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AC High Side Not Building Pressure – Why & What To Do?

Did you check the high side pressure of your car’s AC system just to find out that it isn't building pressure? Do you want to know why this is happening and what you can do about it? You’ve come to the right place, for we have researched this question and have the answer for you.

One common reason why the high side will not build up pressure is that the compressor fails to pressurize the refrigerant before it moves to the condenser. The high side will also have low pressure if your AC system is low on refrigerant.

Let’s talk more about how to determine if your automotive air conditioning system is low on refrigerant or has a failing compressor in the succeeding sections. Learn about the things you can do to fix your compressor below.

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Read on!

The High Side And The Low Side

The air conditioning system inside your car has a high side and a low side. Both sides carry a liquid-state refrigerant and a gas-state refrigerant. The changes in the state of the refrigerant allow the AC system to cool the air inside your car.

Let’s look at this process more closely, so you’ll understand how the system pressurizes the refrigerant of your car.

Checking air flow of an ac

The High-Pressure Side

The compressor compresses gaseous refrigerant and increases its pressure. The increase in pressure also increases the temperature of the refrigerant. This is important so that the temperature of the refrigerant will always be warmer than the air outside of your car.

If the air outside of your car is warmer than the temperature of the refrigerant, then the warm temperature that the refrigerant took from inside the cabin of your car will not dissipate. This will cause the refrigerant to become ineffective. Thus, your car’s air conditioning will also be ineffective.

This high-pressure and high-temperature gas from the compressor goes to the condenser. The condenser conducts the high temperature of the refrigerant and spreads it to its fins.

The condenser looks like a small radiator that you’d find in the front area of your car. Outside air flows through the condenser as you drive your car and blows through the fins of the condenser. The movement of the colder outside air through the condenser’s fins dissipates the warm temperature.

Thus, the high-pressure gaseous refrigerant cools as it passes through the condenser. The release of heat causes the refrigerant to change from a high-pressure gas to a high-pressure liquid. The refrigerant leaves the condenser as a high-pressure liquid and goes to the dryer.

The presence of water vapor in the refrigerant can damage the AC system. Thus, the dryer makes sure that doesn’t happen by removing trace amounts of water vapor in the liquid refrigerant by passing it through a series of desiccants inside the dryer.

The Low-Pressure Side

After the liquid refrigerant leaves the dryer, it goes to the expansion valve. The expansion valve marks the start of the low-pressure side of the AC system.

Between the dryer and the expansion valve is the service port for the high-pressure side of the AC system. This is where you measure the pressure of the refrigerant at the high-pressure side. Therefore, from the compressor to the high-pressure service port, there is nothing else to pressurize the refrigerant.

The expansion valve decreases the pressure of the refrigerant. A drop in the pressure results in a drop in the temperature of the refrigerant. This is to ensure that the low-pressure liquid refrigerant will be cold enough to cool the fins of the evaporator.

Additionally, the expansion valve controls the amount of refrigerant that goes to the evaporator. This allows you to control the temperature that gets into the passenger cabin of your car.

A blower pushes warm air from inside the cabin through the cold fins of the evaporator, moving the heat from the air to the fins of the evaporator. The heat changes the liquid refrigerant to a gaseous form. Moreover, since the heat moves from the air to the evaporator, the resulting air becomes cold as it goes back into the cabin.

The gaseous refrigerant moves out of the evaporator and returns to the compressor.

The low-pressure service port is between the evaporator and the compressor.

Taking Pressure Readings

fill refrigerant.Device and meter liquid cooling in the car by specialist technicians

When you take pressure readings on your AC system, always check it with a pressure chart.

Before you check the pressure chart, always take the ambient temperature of the surrounding air. Take note of the pressure ranges for the low-pressure and high-pressure gauges, not just the high-pressure gauge. The result of both gauges will tell you the most probable cause of the problem.

Once you get a pressure reading for the high-pressure side and the low-pressure side, the next step is to compare the results to the range of values on the chart.

