart indicates is normal then that means you have an overcharged system and if it’s lower it means that you might have a refrigerant leak somewhere in the line If you have zero pressure then it means you have no coolant at all and it's likely leaking entirely from someplace.
What Should the Pressure of your AC System Be?
As we've established, there are two different sides to your AC system that have two different pressures. When you hook up the gauge to your AC system to measure what the pressures are, on the low side the gauge should be giving you a reading somewhere between 25 pounds per square inch and 30 pounds per square inch. That's about average for the low pressure and will only change a little bit based on the ambient temperature around your engine compartment.
When you measure the high-pressure side of your air conditioning system you should be getting anywhere between 200 pounds per square inch and 250 pounds per square inch. If both of your numbers are in these ranges, then everything is working the way it should be and you shouldn't have too much to worry about in terms of your AC system. If you are experiencing a problem with how your AC works, but you also have these normal ranges of low pressures on the low side and high pressures on the high side then it means you have a problem somewhere else in the system. If nothing else, then you have eliminated this issue as the source of your problem so it's not entirely a waste of time to check it out even if it isn't the issue that you're facing.
What Happens if the Readings Aren't Normal?
If you're finding that when you measure the pressure on the low side of your AC and it's coming up above 100, while the pressure on the high side is down around 150 or lower then you have a problem with the low side being high and the high side being low. Obviously, that means something is wrong inside the air conditioning unit itself that's throwing this off.
What Causes High Pressure When the Engine is Off?
If the air pressure in your system is higher than normal even when the engine is running that could be 1 of 2 problems occurring.
Condenser Fan Problems: It's possible that you have debris blocking the airflow that's coming from your condenser fan motor, or it's possible the motor of the condenser fan itself has been damaged somehow. This will reduce the airflow through the condenser and that in turn means it's not going to be able to cool down the AC system. If the condenser fan is under too high pressure it can warp the aluminum and cause leaking from the evaporator coil. This can essentially destroy your entire air conditioning unit.
Overcharged System: The other reason you might be experiencing high pressure is even when the engine is off is that your whole air conditioning system has been overcharged. That's the result of either too much refrigerant being put into the system or too much oil. The pressure is going to need to be released somehow, and it's best to leave that to a mechanic to get the job done safely and properly.
To know for sure that this is the problem you do need to test it when the engine isn't running as pressure goes higher when you do have the engine on, as they're supposed to. If the pressure is high when it's running, then that might just be normal operating procedure. It's also possible that air has gotten into the coolant system and is raising the pressure as well.
What Causes Low Side Pressure to Go Too High?
If you're finding that the low side pressure is higher than it should be while the high side pressure is about normal, then you're probably having an issue with your condenser. Your condenser has a cycle that it goes through in order to allow your air conditioning to run properly. If the condenser runs through this cycle more quickly than it's supposed to then the pressure on the low side is going to increase more than is normal. This could be the result of a problem with your thermostat switch or temperature gauge but you're going to need a mechanic to diagnose this one properly for you. There are a few too many variables for this one to just be handled on your own, or for the reason for this problem to be refined down more than what we've said.
What Causes the Low-Pressure Side to Go Too High and the High Side Pressure to Go Too Low?
Although there are a number of reasons that can make your low side pressure go too high, and the high side pressure go too low, when it comes to both of them happening at the same time you have some limited options.
If you use your gauge and determine that your low side is up over 100 and your high side is down around 150 then odds are a mechanic is going to tell you that the problem you're dealing with is your compressor has failed.
It's possible that you might have a problem with your pressure switch, your expansion valve, or the receiver / dryer also.
What is a Car's AC Compressor?
The compressor is the part of your air conditioning system that actually creates the pressure that the coolant is under in your vehicle. Obviously, this would be a clear place to look if there's a pressure issue in your AC system. The reason you have a high pressure side and a low pressure side at all is because the compressor increases the pressure and as it runs through your AC system to the condenser where it changes from a gas to a liquid and it will eventually lose pressure again as the cooling process takes place. Once it circulates back to the compressor it is repressurized as the whole process continues.
If your compressor it's not working the way, it's supposed to then obviously the pressure in your AC system can be severely compromised as a result. A new compressor may cost you a couple hundred dollars to buy and get repaired.
What is the Pressure Switch in My Car's AC?
If it's not the compressor that's causing trouble for your AC pressure, it may be your pressure switch. The pressure switch is mounted on both the high and low sides of your air conditioning system and it monitors the pressure of the refrigerant as it travels through. The point of the switch is to turn off the compressor to prevent damage if the pressure is too far out of line for the system. If the switch fails, then this is not able to happen and it can result in your pressure being thrown out of whack.
What is An Expansion Valve in My Car's AC?
The expansion valve failing is another thing that can cause the pressure on the low side to go too high and the high side to go too low. Your expansion valve is located at the evaporator and regulates the flow of refrigerant through the AC system as a whole. If you have too much coolant going through the system it could end up freezing and if there's too little then the evaporator may overheat and cause damage. The expansion valve controls the flow so that it stays at optimal levels and keeps the evaporator temperature where it needs to be. If the expansion valve fails then the coolant levels can be thrown off.
What is the Receiver/Drier in My Car's AC?
The receiver / dryer is part of your air conditioning system that stores and dries the AC refrigerant. It has something inside of it called a desiccant, which is similar to those little packets, but you might find in a pair of new shoes or other products that are meant to stay dry. They usually have a label on them that warns you not to eat them. The desiccant in your AC unit does the same job. It absorbs the moisture from the refrigerant so that it doesn't get contaminated with water.
Since the job of the desiccant in the receiver / dryer is to absorb moisture it's possible that it can actually absorb its fill and then no longer be able to do the job it's intended to do. If you have too much water in your system, and the receiver / drier is overloaded then it will not be able to continue doing what it's supposed to do. That in turn can lead to problems with your AC compressor, the expansion valve, and of course the issue with your low side pressure being too high, and your high side pressure being too low.
The Bottom Line
The best way for you to tell if you have a problem with the low side being too high in your air conditioner and the high side being too low is by the pressure gauge that we linked to above. That way you can at least tell for yourself that this is the main issue and not one of the other host of issues that could potentially cause problems with your car's air conditioner. Once you know that your pressure is off and that's the reason your air conditioning is not working the way it's supposed to, then you should hopefully be able to troubleshoot one of the causes that we've gone through which led to this pressure imbalance in the first place.
Getting your air conditioner serviced, especially if you need to drain the refrigerant and put new stuff in, is something that has to be done using the exact right tools by a professional. This isn't really the kind of job you want to do on your own in the driveway by any means. Your best bet if you know that the pressure is off in your air conditioner is to get it to a mechanic so they can narrow down the exact cause of the problem and get it fixed for you safely and quickly.