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Car AC Compressor Stays On All The Time?

The hottest days of summer are over, however you find the air conditioner on your car running all the time, or cycling on and off continuously. If it’s not because of the weather, then AC compressor stays on all the time means there is something wrong with the unit. 

If not troubleshooted as soon as possible, you will face a jaw-dropping electricity bill, plus your AC will be subject to extra wear and tear from unnecessary over-working. 

It should be noted that the compressor has the second highest failure rate among all the components of the air conditioning system. Learn the possible reasons for AC compressor stays on all the time and how to troubleshoot this common maintenance issue.

What does an AC compressor do?

The compressor is the component that powers that air conditioning unit.

When you turn on the air conditioner, the compressor compresses and heats up the refrigerant, a special mixture of fluid. This compressed and hot refrigerant is then sent through the condenser to cool down. 

The refrigerant then goes through the dryer to get rid of contaminants. Once purified, the compressed refrigerant passes through the expansion valve, where it is stripped of its pressure. After traveling through the evaporator, the refrigerant is now super clean, dry and cold. 

To create the gusts of cold air blowing out of your car’s AC, air is sent through this newly cold refrigerant in the compressor tank to cool down and then blown out through the blowers.

car AC compressor stays on all the time
The cold air from the AC is the result of air passing through to dry, cold refrigerant.

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Why does an AC compressor ‘cycle’ on and off?

It is normal for your car’s AC compressor to cycle on and off. For instance, hot hotter days, the cooler you set your AC, the more the compressor has to work to maintain the desired temperature. 

Once the desired temperature is achieved, the AC compressor may cycle off for a while until there is a change in your car’s temperature. Thus the air conditioning system cycling on and off is not a problem, but the frequency of cycling might indicate problems. 

An example of such abnormal cycling frequency is that instead of cycling on and off, the AC compressor stays on all the time.

car ac compressor
The AC compressor usually cycle on and off and is run by a belt.

AC compressor stays on all the time – Common causes 

Auto-climate control

If you have auto-climate control on, the AC compressor will run continuously, summer and winter, if the auto-climate control is on any setting other than off. 

Low refrigerant level

If the amount of refrigerant in the AC compressor reservoir is reduced, most likely due to a leak in the coils, the compressor might not have enough refrigerant to effectively run the AC. Due to this inefficiency, the AC compressor will have to work harder than usual. 

In this case, you’ll likely need a professional to find and repair the leak and restore the refrigerant to the correct level.

Detecting leaks 

Leaks most commonly occur in worn O-rings, seals and hoses of the air conditioning system, followed by leaks in a pinhole of the condenser and evaporator, and leaks in the compressor shaft seal. 

To detect leaks, add refrigerant to restore it to the correct level, then turn the AC on. The most simple but effective method is to spray soapy water to suspected areas and watch for bubbles. 

car ac compressor leak
Leaks most likely occur in worn O-rings, seals and hoses.

If a leak occurs in the O-rings, seals or hoses, simply have them replaced will solve the problem. However, if the issue lies with a leaking condenser or evaporator or the compressor shaft seal, repair is expensive and more troublesome, which should be performed by a professional.

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Dirty or blocked condenser

A condenser full of dirt and debris will not work as effectively as normal to cool down the compressed and hot refrigerant that passes through, resulting in the AC compressor having to overwork.

Dirty evaporator coil 

Similar to the condenser, the evaporator coil may be covered with dirt and debris, impeding its drying process and the system’s overall cooling process, thus making the AC compressor work harder to cool your car’s interior. 

How much to replace an AC compressor?

car ac compressor
A car’s AC compressor.

If you are encountering issues with your AC compressor without knowing exactly the reason, it’s best to have it serviced. Only EPA-certified mechanics may service mobile air conditioner systems.

You might wonder how much replacement costs for AC compressor. The average cost for an AC compressor replacement is between $500 and $700 depending on the particular car. The labor costs alone are estimated to be just over $200.

Rest assured, an AC compressor usually lasts for 8-10 years, which is typically how long an average car owner would keep the car for.