Car AC Turns on and off Randomly (Causes and Fixes)
The AC is one of the few automotive systems that are fairly independent of other components like the engine and electronics. That makes it significantly harder to diagnose and nearly impossible to repair, even for someone with mechanical experience. Still, if you are facing the symptom at hand, at least at the end of this article, you will know what’s causing it and even learn how to fix it if it turns out to be something simple. But more on fixing the issues later; before that, here is a quick rundown of all the possible causes. If your car AC turns on and off randomly, first make sure it’s not just normal AC cycling and whether or not the AC is set to auto. But if that checks out, it could be a worn-out AC relay; the AC is overcharged with freon, there is a refrigerant leak, a faulty interior temperature sensor, or shorted AC controls. Normal AC Cycling First and foremost, it’s important to understand what normal AC cycling is to avoid wasting money on fixing something that isn’t broken. The AC compressor can’t always be on regardless of the setting, auto or manual. The reason is that the AC coils and the evaporator would freeze because of the extremely low temperatures constant running would cause. Instead, the AC compressor has to turn off every half a minute or so to allow the system to warm up and defrost. Normal AC cycling goes something like five to ten seconds on and 15-30 seconds off – if it’s noticeably more often than that, skip to the causes and fixes. Also, if you set your AC to auto, it will again cycle on and off, but it will be much more noticeable because the blower will change speed as well. The reason for that is the compressor is trying to maintain a certain temperature in the cabin. And once the cabin reaches the temperature, it shuts off to save fuel and the environment. Then, the cabin will warm, after which the AC kicks in again, and the cycling continues. Now, how often the AC will do this depends on the difference between the outside temperature and the cabin temperature you set. The bigger the difference, the more often the AC will cycle. Causes and Fixes AC Relay The AC relay works like any other relay. Meaning it’s a switch with a signal wire where when you turn the AC on, the signal from the button closes the relay, which is actually the main AC switch. And that also happens when you turn the AC off. Now, the problem with relays is that they are mechanical components as much as they are electronic, and that’s especially true for older cars. The specific problem is that inside the relay is a thin metal strip that clicks on and off against another metal contact point, and that way, it switches the AC on or off. However, that metal strip wears and thins over time, causing it to overheat, and every time it overheats, it loses contact and switches the AC off. Then, as it cools down, it makes contact again, turning the AC on, and the cycle repeats. It’s also worth mentioning that this will only happen when you manually turn the AC on or have it on auto. Otherwise, a faulty relay won’t cause the AC to turn on by itself. Now, testing the relay is fairly complicated, but you can at least check if it’s worn out fairly easily. First, check in your owner’s manual where the AC relay is located and pull it out. After that, remove the plastic housing/cover and inspect the metal tabs looking for holes or other obvious signs of overheating like a changed color. Also, if you can’t see any tabs, take the relay to the closest electrician and have him test it; it won’t take him longer than 5-10 minutes. And in the end, if it turns out you have to replace the relay, a new one will cost you $10-$25. Overcharged AC Too much refrigerant in the AC system is more detrimental to all the components than running the AC with low refrigerant or a leak. When the AC system is overcharged, it will put more stress on all the components, the compressor could overheat, and the rest of the system is more likely to freeze. When either of the two happens, the AC compressor will turn off until all the temperatures are in order and turn back on again, but keep in mind this will happen much more often than usual. Furthermore, modern AC systems have a refrigerant pressure sensor and will automatically shut down if the pressure is too high, leading the AC to turn on and off randomly. Now, unless you have recharged or topped up the refrigerant using a DIY can, the system is unlikely to be overcharged. However, that can happen in professional settings, too, if the technician working on your car is inexperienced or negligent. Well, whatever the case, if you have only recently recharged the AC, have the system inspected. The technician will check the system pressure, and if the system is overcharged, remove some of the freon using specialized equipment, dispose of it in an environmentally friendly way, and have you on your way in an hour or so. In the end, the whole job shouldn’t cost more than $100
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The AC is one of the few automotive systems that are fairly independent of other components like the engine and electronics. That makes it significantly harder to diagnose and nearly impossible to repair, even for someone with mechanical experience.
Still, if you are facing the symptom at hand, at least at the end of this article, you will know what’s causing it and even learn how to fix it if it turns out to be something simple. But more on fixing the issues later; before that, here is a quick rundown of all the possible causes.
If your car AC turns on and off randomly, first make sure it’s not just normal AC cycling and whether or not the AC is set to auto. But if that checks out, it could be a worn-out AC relay; the AC is overcharged with freon, there is a refrigerant leak, a faulty interior temperature sensor, or shorted AC controls.
