Last Updated: 07.04.2022

To make sure that you are getting the most out of your car’s AC system, it is imperative to ensure that the compressor is working properly.

The compressor is sort of the heart of your car’s air conditioning system. Its main task is to ensure the refrigerant is pressurized before sending it to your vehicle’s evaporator. If anything goes wrong, you will not be receiving that cold burst of air on the hottest days.

You might have encountered a bad AC compressor, and you’ve probably asked yourself, what are the signs of a bad AC compressor in my car?

Let’s discuss this in detail!

What Is an AC Compressor?

vehicle AC air compressor

It is the main part of your car’s air conditioning system, which is mainly responsible for pressurizing the refrigerant.

What Does an AC Compressor Do?

Your car’s AC compressor is at the center of the AC system’s functionality. Its main duty is to ensure the refrigerant is pressurized before it is sent to the evaporator.

That cold burst of air you get on a hot day is the outcome of a thermodynamic development that frees humid air of any hot gases. And it is all thanks to your car’s AC compressor. 

Whenever you push the AC button in your car, you are simply activating the AC compressor, which then goes to work to pressurize the refrigerant.

The AC compressor has a pulley on the outside that free spins with the engine. It’s fitted with an armature that turns the pump on the inside. It has an electromagnetic clutch that closes the gap between the pulley and the armature. When engaged, these two parts will turn together, thereby engaging the AC compressor so that it can cool the system.

Then, the newly adjusted refrigerant is pushed to the condenser where heat is removed. After that, it is a quick stop to the dryer to eliminate impurities and in the expansion valve to be depressurized.

The refrigerant then moves to your air vents, ice-cold and moisture-free.

Where Is It Located in Your Vehicle?

You can find your AC compressor in your car’s engine compartment.

mechanic pointing where the ac compressor is located

It sits on the crank side of your vehicle and it runs off the drive belt. To access it, remove the low-pressure hose, high-pressure hose, electrical connection, and the 10mm bolts. Once all the bolts are out, you can remove your AC compressor for inspection or replacement.

Why Is It So Important?

The AC compressor makes air conditioning possible in your vehicle. Without it, you will not be able to change the temperature of the air from your vents.

An AC system that functions properly makes driving on a hot day comfortable as well as enjoyable. If for any reason the compressor fails, then the air vents in your vehicle will suck in the hot and humid air from the surroundings and blow it back into your car.

Signs of a Bad AC Compressor in My Car 

Now, how do you know your AC compressor is not working? Here are some telltale signs:

  • Vent Blowing Hot Air

The first and most obvious sign of a bad AC compressor is the vent system blowing hot air instead of cold. This is a clear indication that there is something wrong in your AC system.

The hot air may be due to low freon levels. The substant, Freon, is made up of a lubricant and pressurized gas. It is normally used to provide ice-cold air to the cabin of a car. As we mentioned earlier, your AC works effectively by circulating the pressurized refrigerant. So, low levels will end up affecting performance.

The suction pipe of your system is going to feel a little bit warm.

  • Noises Due to a Bad Compressor

When the system starts to mechanically go bad, it can make a loud metallic noise. In most cases, this is a sign that the bearings inside the compressor are failing.

air compressor pulley and clutch

In some cases, the AC compressor pulley may begin to wobble and end up breaking. The serpentine drive belt could also slip off. In worst-case situations, the clutch and the AC compressor pulley could break off.

A clicking or knocking sound could indicate that the mounting bolts are loose. You can do a quick inspection under the hood when the engine is running. A buzzing sound, on the other hand, could be a sign that your AC compressor is wearing out, although it could also be due to an overcharged system.

If a rattling noise is what you hear after pushing the AC button, then you probably have some serious issues to be concerned with. Your compressor pulley or even the serpentine belt may be worn out. In certain conditions, the clutch could produce a rattling uproar. 

  • Stuck Compressor Clutch

It is the work of the compressor clutch to engage and disengage the pulley system from the engine. But a stuck or locked up compressor clutch will not be able to do its job and could possibly kill your car’s AC system. So, a stuck or locked-up clutch should be dealt with immediately after it is diagnosed. 

