It’s not an uncommon problem with AC systems to find the pressure on the “low” side of the compressor being too high. As a result of this, you’ll probably notice your air conditioning not working correctly. And that? That’s frustrating.
You know that feeling when you open the car door on a hot, humid afternoon, and it feels like hell itself comes bellowing out? Since I live in England, that’s not something I have to worry about too much.
However, for those of you from warmer, sunnier parts of the world, it could well be the ultimate kick in the teeth when the AC stops working.
One of the (many) possible reasons for your AC not working is an imbalance in pressure. When the compressor’s low-pressure side is too high, it inhibits the system’s ability to cool the incoming air.
I hope you find this article useful.
Let’s get into it.
How Does An AC System Work?
The modern AC system hasn’t changed much in the last 60-odd years. Sure, it’s considerably more efficient now, but the principles governing it are pretty much the same – a bit like the internal combustion engine.
Here’s a fundamental fact that most people don’t quite grasp. The main job of the AC in a car isn’t to push cold air into the vehicle. Instead, it’s to remove the hot air and associated humidity. Sure, adding cold air in is a by-product, but it’s not the primary purpose.
Here is an example of an AC system. Have a quick watch – this video is 7 minutes long but well worth your time.
The serpentine drive belt is attached to the crankshaft – the “power output” shaft of your engine if it helps to think of it that way. This belt can drive several components, depending on how the company designed the car. These components might include the alternator, the power steering pump, and – such as is relevant to this article – the air conditioning compressor.
From the compressor, two lines lead in/out. These are what we call the high-pressure and low-pressure sides of the compressor. They contain a fluid – the refrigerant.
You can tell which side is which from a quick look at your air conditioning system. The larger-diameter pipes are lower pressure. The pipes with a narrower diameter are high pressure.
To get your head around this, it might help to compare your thinking with a hose pipe. When the nozzle is wide open – just a big hole – the water comes out slowly. It’s low pressure. When the nozzle opening narrows, the water shoots out further with more force: higher pressure.
The compressor compresses the refrigerant gas within it by squeezing it between two points. As a result, it forces scorching, highly-pressurized refrigerant gas into the high-pressure area of the system.
From here, the compressor forces the gas through a condenser. The condenser works in a way very similar to your car’s radiator. As the gas travels through this component, the temperature drops, and it turns back into a liquid.
Immediately after passing through the condenser, the liquid is forced through the dryer. The dryer ensures that the refrigerant is now entirely liquid – there are no gaseous elements to it.
The next component is the thermal expansion valve. This turns the hot liquid into a cold gas. Following this, the gas passes through the evaporator.
You can think of the evaporator as the central part of the system for cooling the air heading into your car’s cabin. Like the condenser or the coolant radiator, air passes through it. However, whereas these have the purpose of removing heat from the vehicle, the evaporator removes heat from the air.
The fan sucks air in towards the cabin. In the process, the air passes through the evaporator, giving you that cool, refreshing feeling as it blows out onto your face.
Following that, the cold refrigerant flows back to the compressor, where the whole process starts all over again.
How Do I Test My AC’s Pressure?
Now we’ve considered how the AC in your car works, let’s consider how to test its pressure.
I should point out, first of all, that you could (and perhaps should) take your car to an air conditioning specialist to have them do this job for you. It’s essential to be very careful when working with AC systems. Some parts of them are quite fragile, and making the wrong mistake could be catastrophic.
If you want to test your car’s AC pressures, it is possible to do it yourself. You should be able to find gauges on Amazon for about $50 to $100. In most situations, you won’t need to shell out hundreds of dollars for a top-of-the-range model – you’re only doing a quick, cheap check. That being said, make sure that you buy a reliable gauge.
Before starting the testing procedure, you should find what the correct required pressures are. Some manufacturers list these in the owner’s manual. For lots of cars, however, you’ll have to look them up online. When doing this, make sure to use a factual vehicle directory rather than random people’s random guess on a random forum.
Use the expected ambient temperature or general readings for comparison.
When you’ve got your AC pressure gauge, hook it up. The AC system has two caps on it. One of these caps is on the low-pressure side, the other on the high-pressure side. Remember that the high-pressure side of the AC has a narrower diameter and vice versa.
In your owner’s manual, there should be a page detailing which AC cap is which, as well as where to find them. On some vehicles, just accessing these ports can be quite the chore. They can be tucked away down the back of the engine block sometimes. Yep. Manufacturers do seem to just like making things difficult occasionally.
The gauge usually differentiates between the high- and low-pressure hoses with colors. Blue represents low pressure, whereas red represents high pressure.
Remove the caps and attach the hoses, ensuring you put each hose on the right side. You’ll have to pull the sleeve up, push it on, and then release the sleeve before tightening it on. It’s easy not quite to attach it properly, so make sure it’s on correctly.