There are only two possible outcomes in the chart where you have a high-pressure side value that is below the range of values.

The first outcome is when you get a value for the low-pressure side and the high-pressure side that are below the provided ranges for both. This combination means that your AC system is low on refrigerant.

However, a mechanic can recharge your refrigerant, they'll need to look for a possible leak. If you recharge your refrigerant without plugging the leak, then you will end up losing the refrigerant again and quickly face the same issue.

The second outcome comes from a combination of a high-pressure side value that is lower than the range and a low-pressure side value that is higher than the range. In this situation, the problem is the compressor. You need to service the compressor or replace it.

How to fix a failing AC compressor?

mechanic's hand that is giving special tools. Remove the pulley. Repair the air compressor of the car in the garage, AC High Side Not Building Pressure - Why & What To Do?

In most cases, the compressor fails when the clutch fails.

The compressor of your AC receives power from the serpentine belt of your car. The serpentine belt goes to different components of your car to provide them with the power that they need. This power comes from the engine, providing the compressor power as long as the engine is running.

However, even if you don’t use the air conditioner, the compressor’s pulley keeps on spinning. To reduce wear on the AC, the compressor has a clutch system that disengages the compressor from its pulley if you turn off your AC.

If the clutch doesn’t engage when your AC system needs it, then the compressor will not be able to do its job.

Compressor Clutch Relay

ac compressor relay regulator

The first thing you can check is the compressor clutch relay. You can usually find it in the fuse block inside the engine bay.

A relay is an electrical component that allows small current circuits to serve as a switch for large currents. This allows the AC controller inside the car to activate the current that powers the electromagnet that activates the compressor clutch.

The cover of the fuse block should have a diagram of the different relays and fuses inside the block. A clutch relay can have several different labels, like MGCLT (magnetic clutch). Check the user manual to find out the exact name of the compressor clutch relay of your AC system.

If the relay fails, the compressor clutch will no longer be responsive to the controls inside the car.

Fortunately, replacing a relay is similar to replacing a fuse. Just pull out the failing relay and plug in the replacement. Make sure you get the exact replacement relay.

Compressor Pulley Bearing

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A failing compressor pulley bearing can cause the pulley to seize or slow down its movement. When this happens, turning on the AC will push the clutch into place, but the pulley is barely moving. This means that the pulley will not be able to get enough power from the serpentine belt to make the compressor do its job.

Replacing the bearing of the compressor will fix this issue. The bearing is what makes the pulley move more freely when the serpentine belt rotates it. A worn bearing makes it harder for the pulley to rotate.

AC Compressor Speed Sensor

The compressor speed sensor is responsible for monitoring the speed of the compressor and reporting it to the AC amplifier or ECU. You’ll find it at the back or bottom of the AC compressor.

The AC amplifier compares the speed of the compressor and the engine. The ratio between the engine speed and the compressor speed should be constant. Some car models inform the driver that there is a variance in the speed of the engine and the compressor.

A variation in the RPM of the engine and the compressor can mean two things.

First, it could mean that there is a serpentine belt slippage. A worn serpentine belt can start to slip and lose grip. Replacing the serpentine belt will fix this issue.

Second, it could mean that the compressor clutch is slipping. A problem with the bearing can lead to this issue.

Additionally, a lack of power supply can also cause this problem. Low electrical energy can cause the electromagnet to become less effective in keeping the compressor clutch in place.

Check both possible issues to identify the best solution.


mechanic's hand that is giving special tools. Remove the pulley. Repair the air compressor of the car in the garage

Checking the pressure reading from both the high-pressure side and the low-pressure side will give you an idea of what is causing the high side to not build up pressure.

If you enjoyed reading this article, you might find the articles below equally enjoyable to read:

What Does An Overcharged AC Sound Like? [What Other Problems Have Similar Sounds?]

Can Overcharging Car AC Damage The Compressor?