AC Pressure Switch

AC Pressure Switch – Keeping Your Aircon Compressor Safe?

Did you know, that Packard became the world’s first automaker to include air conditioning in their cars? Granted, this was back in 1939, which would’ve cost the would-be buyer the equivalent of $4,700 in today’s money for an AC unit. Since then, we’ve never looked back. Chilled as we are, we take for granted the many bits and pieces – like the AC pressure switch – that make an AC tick.

But let’s slow things down, and talk more about that AC pressure switch, shall we? What does this switch do in your car’s air conditioning system, anyway? Well, it appears as though the AC pressure switch ensures that the compressor – one of the most expensive items in any car – doesn’t explode any time the AC is a bit wonky. So, here’s all you need to know about the AC pressure switch…

What Do You Need To Know About The AC Pressure Switch?

First, what is the AC pressure switch, and how does it work? As we’ve hinted at earlier, this switch is a safety system built into your car’s air conditioning compressor. It’s also sometimes referred to as the ‘refrigerant pressure switch’. Its goal is simple. This being to prevent excessive pressure build-up inside the air conditioning system from damaging itself. It also includes vital components such as the compressor.

It basically monitors the pressure that’s cycling through the AC loop, and more specifically, the car’s refrigerant. As the refrigerant – or ‘Freon’, as some might call it – circulates the aircon, it can pressurize. Ordinarily, this pressure is normal, as it’s a sign of there being a flow of refrigerant, and is regarded as healthy. However, there are times when this pressure is thrown out of whack.

At some point, the refrigerant’s pressure might be too high, while at other times (or at varying parts of the AC system), be too low of a pressure. At any one time, there could also be a scenario where the refrigerant is both at too high or too low of pressure. This creates an imbalance, where two sides of the air conditioning loop aren’t at equal pressure – which can damage the AC system.

How Does The AC Pressure Switch Prevent Damage To The Compressor?

To make sure your compressor doesn’t break itself, the AC pressure switch steps in to even this out. Usually, most cars have two pressure switches – one for high, while the other for low pressure. That said, some cars only have one or the other. But how does it balance out the pressure variation in the AC system to prevent it from catastrophe?

Well, here are both sides of the story…

  1. What if the refrigerant pressure is too low? In this scenario, your AC pressure switch (low pressure) disengages the compressor clutch. When it does this, it works alongside the suction section of the air conditioning system, thus preventing strain on the compressor.
  2. What if the refrigerant pressure is too high? If this is the case, the AC pressure switch (high pressure) also does the same thing, by disengaging the compressor clutch. But this time, the switch does so by the discharge side of the AC loop and hence turning off the compressor.

Your AC pressure switch is a sort of transducer, which can alter the internal resistance when there’s a change in pressure. It monitors pressure through the evaporator outlet, and can often be found mounted on the aircon’s accumulator. The switch is then alerted, through the aforementioned input, whenever there is an excessive increase or decrease in pressure within the loop.

The AC pressure switch receives its signals from the compressor, which can alert it if the pressure is too high or too low. If the pressure is too high, it can cause the compressor to overheat. On the flip side, and if the pressure is too low, the compressor could be damaged due to a lack of oil, and thus, lubrication. Should the switch fail to function, nothing can stop the compressor from overloading.

What Happens If The Pressure Does Get Too High Or Too Low?

We already told you that having the pressure inside the AC be too low or high can break apart the compressor. But how exactly does it do this? First, we’ll have to learn more about how the AC works, and we can begin with those “sides”. Your car’s aircon has two halves, a low and high-pressure side, respectively. This is, in a nutshell, how your Freon can cool you down.

First, a bit of chemistry… Refrigerant (or Freon) is a gas at room temperature but turns into a liquid when it’s chilled or compressed. This is where your compressor comes along to transform the Freon into that chilly gust that brushes through your hair. On the low-pressure side of the aircon, Freon is in its gaseous state, until it passes through the air conditioning compressor.

When this happens, the compressor liquefies it and lets it flow across the rest of the system into a component either called the ‘receiver’ or ‘drier’. This is where air passes through, is cooled down, and blows into your face as a frosty mist. This refrigerant exits the system under high pressure and is cycled back to the low-pressure side of the AC loop. Here, it turns once more into gas.

If the pressure inside the system is too low, your compressor can burn out if there isn’t a sufficient amount of lubrication. It’ll then fail to compress the refrigerant and subsequently causes the whole AC system to fail. On the other hand, when the pressure gets too high, the compressor could burst under pressure. This is enough to rupture the system, causing a refrigerant leak to spring up.

What Are The Symptoms Of A Faulty AC Pressure Switch That You Need To Look Out For?

Seeing how important the AC pressure switch is, it’s no wonder why it’s been engineered to last for a very long time. How long, exactly? Unlike other components, the lifespan of the switch isn’t counted by mileage or years, but by cycles. This being how many cycles your car’s aircon is running – turning it on, and then off is considered as one cycle.