Now, turn on the engine and leave it for a little while. Doing this should give the system sufficient time to get the refrigerant flowing around and working efficiently. At this point, read the pressures on the gauges.
The low-pressure (blue) gauge should show something in the range of 20 PSI to 30 PSI, or 1.3 to 2.1 Bar. You should expect much higher readings on the high-pressure gauge of between 200 PSI and 250 PSI (13.8 to 17.2 Bar).
These readings will vary depending on the manufacturer, make, and model. The figures provided above are a general estimate only.
If either of the pressures is too high or too low, it indicates a problem somewhere within the AC system.
What Causes AC Low Side Pressure To Be Too High?
This article will consider what happens when the pressure on the AC’s low side is too high.
There are two different possible situations here.
- The AC low side pressure is too high, and the high side pressure is also high.
- The AC low side pressure is too high, and the high side pressure is too low.
Depending on what your gauges show, different problems are likely. Therefore, there may be different steps you need to take to get it fixed.
If The Low Side Pressure Is Too High And The High-Pressure Side Is Normal…
The high-pressure side is what comes out of the AC belt-driven compressor. If the readings here are still high – that is, within their normal operating range – it indicates that this part of the system is working correctly.
The problem probably lies somewhere in the transition between the high- and low-pressure sides.
There could be something wrong with the condenser or the condenser fan. These are the most likely issues, although that’s far from an exhaustive list.
The condenser is the first component in the AC system after the compressor. It should reduce the temperature and pressure as it heads up to the expansion valve. Suppose there’s an issue with it or the fan. In that case, it might not cool the refrigerant sufficiently, meaning the pressure is still higher than it should be after it’s passed through the evaporator.
The other potential reason for this situation is an overcharged system. When the AC system is re-gassed, every car is designed to take a certain quantity of refrigerant and oil. Like all the fluids you need in a vehicle, you have to put the right amount of them in.
It’s possible for a mechanic to accidentally put too much refrigerant or oil into your vehicle for various reasons.
The result is an overall increase in pressure in the system as a whole.
If The Low Side Pressure Is High And The High Side Pressure Is Low…
In this situation, where the high-pressure side of the AC is too low, and the low side pressure is too high, the problem is probably compressor-related.
It could also be related to the AC pressure switch, the expansion valve, the evaporator, or the dryer. There could also be a leak in the AC system.
The AC pressure switch regulates the flow of refrigerant through the compressor. Sensors on the high and low sides monitor their pressures. If either side is too high or too low, it should switch off the compressor to prevent any damage and keep the system working. Should these sensors or the switch develop faults, this can lead to the refrigerant pressures being too high or too low.
If the expansion valve isn’t working – or at least isn’t working correctly – the pressure and temperature will be affected as the refrigerant goes through the evaporator. As a result, the gauges might show that the system’s pressures aren’t what they should be.
The dryer’s job is to remove moisture from the refrigerant. If, for some reason, it stops working or gets overloaded, it won’t be removing moisture. This moisture leads to the high-pressure side being too low and the low-pressure side being too high.
How Do I Solve AC Low Side Pressure Being Too High?
So then, we’ve looked at the potential problems (or at least some of them). Now, how do we fix it?
If you’re thinking of trying to re-gas your own AC system or otherwise attempt to fix it, think again.
This kind of work is almost solely limited to professionals. While you may be able to find tutorials on the internet that show you how to do it, believe me – it’s probably not worth it.
Re-gassing or fixing an AC system requires specialized equipment and chemicals that you should need licenses to acquire. This is especially the case when it comes to the refrigerants used in your car’s AC.
And that’s before we get into all the specialist knowledge you, honestly, can’t do without.
On this occasion, you’re going to have to swallow the cost and take your car to the nearest qualified AC technician. Ring around your local shops to see who can do it and give you the best quotes.
It’s reasonable to expect to pay out between $80 and $150 for an air conditioning re-gas. Depending on your system, it could be slightly more than this.
More in-depth AC work – for example, if the technician finds a leak somewhere in the system – will cost more. It’s hard to put an exact figure on it because it very much depends on where the leak is. The leak could also be a hairline crack in one of the pipes that the technician can only find with CSI-style UV lamps.
All of this – the “finding” and the “fixing” – takes time. Even if the cost for materials isn’t too high, expect to pay somewhere in the hundreds of dollars. As a reasonable guess, anywhere from $200 to $1,000.
What Else Should I Know About AC Systems?
Here, we’ll look at some other common questions about the air conditioning systems you find in cars.
Keep on reading to learn more about these fascinating systems.