  • Mysterious Leaks

It is advisable to check your car for any visible signs of leaks, which are known to cause an AC compressor to fail.

Unlike other signs that are easy to spot, leaks require some effort to spot. You need UV dye refrigerant when looking for leaks. Make sure your car is on and your blowers are at max setting. Also, the compressor clutch has to be engaged.

Start by locating your low-pressure port to add the UV dye refrigerant. Connect the tube that is attached to the bottle to the low-pressure port. Then discharge the refrigerant dye into the AC system. 

It should take you roughly 5 to 10 minutes to empty the bottle. You need a UV leak detection kit to check out the leaks. You should probably do this at night for better results.

If there is a leak or leaks in the system, you will see the UV dye. Make sure you check all the areas properly, especially those with gaskets.

Alternatively, you can use an AC leak detector. This device is uniquely designed to detect any escaping refrigerant vapors.

A spray bottle filled with soapy water can be used as well since your refrigerant would be leaking as a gas. The soap would bubble up where the leak is happening. 

  • Clogged Cabin Air Filter

If the cabin air filter is clogged, it will limit airflow and not allow enough cold air to make it to the vents. This will force your AC system to work extra hard to keep the cabin cool. In some cases, it can cause some sort of rancid smell to come out of the vent

The cabin air filter is responsible for filtering and retaining exhaust gases, pollen, bacteria, and dust that may pass into your ventilation system. This will make the interior of your car healthier and more comfortable. You can find the cabin air filter under the hood of your car, under the dashboard, or inside the glove compartment.

The cabin air filter is a maintenance item that you have to service according to your owner’s manual. Do the maintenance as often as necessary to keep the air you breathe inside your vehicle free from allergens and other impurities.

  • Bad Blower Motor

This is a very important part of your car’s AC system that could be causing your AC problems and not the AC compressor. You cannot effectively control your cabin temperature without the blower motor.

This small rotary motor is responsible for driving a fan that is situated behind the vents. This is the mechanism that pushes ice-cold air at varying speeds.

There are a few things that can tell you of a bad blower motor. The first and most common sign is weak airflow. This is probably because the blower motor is worn out over time and it isn’t strong enough to push enough air.

You may experience a burning smell or smoke in your cabin while driving. If that’s the case, then you probably have a burned blower motor. Any blown fuse in the circuit of your blower motor could indicate that the circuit is overloaded.

So, before blaming your AC compressor for the lack of cold air in your car, you should first check for signs of a bad blower motor.

How to Diagnose an AC Compressor

The proper way of diagnosing problems with your AC system is by using an AC manifold gauge set.

What is an AC manifold gauge? 

This is a unique system engineered to test AC systems. A typical gauge set has the low side and high side pressure reading to your car’s AC system. You can use it to test, evacuate, as well as fill AC systems.

What You Will Need

  • Manifold gauge

Step-By-Step Instructions

Step 1: Set Up the Manifold Gauge

Most cars use R134A refrigerant. So, this setup is for the use of R134A.

Start by taking a look at the valves and make sure they are in the correct position before you begin the setup. The manifold gauge knobs and hoses should be closed. Now you are ready to hook up the manifold gauge.

The high side gauge has a red hose and the low side gauge has a blue hose. The yellow hose at the middle is for adding refrigerant into the system. Locate the low and high side fittings of your AC unit and connect the hoses. Turn the knobs on for the gauge to work.

Step 2: Start Your Car and Read the Pressure on High and Low Sides

Start the car and turn your AC on full blast. Remember to roll down all your windows. Allow your AC system to run for around five minutes. This will allow the system to cycle and run properly. Take the pressure readings on the low and high sides. What you are looking for are general readings.

When the AC is off, the static pressures are going to be equal on both sides. But the readings will differ when you turn the AC on. On average, you are going to see pressures that are relative to ambient temperature. 

A good AC system is going to be right in the range of 35 to 25 on the low side and above 150 on the high side. If the low side is really low like 20, 15, or 10, and the high side is under 100, that is usually an indication of undercharging, which means, there is not enough Freon in your system.