AC pressure switches have been rated to last as long as 50,000 cycles on average. For most people, this could easily be the entire lifetime of their cars. Therefore, it’s likely that the lot of you reading this won’t have to worry about replacing the switch anytime soon. However, repeatedly turning on and off the AC, even for a brief moment, can wear down the switch prematurely.

AC pressure switch failure symptoms.

We’ve already discussed the dangers of what was to happen if the switch fails, and a pressure build-up in your AC occurs. It will result in significant harm to your AC compressor, hoses, as well as other auxiliary components. All of them, mind you, aren’t cheap to repair or replace. Thankfully, there are a lot of clear symptoms when the switch exhibits signs of failure or has already failed.

Here are some tell-tale signs that you need to consider replacing your aircon’s refrigerant pressure switch…

1. Aircon Fails To Turn On

Imagine turning on the AC knob on a hot and sticky summer day, only to find it not working at all. If the aircon isn’t turning on, it naturally indicates that something’s amiss with the air conditioning, but which one? Numerous parts contribute to its operations, so it’s hard to diagnose precisely where the point of failure is. Among them would be the AC pressure switch.

The good news here is that it might not be too late. Some of those switches have a failsafe mode, so it could automatically turn the compressor off if the pressure is too low or high. They can override the climate controls in your car, and keep the whole system turned off until it’s been fixed. By making sure the compressor doesn’t activate, the switch is still preventing it from burning out.

Nevertheless, this backup feature will only work if the switch is merely failing, not failed altogether. There will also be occasions where the pressure switch might not be able to read the pressure, and couldn’t relay those readings to switch off the compressor. If your car’s aircon can’t turn on, it’s a good idea to send it off for a check-up by a technician to see what’s causing it.

2. AC Doesn’t Blow Any Cold Air

The AC pressure switch isn’t just the last-man-standing or last resort safety toggle for your air conditioning system, remember. One of its most important roles is helping to actively manage the pressure inside the system, keeping it just about perfectly balanced. If the switch is failing but is nonetheless still somewhat operational, it can still enable the aircon to keep on working.

That said, it might not work at its best. If the switch isn’t in tip-top shape, it can receive incorrect pressure readings, or mismanage them entirely. When this happens, your car’s aircon can’t work at its most efficient and optimal state. You can immediately feel this. You may turn on the aircon to still find cold air gushing out. However, it might not be as strong as it used to be.

Your car’s AC may take longer to chill down your car, which can be exceedingly noticeable on a hot day. Or, that failing switch – or switches, if both of them are on their way out – could manifest as the aircon being weak. It’s still turning on when you ask it to, at least. But now, and even after you’ve cranked the aircon to its coolest setting, you can barely feel cold air trickling out of the vents.

3. Aircon Turning On And Off Repeatedly

You’ll have to understand your car more thoroughly to notice if this is, or isn’t a problem. With some cars, the AC will automatically turn the aircon off when it’s reached the desired temperature. With that in mind, this automated system is often quite gradual and seamless. This won’t be the case if the AC pressure switch fails, however, as the intermittent turning on and off will be quite jarring.

You’ll turn on the climate control, only to find the aircon turning itself off immediately thereafter. In a few seconds or so, it’ll turn itself back on… And then off again. When the aircon intermittently turns itself on or off in rapid succession, it’s a sign that the pressure switch might be failing. On the bright side, at least this symptom won’t generally be exhibited if the switch has failed completely.

Aircon turning on and off intermittently.

Still, it’s annoying. You’d set a temperature that you want the climate control to chill it down to. What you’d find instead is the aircon continuously in a cycle of switching on, and then off every few seconds. It’ll keep on going until you switch the whole AC system off. This, once again, happens as the refrigerant pressure switch can’t effectively monitor and regulate the AC pressure.

4. Odd Noises When The AC Is Turned On Or Running

This is more or less a continuation of our previous point, which can be just as serious. The only thing you should be hearing of your aircon is the howling gust of chilly air. Following decades of evolution and refinement, automotive air conditioners are getting ever quieter and more civilized. Although, and if you hear closely, you could sense the odd clicking sounds deep in the bowels of the AC loop.

These ‘clicks’ are the compressor, turning itself on and off. It should be audible when you’re turning the climate control on or off. But if you keep hearing those clicks going off like machinegun fire, then it may be correlated to the AC pressure switch failing. This happens as it intermittently switches the compressor on and off, should the switch be faulty, and can’t adequately read the pressure.

What OBD Error Codes Should You Be Wary Of For A Bad AC Pressure Switch

If you spot these symptoms of a damaged, failing, or utterly failed AC pressure switch, you shouldn’t put off any repairs. It may be safe to keep on driving for a little bit after the switch is gone, as you head down to the local mechanic. Having said that, and should you keep using the AC for extended periods of time with a bad pressure switch, it can put a lot of strain on the compressor.

That, along with other aircon-related componentry, can wear out and fail far quicker than it should, thus prompting you to shell out for overly complex, and needlessly expensive repairs. If you happen to have an OBD scanner, you could plug it into your car to find any errors codes. This could, at least, help you narrow down the source of your AC-related issues, whether it’s the switch or else.