History Of AC Systems
AC systems have been used in cars since the American company Packard first introduced them in 1939. However, they never really became popular with consumers due to things like the high cost and large amount of space required.
Packard discontinued vehicles with AC units in 1941. The market remained AC-less until Chrysler brought them back onto the scene in the mid-1950s, offering them as an optional extra on the 1953 Chrysler Imperial.
This system had a switch on the dashboard with three different positions – low, medium, and high. As well as cooling the passenger cabin quickly, it also filtered out particulates such as pollen and dust and drew in more air from the outside than traditional systems. This meant that the car felt fresher on the inside.
After this year, Cadillac, Buick, and Oldsmobile also offered AC extras on a range of their vehicles. These AC units were provided by Frigidaire, a company that also produced units for Jaguar, Rolls Royce, and Bentley.
The real benchmark system – the one that, basically, we still use today – was developed by Nash. On the Nash Ambassador, the AC system was compact enough to fit under the hood alone, instead of across the hood and the trunk like most contemporary technologies. Perhaps most importantly, the company combined the AC and the heating systems to produce a basic version of today’s advanced climate control systems.
Demand for ACs in cars grew in the 1960s, and, before long, many of the USA’s cars had these systems as standard. This was mostly the case in hotter parts of the country – the South.
Here is a video on the history of Nash, if you’re interested.
Why Does The Refrigerant Need To Be Cooled And Then Re-Heated?
Another way of phrasing this question might be, “why would you not just have cold liquid or gas constantly flowing through the air conditioning system? Why does it have to change from cold to hot constantly?”.
It’s a valid question. The answer might take a little head-scratching, but I’ll do my best to answer it here.
First of all, let’s briefly look at some important concepts you probably learned in high school physics and chemistry. Liquids are virtually impossible to compress.
You can test this theory for yourself by taking a syringe, half-filling it with water, and then putting your finger over the end. Is it possible to compress the water in the syringe? No. (Also, please don’t hurt yourself attempting this.)
Gases, however, are by comparison easy to compress.
This is due to the inherent molecular structures of liquids and gases.
The compressor must compress the refrigerant to keep the whole system flowing. In turn, this keeps everything cool.
If the compressor attempted to compress a liquid, it would simply break, for the reasons shown above. By the same principles, your pistons and con-rods can warp if coolant gets into the combustion chamber. The liquid can’t be compressed in the compression (or “squeeze”) stroke. Something has to give.
Secondly, as mentioned right at the start of this article, the AC’s main job is to remove heat from the air going into the cabin instead of just pushing cold air in. Where does that heat go? Into the system at the evaporator. From here, it goes back through the compressor and around to the condenser, where the environment cools it.
Does Using The AC Affect Fuel Economy?
This question is one of the most hotly debated and has been for some time. While I don’t presume to settle this debate once and for all, I can give you a brief overview as to why there are so. Many. Different. Opinions.
As you’ll remember from the start of this article, the AC is driven by the fan belt, which loops around the crankshaft pulley and the AC compressor pulley. Because of this, it’s inevitable (because, physics) that the AC compressor pulley puts drag on the crankshaft.
Since the crankshaft is the “power output” shaft of the engine, drag to it technically reduces the engine’s maximum power. By association, your fuel economy is also affected.
In old cars, the AC compressor could apply a lot of drag to the system. Yet, in newer cars, this effect is barely noticeable at all under normal conditions. The pulleys and the fan belt – not to mention the AC system overall – are all much more efficient these days.
The EPA released a statement concluding that driving with your windows down resulted in a more significant fuel economy decrease than driving with the AC switched on.
Despite all this, the following factors can influence how much power the AC draws and, therefore, how much drag it puts on the system.
- How hot it is outside, and how cold you want the incoming air to be. The greater the temperature differential, the more “work” the AC compressor needs to do.
- Having the correct amount of oil for lubricating the system.
- Making sure the system is correctly pressurized.
There’s one final point to throw in here: keeping your AC at all times is good for the system. The rubber O-Rings and seals in the system are designed to be lubricated by the oil. If you leave your AC off for extended periods – for example, over winter – these rubber components could become brittle and stop working. This leaves you with an extensive leak.
Personally, I leave my AC on all year round, except on especially cold days. This keeps the system healthy and my repair bills down – at least, so far.
AC Low Side Pressure Too High – Conclusion
I hope you’ve found this article useful and that it’s given you a decent beginner’s understanding of an automotive AC system.
If there’s one thing to take away, I would suggest taking your car to a mechanic you trust for almost every AC issue you come across. These systems are tricky to navigate and surprisingly easy to break, and so if you aren’t sure what you’re doing, it’s better to be safe than sorry.
Thanks for reading, and I hope you manage to successfully get your AC working once again.
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