If the low side is up at around 45, 55, almost 60 and your high side is creeping up above 250 and 300, then that’s an indication of an overcharge, meaning there’s too much freon in the system.

How to Test AC Compressor Clutch Function?

The AC compressor clutch is usually located at the front of the engine by the drive belt. It only turns when it is called upon to circulate refrigerant through the AC system. So, the clutch turns on and off as needed to deliver cool air.

If the air conditioner control button on your car ever starts flashing, then that’s a warning sign that the clutch in the compressor isn’t turning as it should. You can, however, check the coil resistance of the AC compressor clutch.

The clutch coil refers to an electromagnetic coil that usually closes the gap between the pulley and the clutch hub. You can easily diagnose a defective or bad clutch coil.

What You Will Need

  • Ohmmeter
  • Multimeter

Step-By-Step Instructions

Step 1: Check Clutch Coil Resistance

Start by unplugging the coil connector and reading the resistance with an ohmmeter. The clutch coil has positive and negative signs on the backside. Use alligator clips to attach the positive and negative terminals and take the readings.

A bad clutch coil normally gives a reading that is less than 3 ohms or more than 5 ohms. But a good coil usually gives a reading of more than 3 ohms or less than 5 ohms. A new AC compressor clutch will give a reading of around 4.5 ohms.

Step 2: Check Operational Voltage

Start the engine of your car and initiate your AC system by pressing your AC button. Using a multimeter, connect the positive probe to the positive side of the clutch coil connector and the negative probe to the negative side of the clutch coil connector.

The clutch coil’s measurement should be within one volt of charging voltage. A significant drop in the available voltage can cause premature failure of the AC compressor clutch. You may have to use a wiring diagram to find why you’ve got excessive resistance if the operational voltage isn’t correct.

How to Replace an AC Compressor Clutch?

We have provided you with all the possible signs of a bad AC compressor in the car. If you have observed all the aforementioned signs, especially AC compressor leaks, then we think it is time you replaced your AC compressor. The process may be daunting for first-timers, but you can do it yourself with proper tools and guidelines.

What You Will Need

  • A socket set
  • Extensions and ratchet
  • A breaker bar
  • Torque wrench
  • Metal wire brush
  • Socket universal joint

Step-By-Step Instructions

Step 1: Empty the System of Refrigerant

Connect the manifold gauge and take the readings. If the system has no refrigerant, the low and high sides of the gauge should read 0 psi. You can use a refrigerant recovery machine to recycle any refrigerant, or you can just have a mechanic recycle the refrigerant for you.

Step 2: Remove the Belt and Refrigerant Lines

The next thing you have to do is pry on the fan belt tensioner and pull the fan belt off. Access the bolts and remove the refrigerant lines on top of the compressor by unscrewing the bolts and pulling them off. 

Put paper towels in the openings of the two hoses to prevent the entry of debris. Remove the electrical connection and the bolts that attach the compressor to the engine.

Step 3: Transfer the Wiring Harness

The new compressor will have one wiring harness. So, you have to take off the wiring harness from the old compressor and transfer it to the new one. Make sure the compressor is prefilled with oil or you may have to do it yourself.

Step 4: Mount the New Compressor to the Engine

Compare the two compressors and make sure that everything is properly configured. Clean the threads on the bolts that hold the compressor to the engine.

Position the new AC compressor into place and insert the bolts. Then, torque (18 foot-pounds) the bolts in the correct order. Insert the electrical connector and make sure it pops into place. Attach the hoses and put the belt on and you are done.

Related: Why Does My Car Shake When the AC Is On?

The Bottom Line

As we mentioned at the beginning of this article, the AC compressor is the heart of your air conditioning system. It is the mechanism that controls the pressure, and without it, you will have no cold air coming out the vents.

So, you should look out for signs of a bad compressor if the AC system doesn’t cool the car. But you shouldn’t be too quick to blame the AC compressor for the lack of cool air. Sometimes, other components could be causing the problem. The issue could be because of a clogged cabin air filter, clogged condenser, or faulty cooling fan.