After all, it could just be a blown fuse, or perhaps some faulty wiring. Before you go ahead and pay up for a replacement, you should diagnose your car beforehand to know if the pressure switch really is the cause. Here are some error codes to look out for, if you’d like to determine that the pressure switch is the singular point of failure…

  • P0745 – “Pressure Control Solenoid Malfunction”
  • P0746 – “Pressure Control Solenoid Performance Or Stuck Off”
  • P0747 – “Pressure Control Solenoid Stuck On”
  • P0748 – “Pressure Control Solenoid Electrical”
  • P0749 – “Pressure Control Solenoid Intermittent”

What Other Error Codes Could There Be?

A little note on the last two – P0748 and P0749 – OBD error codes, they do point towards a failure or damage with the AC pressure switch. Yet, it may also indicate faults within the electrical system or wiring around the aircon loop. Before you consider a replacement of the switch, do check the wires and cabling too, just to make sure they’re not the ones prompting the error codes to appear.

On top of those earlier five trouble codes, you should also be on the lookout for these other four to appear. These codes down below are namely to alert you of faults with other parts of the AC system. These may include defects or damage with the wiring connections, refrigerant pressure charges, or elsewhere. However, they may also be related to the AC pressure switch by some coincidence…

  • P0530 – “A/C Refrigerant Pressure Sensor Circuit Malfunction”
  • P0531 – “A/C Refrigerant Pressure Sensor Circuit Range/Performance”
  • P0532 – “A/C Refrigerant Pressure Sensor Circuit Low Input”
  • P0533 – “A/C Refrigerant Pressure Sensor Circuit High Input”

How Much Does An AC Pressure Switch Replacement Cost?

Given that we’re now better acquainted with what the AC pressure switch is, and how it behaves when it fails, is there anything we can do to fix it? Unfortunately, the switch is an enclosed unit and isn’t easily fixable. When bringing up the issue with any technician, they’ll likely tell you that it needs a swap. In other words, you’ll have to replace the switch with a brand new one.

But how much does it cost to replace it? There’s at least some good news here, as the cost for a new switch isn’t too exorbitant. It’s relatively cheap compared to the rest of the AC unit, in fact. For the switch itself, you can find one for between $20 to $100. Although, it does ramp up once you include labor costs, should you opt for a professional replacement job instead of doing it DIY-style.

The hourly labor rates shouldn’t be painful, as replacing the switch shouldn’t take you any longer than an hour with most cars. You can expect to add another $50 to $200 for the hourly labor rates. In contrast, replacing the AC compressor could leave you knocking on the door of $1,000 or more. With that huge risk in mind, why would you ignore or put aside replacing the AC pressure switch?

How Can You Replace The AC Pressure Switch DIY?

Aha, but what if you need to replace the AC pressure switch, but want to skirt the oft high labor rates? Well, in that case, we have even better news! Replacing the refrigerant pressure switch is one of the easier automotive repairs that you can do by your lonesome. You won’t even need too many tools, nor must you invest too much time and effort into it.

In just three simple steps, you can replace the AC pressure switch in no time…

Step 1:

Once you have the compatible switch on hand, you’ll have to find out where it goes. Remember that the aircon compressor has two distinct switches – one for high pressure, and one for low pressure. You have to make sure you both find, and insert the correct ones in, correspondingly.

Then, you’ll have to locate where the switches are mounted. You can refer to a repair manual on your car to expedite the process. Generally, the switches are installed on the aircon pressure line, compressor, or the accumulator (sometimes referred to as the ‘drier’).

Step 2:

Next up, you can start to remove the old pressure switch(s). Before that, you have to disconnect the battery and then move on to anything else. Removing the battery – and thus cutting off the electrical supply – is a to-do item that you have to tick off anytime you’re working on your car.

Now that safety concerns are out of the way, you can start to remove the old switches. First, remove the connections that plug into the AC pressure switch. You could then loosen the switch with a socket or wrench, and thus carefully unscrew it from the mount.

Important Note: Commonly, you don’t have to purge the AC system before removing the switches. There’s a Schrader valve built into most AC compressors, which prevents a refrigerant leak through the switch’s mounting points. However, and just to be safe, you should consult a repair manual for your car, in particular, to make sure.

Step 3:

Now, you can repeat the whole process in reverse. First, let’s re-mount the new pressure switch. You should also be certain that you’re attaching the right switches – for low or high pressure – where they need to be. Screw it in, and tighten it with a socket or wrench until it’s just about snugly in place.

Next up, reinstall the electrical connectors, and then re-connect the battery. The final step of the puzzle is trying out the AC system, to see if you’ve done things right. It should start working right away, so gradually crank up your aircon, and check to make sure it’s working

AC Pressure Switch – Final Thoughts

In all, this perfectly summarises our look into AC pressure switches. It’s such a small component, and one could almost call it inconsequential. Yet, and when it does fail, your entire aircon not only goes haywire but could also start wearing itself down. If and when you do sense that something is wrong with those switches, never hesitate to have them replaced, as soon as possible.

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