Normal AC Cycling
First and foremost, it’s important to understand what normal AC cycling is to avoid wasting money on fixing something that isn’t broken. The AC compressor can’t always be on regardless of the setting, auto or manual.
The reason is that the AC coils and the evaporator would freeze because of the extremely low temperatures constant running would cause. Instead, the AC compressor has to turn off every half a minute or so to allow the system to warm up and defrost.
Normal AC cycling goes something like five to ten seconds on and 15-30 seconds off – if it’s noticeably more often than that, skip to the causes and fixes.
Also, if you set your AC to auto, it will again cycle on and off, but it will be much more noticeable because the blower will change speed as well. The reason for that is the compressor is trying to maintain a certain temperature in the cabin.
And once the cabin reaches the temperature, it shuts off to save fuel and the environment. Then, the cabin will warm, after which the AC kicks in again, and the cycling continues. Now, how often the AC will do this depends on the difference between the outside temperature and the cabin temperature you set. The bigger the difference, the more often the AC will cycle.
Causes and Fixes
AC Relay
The AC relay works like any other relay. Meaning it’s a switch with a signal wire where when you turn the AC on, the signal from the button closes the relay, which is actually the main AC switch. And that also happens when you turn the AC off. Now, the problem with relays is that they are mechanical components as much as they are electronic, and that’s especially true for older cars. The specific problem is that inside the relay is a thin metal strip that clicks on and off against another metal contact point, and that way, it switches the AC on or off.
However, that metal strip wears and thins over time, causing it to overheat, and every time it overheats, it loses contact and switches the AC off. Then, as it cools down, it makes contact again, turning the AC on, and the cycle repeats. It’s also worth mentioning that this will only happen when you manually turn the AC on or have it on auto. Otherwise, a faulty relay won’t cause the AC to turn on by itself.
Now, testing the relay is fairly complicated, but you can at least check if it’s worn out fairly easily. First, check in your owner’s manual where the AC relay is located and pull it out. After that, remove the plastic housing/cover and inspect the metal tabs looking for holes or other obvious signs of overheating like a changed color.
Also, if you can’t see any tabs, take the relay to the closest electrician and have him test it; it won’t take him longer than 5-10 minutes. And in the end, if it turns out you have to replace the relay, a new one will cost you $10-$25.
Overcharged AC
Too much refrigerant in the AC system is more detrimental to all the components than running the AC with low refrigerant or a leak. When the AC system is overcharged, it will put more stress on all the components, the compressor could overheat, and the rest of the system is more likely to freeze.
When either of the two happens, the AC compressor will turn off until all the temperatures are in order and turn back on again, but keep in mind this will happen much more often than usual.
Furthermore, modern AC systems have a refrigerant pressure sensor and will automatically shut down if the pressure is too high, leading the AC to turn on and off randomly. Now, unless you have recharged or topped up the refrigerant using a DIY can, the system is unlikely to be overcharged. However, that can happen in professional settings, too, if the technician working on your car is inexperienced or negligent.
Well, whatever the case, if you have only recently recharged the AC, have the system inspected. The technician will check the system pressure, and if the system is overcharged, remove some of the freon using specialized equipment, dispose of it in an environmentally friendly way, and have you on your way in an hour or so. In the end, the whole job shouldn’t cost more than $100.
Faulty Temperature Sensor
This goes for cars that have an automatic climate control button. In the auto setting that we have explained earlier, the AC maintains the cabin temperature automatically, and for that, it needs an interior temperature sensor.
The sensor is usually located behind a small grill near the climate controls or next to one of the even front-facing vents. Now, if the sensor is faulty, it will send wrong and erratic information to the AC, causing it to turn on and off sporadically and even change the vent air temperature randomly.
However, keep in mind that these sensors rarely fail, but even so, the symptoms we mentioned can hardly be attributed to anything else. So, if you notice the two symptoms, it’s a good idea to remove them and have the sensor tested before buying a new one.
A new temperature sensor rarely costs more than 15 bucks, but it’s also worth mentioning that it’s not removable in some older cars. Instead, the sensor is integrated into the AC controls, and you have to replace the whole unit. Still, with a quick google search, you can easily check what case is with your car.
Refrigerant Leak
If your car has a refrigerant leak or is naturally low on freon, in case you haven’t recharged it in over two years, the system will have to cycle much more often to cool the cabin. That will make it seem as if the AC is turning on and off randomly, but instead, it’s struggling to maintain the set temperature. Furthermore, if the system is critically low on freon, the compressor will turn off entirely instead of cycling.
And in the end, if you don’t fix the leak or recharge the system, the AC will refuse to start altogether, and you will see it turn off by itself about a second or two after you turn it on. And another giveaway that the AC is low on freon is it blows lukewarm air out the vents, or at least the air is noticeably warmer than what you are used to.
Now, recharging the AC costs between $50 and $150, depending on how much freon is missing. However, if the system has a leak, it’s impossible to predict how much it will cost to fix. Still, we can say that it will cost about $50 to detect the leak, then another $150 to recharge the system and anywhere between $100 and $500 to fix the leak. That brings the total to at least $300.
AC Controller
The last possible cause, the AC controller, is the least likely. The controllers are fully electronic, and since they are inside the cabin, they are well protected from moisture, heat, and other contaminants. Still, every once in a while, the AC controls can fail, especially if your car gets wet for one reason or another.
Maybe you forgot to close the window, and it started raining, maybe the car was flooded, or it could be that one of the drains is clogged and is leaking water into the cabin. But ultimately, it could also be a factory defect.
Now, a faulty AC controller will quite obviously turn the AC on and off randomly, and besides that, you won’t notice any other symptoms. It’s also important to mention that the AC will turn on without you first turning it on manually, which doesn’t happen with the other causes. Besides the AC turning on and off, you could also notice other functions and settings change without your input, and that’s the most concrete giveaway the AC controls are faulty.
Now, the cost to replace AC controls is widely different between vehicles, but the best thing you can do is buy one from a junkyard because, again, these rarely ever fail. That way, it will cost you no more than 50 bucks, and with the help of some youtube videos, you can replace it at home fairly easily.
Can a Car Run Without an AC Relay?
Yes, a car will run without an AC relay. The AC system in a car is independent of all other systems, and the AC relay is nothing more than an on-and-off switch for the AC, and the car will also run without an AC fuse. So, without an AC relay, the AC won’t work, but it won’t affect anything else.
What Happens If Car AC Has Too Much Refrigerant?
The worst thing that can happen if a car’s AC has too much refrigerant is it can destroy the compressor. When the system is overcharged, the compressor temperature will reach freezing, and the excess freon can collect in the compressor and destroy the internal components. Other than that, the AC evaporator can freeze, and the condenser. Both will cause the AC to stop working either randomly or indefinitely.
What Usually Leaks on the Car AC System?
The most common leaks in a car AC system can be found on the two valves used for charging the AC, which is also the cheapest to fix. Other common spots are the condenser and the evaporator. Of the two, the condenser is cheaper to replace because it sits in front of the radiator and is easier to access. The evaporator, on the other hand, is inside the dashboard, so it takes much more time and money to replace.
How Often Should Your Car AC Be Charged?
How often you should recharge your car AC depends on how often you use it. For example, if you live in a hot climate and use the AC daily, recharging it every one to two years is ideal. However, if you live in a colder climate, recharging it every two to three years will be enough.
Can You Overuse Car AC?
No, you can’t overuse car AC. The car AC is made to last the lifetime of a vehicle provided you service and recharge the system regularly. That way using it more often or overusing it won’t lower the AC’s lifespan noticeably.
Why Does My Car AC Randomly Stop Cooling?
If your car’s AC randomly stops cooling, in other words, randomly starts blowing hot air, there is likely moisture in the system mixed with the refrigerant. To fix this, you have to take your car for a service where the technician will remove all the moisture using a vacuum pump and recharge the refrigerant if needed.
Does Driving With AC on Use More Fuel?
Yes, driving with AC on uses more fuel. The AC compressor is run by the engine using a belt. Now, when the AC is off, the AC compressor clutch is not engaged, and the engine belt spins only the pulley. But when the clutch is engaged, the AC compressor consumes about 5 to 15 percent of the engine’s power and increases the fuel consumption by 10 to 20 percent.
What Is the Most Common Cause of AC Failure in a Car?
The most common cause of AC failure or the AC not cooling is low refrigerant. The AC system in modern cars is pretty sturdy, but like all systems, it will lose some refrigerant over time, and when it does, the AC won’t be as efficient. The second most common cause is refrigerant leaks, and after that comes component failures like the compressor, evaporator, expansion valve, drier, etc.
Conclusion
If you are sure your AC isn’t just cycling and is in fact randomly turning on and off, you should start by checking the AC relay. To do that properly, remove it and take it to an electrician, who will quickly test it using a multimeter.
Other than that, try to remember when was the last time you recharged the AC system. If it was recent, it could be the AC is overcharged, or if it’s been a while, the AC could be low on refrigerant or be leaking. And if everything checks out, the only thing left is to test your interior temperature sensor and the climate controls.
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